U.S. patent application number 13/745028 was filed with the patent office on 2014-07-24 for low-loss uv to mid ir optical tellurium oxide glass and fiber for linear, non-linear and active devices.
This patent application is currently assigned to NP PHOTONICS, INC.. The applicant listed for this patent is NP PHOTONICS, INC.. Invention is credited to Arturo Chavez-Pirson, Daniel Larry Rhonehouse.
Application Number | 20140205258 13/745028 |
Document ID | / |
Family ID | 51207747 |
Filed Date | 2014-07-24 |
United States Patent
Application |
20140205258 |
Kind Code |
A1 |
Rhonehouse; Daniel Larry ;
et al. |
July 24, 2014 |
LOW-LOSS UV TO MID IR OPTICAL TELLURIUM OXIDE GLASS AND FIBER FOR
LINEAR, NON-LINEAR AND ACTIVE DEVICES
Abstract
A tellurium oxide glass that is stable, strong and chemically
durable exhibits low optical loss from the UV band well into the
MIR band. Unwanted absorption mechanisms in the MIR band are
removed or reduced so that the glass formulation exhibits optical
performance as close as possible to the theoretical limit of a
tellurium oxide glass. The glass formulation only includes glass
constituents that provide the intermediate, modifiers and any
halides (for OH-- reduction) whose inherent absorption wavelength
is longer than that of Tellurium (IV) oxide. The glass formulation
is substantially free of Sodium Oxide and any other passive glass
constituent including hydroxyl whose inherent absorption wavelength
is shorter than that of Tellurium (IV) oxide. The glass formulation
preferably includes only a small residual amount of halide.
Inventors: |
Rhonehouse; Daniel Larry;
(Tucson, AZ) ; Chavez-Pirson; Arturo; (Tucson,
AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NP PHOTONICS, INC. |
Tucson |
AZ |
US |
|
|
Assignee: |
NP PHOTONICS, INC.
Tucson
AZ
|
Family ID: |
51207747 |
Appl. No.: |
13/745028 |
Filed: |
January 18, 2013 |
Current U.S.
Class: |
385/142 ;
501/37 |
Current CPC
Class: |
G02B 6/02 20130101; C03B
2201/60 20130101; C03C 3/253 20130101; C03C 3/122 20130101; C03C
3/23 20130101; C03B 2205/14 20130101; Y10T 428/2933 20150115; C03C
13/048 20130101; C03C 4/0071 20130101; G02B 6/102 20130101; C03C
4/10 20130101 |
Class at
Publication: |
385/142 ;
501/37 |
International
Class: |
C03C 13/04 20060101
C03C013/04; G02B 6/00 20060101 G02B006/00 |
Goverment Interests
GOVERNMENT RIGHTS
[0001] This invention was made with United States Government
support under Contract Number N68335-11-C-0035 with the Naval Air
Warfare Center AD (LKE). The United States Government has certain
rights in this invention.
Claims
1. An oxide glass composition, comprising: a glass network former
of Tellurium (IV) oxide TeO.sub.2 of 65 to 90 mole percent, said
Tellurium (IV) oxide having an absorption feature with an inherent
absorption wavelength corresponding to its phonon energy peak; a
glass intermediate of Lanthanum (III) oxide La.sub.2O.sub.3 of 0.2
to 15 mole percent, said Lanthanum (III) oxide having an absorption
feature with an inherent absorption wavelength longer than that of
Tellurium (IV) oxide; and a glass modifier MO of 0.2 to 35 mole
percent where M is selected from Mg, Ca, Sr, Ba, Zn, Pb and Cd,
said modifier having an absorption feature with an inherent
absorption wavelength longer than that of Tellurium (IV) oxide; a
halide of 0 to 5 mole percent, any said halide having an absorption
feature with an inherent absorption wavelength longer than that of
Tellurium (IV) oxide; and wherein said oxide glass composition is
substantially free of each of the following, Sodium Oxide
Na.sub.2O; Hydroxide OH--; and any other passive glass constituent
having an absorption feature with an inherent absorption wavelength
shorter than that of Tellurium (IV) oxide.
2. The oxide glass composition of claim 1, wherein the Lanthanum
(III) oxide concentration is at least 5 mole percent; the halide
comprises a non-zero amount of Fluoride less than 0.5 mole percent,
and the modifier MO comprises ZnO.
3. The oxide glass composition of claim 1, wherein the Lanthanum
(III) oxide is 5 to 10 mole percent.
4. The oxide glass composition of claim 1, wherein the glass
composition is a passive glass free of any active dopants.
5. The oxide glass composition of claim 1, wherein the glass
composition comprises an active dopant selected from Er, Ho, Tm,
Dy, Sm, Fe and Cr for stimulated emission at a wavelength between 2
microns to 4.5 microns.
6. The oxide glass composition of claim 5, wherein the active
dopant is at least 2.5 weight percent.
7. The oxide glass composition of claim 1, wherein the halide
comprises a non-zero amount of a halide less than 5 mole
percent.
8. The oxide glass composition of claim 7, wherein the non-zero
amount of the halide is less than 1 mole percent.
9. The oxide glass composition of claim 1, wherein substantially
free is less than 0.1 mole percent.
10. The oxide glass composition of claim 1, wherein said glass
exhibits an optical loss of less than 2 dB/m over the entire
spectral range from 0.6 microns to 4.5 microns.
11. The oxide glass composition of claim 10, wherein said oxide
glass exhibits an optical loss of less than 0.5 dB/m over a
spectral sub-range from 0.65 microns to 4.2 microns.
12. The oxide glass composition of claim 11, wherein said oxide
glass exhibits an optical loss of less than 0.3 dB/m over a
spectral sub-range from 2 microns to 4 microns.
13. The oxide glass composition of claim 10, wherein said oxide
glass exhibits an optical loss of less than 0.3 dB/m due to OH-- at
approximately 3 microns corresponding to the peak absorption for
OH--.
14. The oxide glass composition of claim 1, wherein said oxide
glass exhibits a tensile strength exceeding 50 KPSI.
15. The oxide glass composition of claim 1, wherein said oxide
glass exhibits a .DELTA.T=Tx-Tg of at least 100.degree. C. where Tx
is the crystallization temperature of the glass and Tg is the glass
transition temperature.
16. The oxide glass composition of claim 1, wherein the glass is in
the form of an optical fiber.
17. The oxide glass composition of claim 1, wherein the glass
composition comprises an active dopant selected from Ce, Pr, Eu,
Tb, Lu, Bi, Ti, Er and Yb may be used for stimulated emission at
wavelengths less than 2 microns.
18. An oxide glass composition, comprising: a glass network former
of Tellurium (IV) oxide TeO.sub.2 of 65 to 90 mole percent, said
Tellurium (IV) oxide having an absorption feature with an inherent
absorption wavelength corresponding to its phonon energy peak; a
glass intermediate of Lanthanum (III) oxide La.sub.2O.sub.3 of 5 to
15 mole percent, said Lanthanum (III) oxide having an absorption
feature with an inherent absorption wavelength longer than that of
Tellurium (IV) oxide; and a glass modifier MO of 0.2 to 35 mole
percent where M is selected from Mg and Zn, said modifier having an
absorption feature with an inherent absorption wavelength longer
than that of Tellurium (IV) oxide; a non-zero amount of fluoride up
to 0.5 mole percent, said fluoride having an absorption feature
with an inherent absorption wavelength longer than that of
Tellurium (IV) oxide; and wherein said oxide glass composition is
substantially free of each of the following, Sodium Oxide
Na.sub.2O; Hydroxide OH--; and any other passive glass constituent
having an absorption feature with an inherent absorption wavelength
shorter than that of Tellurium (IV) oxide; wherein said oxide glass
exhibits an optical loss of less than 2 dB/m over the entire
spectral range from 0.6 microns to 4.5 microns and less than 0.5 dB
over a spectral range of 2 microns to 4 microns; wherein said oxide
glass exhibits a tensile strength of at least 50 KPSI; and wherein
said oxide glass exhibits a .DELTA.T=Tx-Tg of at least 100 where Tx
is the crystallization temperature of the glass and Tg is the glass
transition temperature.
19. A method for forming an oxide glass, comprising: providing a
mixture of powdered oxides comprising, a glass network former of
Tellurium (IV) oxide TeO.sub.2 of 65 to 90 mole percent, said
Tellurium (IV) oxide having an absorption feature with an inherent
absorption wavelength corresponding to its phonon energy peak; a
glass intermediate of Lanthanum (III) oxide La.sub.2O.sub.3 of 0.2
to 15 mole percent, said Lanthanum (III) oxide having an absorption
feature with an inherent absorption wavelength longer than that of
Tellurium (IV) oxide; and a glass modifier MO of 0.2 to 35 mole
percent where M is selected from Mg, Ca, Sr, Ba, Zn, Pb and Cd,
said modifier having an absorption feature with an inherent
absorption wavelength longer than that of Tellurium (IV) oxide; and
a halide of 0.2 to 7 mole percent, any said halide having an
absorption feature with an inherent absorption wavelength longer
than that of Tellurium (IV) oxide, said mixture comprising a
non-zero amount of a hydroxyl (OH--) impurity; applying heat to the
mixture of powdered oxides in a gas atmosphere to melt the powdered
oxides to form an oxide glass, said halide reacting with the
hydroxyl to remove the hydrogen during the melt; and annealing the
oxide glass to form the finished oxide glass that comprises a
non-zero amount of said halide less than 1 mole percent, said
finished oxide glass substantially free of each of the following,
Sodium Oxide Na.sub.2O; Hydroxide OH--; and any other passive glass
constituent having an absorption feature with an inherent
absorption wavelength shorter than that of Tellurium (IV)
oxide.
20. The method of claim 19, wherein the glass modifier MO comprises
ZnO and the halide comprises Zinc Fluoride, the reaction of Zinc
Fluoride with the hydroxyl removing hydrogen fluoride and producing
an additional amount of ZnO.
21. The method of claim 19, wherein the Lanthanum (III) oxide
concentration is at least 5 mole percent.
22. The method of claim 19, wherein the finished oxide glass
exhibits an optical loss of less than 2 dB/m over the entire
spectral range from 0.6 microns to 4.5 microns and less than 0.5
dB/m over a spectral range of 2 microns to 4 microns.
23. The method of claim 19, further comprising: adding an active
dopant of at least 2.5 weight percent to the glass composition,
said active dopant selected from Er, Ho, Tm, Dy, Sm, Fe and Cr for
stimulated emission at a wavelength between 2 microns to 4.5
microns.
24. An optical device, comprising: a passive fiber configured to
transport optical energy with an optical loss of less than 2 dB/m
over a spectral range from 0.6 microns to 4.5 microns and less than
0.5 dB/m over a spectral range of 2 microns to 4 microns, a
transmissive core of said fiber comprising an oxide glass with a
glass network former of Tellurium (IV) oxide TeO.sub.2 of 65 to 90
mole percent, a glass intermediate of Lanthanum (III) oxide
La.sub.2O.sub.3 of 0.2 to 15 mole percent, a glass modifier MO of
0.2 to 35 mole percent where M is selected from Mg, Ca, Sr, Ba, Zn,
Pb and Cd and a non-zero amount of halide less than 5 mole percent,
said oxide glass substantially free of Sodium Oxide Na.sub.2O,
Hydroxide OH-- and any other passive glass constituent having an
absorption feature with an inherent absorption wavelength shorter
than that of Tellurium (IV) oxide; a source of optical energy in
the Mid IR band between 2 and 4.5 microns optically coupled to the
passive fiber; and a source of optical energy in the UV or Visible
bands optically coupled to the passive fiber.
25. An optical device, comprising: an active fiber comprising an
active core formed of an oxide glass with a glass network former of
Tellurium (IV) oxide TeO.sub.2 of 65 to 90 mole percent, a glass
intermediate of Lanthanum (III) oxide La.sub.2O.sub.3 of 0.2 to 15
mole percent, a glass modifier MO of 0.2 to 35 mole percent where M
is selected from Mg, Ca, Sr, Ba, Zn, Pb and Cd, a non-zero amount
of halide less than 5 mole percent and an active constituent, said
oxide glass substantially free of Sodium Oxide Na.sub.2O, Hydroxide
OH-- and any other passive glass constituent having an absorption
feature with an inherent absorption wavelength shorter than that of
Tellurium (IV) oxide; and an energizing source coupled to the
active fiber to energize the active constituent to generate light
at a wavelength in the Mid IR band between 2 microns and 4.5
microns.
26. The optical device of claim 25, wherein the amount of the
active constituent is at least 2.5 weight percent.
Description
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to optical tellurium oxide glasses
and fibers for Mid Infrared (MIR) devices, and more particularly to
a tellurium oxide glass composition that improves optical
performance in the MIR band without sacrificing stability, strength
or chemical durability.
[0004] 2. Description of the Related Art
[0005] Glass and glass optical fiber have become critically
important engineering materials over the last half century. Glassy
materials are generally capable of transmitting light in one or
more of several wavelength "bands" loosely defined as Ultraviolet
(UV) from 0.01-0.39 microns, Visible (Vis) from 0.39-0.750 microns,
Near Infrared (NIR) from 0.750-2.0 microns, Mid Infrared (MIR) from
2.0-5.0 microns, and Long Wave IR>>5.0 microns. The glasses
and optical fibers are used in many applications including but not
limited to communications, spectroscopy, chemical sensing, medical
applications, generating and/or guiding laser light and other
optical sources.
[0006] Generally speaking, the glasses and optical fibers may be
used in linear, non-linear and active devices. In linear devices,
light is transmitted without changing the properties of the light.
Typical linear devices include light guiding structures such as
transport fiber, single and multimode fibers and waveguides. In
non-linear devices, the optical output is a nonlinear function of
the intensity of the light, often resulting in creation of
frequencies not present in the input. Typical non-linear devices
include Raman lasers and amplifiers, optical switches and
supercontinuum sources. In active devices, the glass includes an
active dopant that provides optical gain. For each pump photon
absorbed, the dopant re-emits multiple photons. Typical active
devices include lasers and amplifiers.
[0007] To be useful, a glass must exhibit low optical losses,
typically measured in dB/m, for the band or bands of interest.
Absorption at wavelengths within the band produces optical loss.
Furthermore, the glass and glass optical fiber must be strong,
chemically durable and stable (i.e. not prone to
crystallization).
[0008] In the 1970's silica glass and fiber was developed. Silica
based optical fiber became the material of choice for long haul
data transmission due to exceptional low loss in the so-called
telecom window near 1.5 micron wavelength. Silica fiber however is
limited by inherent absorption of the silica glass at wavelengths
longer than about 2.3 microns.
[0009] U.S. Pat. No. 3,883,357 to Cooley proposed a laser glass
composition comprising Tellurium (IV) Oxide TeO.sub.2, Lanthanum
Oxide La.sub.2O.sub.3 and Zinc Oxide ZnO and an effective lasing
amount of Nd.sub.2O.sub.3 for stimulated emission at a wavelength
of about 1.06 microns. As stated at col. 3, lines 29-53, this glass
exhibited unexpectedly high fluorescent activity at 1.06 microns
and thus the potential for enhanced gain compared to silica glass.
These and other Zinc Tellurite glasses did not capture a large
market due to the very mature low-cost process for making high
quality silica based fiber with exceptionally low loss.
[0010] In the 1990's and early 2000's tellurite glass again gained
favor as the need for telecom amplifiers drove research into erbium
doped tellurite lasers operating at 1.5 microns (U.S. Pat. Nos.
5,251,062; 6,266,191; 6,413,891). Tellurite glass possesses broad
glass forming regions and excellent rare earth solubility as
compared to silica to support higher dopant concentrations. Over
the next decade these active devices moved to slightly longer
wavelengths up to 2 microns by incorporating Thulium and Holmium
dopants thus covering the NIR wavelength range. These glasses often
suffered due to losses caused by water (hydroxyl OH--) incorporated
into the glass during melting which quenches gain by these active
NIR dopants.
[0011] Researchers continue to search for glasses and optical glass
fiber that exhibit low loss well into the MIR band and possess the
requisite glass transition temperature Tg, stability, strength and
durability. MIR glass has been developed in several distinct
families including halide, chalcogenide, and oxide types.
[0012] Of the possible halides including fluorine, chlorine,
bromine and iodine, only fluoride-based glasses have gained some
commercial use due to a severe lack of chemical durability and
potential toxicity of chlorides, bromides and iodides. Fluoride
glass exhibits good transmission characteristics to greater than 5
microns but has not gained widespread acceptance due to low
chemical durability, low physical strength, difficulty in achieving
low loss fusion splices, low melting temperatures that make them
not suitable for coating with anti reflective coatings by common
vapor phase methods and difficulty in routinely producing very long
lengths without defects. The inherent low melting temperatures of
fluoride glasses also limit laser damage thresholds and maximum
average power handling capability.
[0013] Chalcogenide glass based on the elements Sulfur, Selenium
and Tellurium has good transmission from the near infrared to long
infrared region but does not transmit in the visible. These glasses
do not contain oxygen but are made up of inter-metallic structures
such as As.sub.2S.sub.3, As.sub.2Se.sub.3, GeS.sub.2, GeSe.sub.2,
etc. . . . Chalcogenides also have distinct absorption peaks in the
MIR region. Chalcogenides are physically extremely weak leading to
fiber breakage during manufacturing of cables as well as in use in
high vibration environments. Chalcogenides possess very low melting
temperatures making them not suitable for common vapor phase
coating processes.
[0014] Oxide glasses based on Tellurium can theoretically transmit
light with low loss to beyond 5 microns. The optical losses into
the MIR are inherently higher with tellurium oxide than with either
the halides or chalcogenides because of its higher phonon energy.
However, known formulations of oxide glasses for NIR applications
possess the required stability, strength and chemical durability
lacking in the halide and chalcogenide glasses. These formulations
have typically relied on the incorporation of Lead (Pb), Germanium
(Ge), Tungsten (W), Niobium (Nb) and Sodium (Na) as well as various
others atomic species to overcome the tendency towards
crystallization. Lead being toxic is avoided when possible.
[0015] To improve optical performance of these Tellurium oxide
glasses into the MIR band, researchers have directed their efforts
to developing glass formulations and processing that reduce the
amount of hydroxyl (OH--) that is entrapped in the glass during
melting.
[0016] U.S. Patent Pub. No. 2003/0045421 to Berger describes an
optical tellurite glass for waveguide amplifiers and oscillators
comprising TeO.sub.2, ZnO, PbO, Nb.sub.2O.sub.5, La.sub.2O.sub.3
and/or other rare earth oxides (dopants) and metal halides that
have good melting and processing properties and a high
crystallization stability and a low water content. In [0031] Berger
claims a "surprisingly low OH-group absorption of the glasses of
less than 3.5 dB/cm at 3,200 nm."
[0017] Liao et. al. "Preparation and characterization of new
fluorotellurite glasses for photonics application", Journal of
Non-Crystalline Solids 355 (2009) 447-452 fabricated new glasses
based on TeO.sub.2--ZnF.sub.2--PbO--Nb.sub.2O.sub.5 for
mid-infrared lasers. The addition of ZnF.sub.2 changed
significantly the glass optical properties. In particular, the
absorption loss in the visible and infrared regions of the
fluoro-tellurite glasses (e.g. 10 mol % ZnF.sub.2) was much reduced
compared with that of the tellurite glass, which was because the
hydroxide (OH) groups decreased markedly.
[0018] Jonathan Massera et al. "Processing of Tellurite-Based Glass
with Low OH Content" J. Am. Ceram. Soc. 1-7, 2010 reported on the
processing and characterization of tellurite-based glass in the
TeO.sub.2--Bi.sub.2O.sub.3--ZnO (TBZ) glass family and specifically
on efforts to reduce their absorption loss due to residual (OH)
content. Massera replaced the 20 mol % ZnO of the control glass
with 20 mol % ZnF.sub.2 and added a fluorinating agent NH4F--HF to
reduce hydroxyl. After melt, the final glass composition includes
greater than 7 mol % of Zinc Fluoride. Massera also was able to
reduce hydroxyl content by performing the melt in an oxygen rich
atmosphere.
[0019] Heike Ebendorff-Heidepriem et al. "Extruded tellurite glass
and fibers with low OH content for mid-infrared applications"
Optical Materials Express, Vol. 2, Issue 4, pp. 432-442 (2012)
reports a fluoride-free process using a dry atmosphere for the
glass melt that enables the absorption at the OH peak at 3.3
microns to be reduced by more than an order of magnitude compared
with glasses melted in open air. They reported an OH absorption
peak of 40-50 dB/m at 3.3 microns for a fluoride-free glass
composition of 73 T.sub.3O.sub.2-20ZnO-5Na.sub.2O-2La.sub.2O.sub.3
(in mole %).
SUMMARY OF THE INVENTION
[0020] The following is a summary of the invention in order to
provide a basic understanding of some aspects of the invention.
This summary is not intended to identify key or critical elements
of the invention or to delineate the scope of the invention. Its
sole purpose is to present some concepts of the invention in a
simplified form as a prelude to the more detailed description and
the defining claims that are presented later.
[0021] The present invention provides a tellurium oxide glass
having low optical loss from the UV band well into the MIR band
that is stable, strong and chemically durable.
[0022] This is accomplished by removing or reducing unwanted
absorption mechanisms in the MIR band so that the glass formulation
exhibits optical performance as close as possible to the
theoretical limit of a tellurium oxide glass while simultaneously
providing glass stability, strength and chemical durability. The
glass formulation only includes glass constituents that provide the
intermediate, modifiers and any halides (for OH-- reduction) whose
inherent absorption wavelength is longer than that of Tellurium
(IV) oxide. The glass formulation is substantially free of Sodium
Oxide and any other passive glass constituent including hydroxyl
whose inherent absorption wavelength is shorter than that of
Tellurium (IV) oxide. The glass formulation preferably includes
only a small residual amount of halide. The pre-melt glass
formulation suitably includes only a sufficient amount of halide to
react with and carry away the hydroxyl during the melt. A non-zero
residual amount of halide in the glass indicates that substantially
all of the hydroxyl is removed. A small residual amount does not
degrade the chemical durability of the glass nor does it lead to
breakage of tellurium oxygen bonds that would increase absorption
in the MIR band and weaken the glass.
[0023] In an embodiment, an oxide glass composition comprises a
glass network former of Tellurium (IV) oxide TeO.sub.2 of 65 to 90
mole percent. Tellurium (IV) oxide has an absorption feature with
an inherent absorption wavelength corresponding to its phonon
energy peak. The glass composition also includes a glass
intermediate of Lanthanum (III) oxide La.sub.2O.sub.3 of 0.2 to 15
mole percent, a glass modifier MO of 0.2 to 35 mole percent where M
is selected from Mg, Ca, Sr, Ba, Zn, Pb and Cd and a halide of 0 to
5 percent. Each of the Lanthanum (III) oxide, modifier and halide
has an absorption feature with an inherent absorption wavelength
longer than that of Tellurium (IV) oxide. The oxide glass
composition is substantially free of each of Sodium Oxide
NaO.sub.2, Hydroxide OH-- and any other passive glass constituent
having an absorption feature with an inherent absorption wavelength
shorter than that of Tellurium (IV) oxide. The oxide glass may be a
passive glass free of any active dopants or may be an active glass
including active dopants selected from Er, Ho, Tm, Dy, Sm, Fe and
Cr for stimulated emission at a wavelength between 2 microns to 4.5
microns. Alternately, active dopants selected from Ce, Pr, Eu, Tb,
Lu, Bi, Ti, Er and Yb may be used for stimulated emission at
wavelengths less than 2 microns. The oxide glass exhibits
solubility to support high concentrations of active dopants up to
and exceeding 5 mol %.
[0024] In an embodiment, the oxide glass comprises a non-zero
amount of the halide at a concentration of less than 5 mol %,
suitably less than 1 mol % and preferably less than 0.5 mol %.
[0025] In an embodiment, the oxide glass has an optical loss of
less than 2 dB/m over the entire spectral range from 0.6 microns to
4.5 microns. The glass exhibits an optical loss of less than 0.5
dB/m over a spectral sub-band from 0.65 to 4.2 microns. The glass
exhibits an optical loss of less than 0.3 dB/m over a sub-band from
2 microns to 4 microns including an optical loss of less than 0.3
dB/m due to OH-- at approximately 3 microns corresponding to the
peak absorption of OH--.
[0026] In an embodiment, the oxide glass is in the form of an
optical fiber. In an embodiment, the optical fiber is used as a
passive transport fiber for multiple optical sources including at
least a MIR source. In an embodiment, the optical fiber is used as
the gain media for a MIR laser. In another embodiment, the optical
fiber is used in a Raman amplifier/laser or a Supercontinuum
source.
[0027] These and other features and advantages of the invention
will be apparent to those skilled in the art from the following
detailed description of preferred embodiments, taken together with
the accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a table of phonon energies and absorption
wavelengths for glass constituents.
[0029] FIG. 2 is a diagram of a tellurium oxide glass composition
in accordance with the present invention;
[0030] FIG. 3 is a plot of absorption coefficient versus wavelength
for different tellurium oxide glasses;
[0031] FIG. 4 is a process diagram for an embodiment of forming a
bulk tellurium oxide glass and then drawing a fiber;
[0032] FIGS. 5a and 5b are plots of transmission and propagation
loss, respectively versus wavelength for a fiber having a core
tellurium oxide glass composition in accordance with the present
invention;
[0033] FIG. 6 is a diagram of a system illustrating the use of a
passive tellurium-oxide fiber to transport optical signals for
sources spanning UV-Vis-MIR; and
[0034] FIG. 7 is a diagram of a MIR laser source using an active
tellurium oxide fiber as the gain media.
DETAILED DESCRIPTION OF THE INVENTION
[0035] The present invention provides a tellurium oxide glass
having low optical loss from the UV band well into the MIR band
that is stable, strong and chemically durable. The tellurium oxide
glass provides a base glass formulation for use in linear,
non-linear and active devices as a bulk glass or fiber in the UV,
Visible and MIR bands.
[0036] To achieve optical loss performance as close as possible to
the theoretical limit of tellurium oxide glass all of the unwanted
absorption mechanisms, particularly those in the MIR band, must be
either removed or substantially reduced without compromising glass
stability, strength and chemical durability. The proper formulation
of the tellurium oxide glass concerns both what glass constituents
are included and what glass constituents are excluded or
minimized.
[0037] To review, the known tellurium oxide glass formulations for
MIR used standard tellurite glass compositions with proven
stability, strength and chemical durability and modified the
composition and melt in an attempt to reduce hydroxyl (OH--)
content.
[0038] Burger reported an absorption loss of less than 3.5 dB/cm
(350 dB/m) at 3,200 nm, which he described as a "surprisingly low
OH-group absorption". Burger's glass included 0.5-12 mol %
Nb.sub.2O.sub.5 which has an inherent absorption wavelength shorter
than that of Tellurium (IV) oxide. Burger's glass also includes
lead oxide PbO.
[0039] Liao's glass also included Nb.sub.2O.sub.5. Liao used 10 mol
% ZnF.sub.2 to reduce the hydroxide (OH) groups. As shown in FIG.
3, Liao's glass has a minimum absorption at approximately 2 microns
and exhibits absorption features between 2.7 and 3.5 microns. The
wavelength corresponding to the theoretical minimum absorption of
tellurium oxide glass occurs at approximately 3 microns. It is
clear that Liao has not substantially eliminated the residual OH
and it is likely that the high concentration of ZnF.sub.2 has
created absorption features by breaking the tellurium oxide bonds
creating non-bridging oxygen. The high residual concentration of
ZnF.sub.2 may also weaken the glass and degrade its chemical
durability. Liao's glass also includes lead oxide PbO.
[0040] Massera uses a very high concentration of ZnF.sub.2, 20 mol
%, to reduce hydroxyl. As shown in FIG. 3a, melting the TBZ glass
having 20 mol % ZnF.sub.2 substituted for the ZnO in an oxygen
environment after pretreatment with a fluorinating agent 32 did
significantly lower the absorption coefficient as compared to the
base TBZ glass 30. This glass has a minimum absorption at
approximately 2.5 microns but exhibits absorption features between
2.5 microns and 3.5 microns. Again, the high concentration of
ZnF.sub.2 may also weaken the glass and degrade its chemical
durability by breaking the tellurium oxide bonds creating
non-bridging oxygen.
[0041] Ebendorff-Heidepriem uses a "fluoride-free" process for the
glass melt. However, Ebendorff's glasses contain alkali metal in
particular sodium, which is a network modifier in glass. Adding
sodium depolymerizes the glass network by breaking bridging
tellurium-oxygen-tellurium bonds. In doing so, a larger proportion
of TeO.sub.3 units occur which leads to an increase in non-bridging
oxygen sites and an increase in absorption at the longer range of
tellurium-oxygen phonon energy at 740 cm.sup.-1 versus the peak
near 650 cm.sup.-1 for TeO.sub.4 units and a large decrease in Tg
values. This increase at 740 cm.sup.-1 will be reflected in an
increase in absorption at the short wavelength IR edge for the
glass. This is to be avoided as much as possible in order that a
fiber retains maximum transmission capability for MIR wavelengths.
Additionally hydrogen ions can then associate with these
non-bridging oxygen ions forming hydroxyl groups giving rise to
unwanted absorption at the fundamental hydroxyl absorption peak at
approximately 3 microns.
[0042] The reported results clearly demonstrate that although
progress has been made in reducing OH-- and improving optical
losses into the MIR band the present tellurium oxide glass
formulations and melting methods do not provide performance close
to the theoretical limit of a tellurium oxide glass while
simultaneously providing glass stability, strength and chemical
durability.
[0043] Our work to improve the optical performance of tellurium
oxide glass towards its theoretical limit without sacrificing
stability, strength and chemical durability has revealed two
fundamental principles. First, the oxide glass formulation should
not include any passive glass constituents having an absorption
feature with an inherent absorption wavelength shorter than that of
Tellurium (IV) oxide. Even though the inherent absorption
wavelength of Tellurium (IV) oxide lies at 15.4 microns, well above
the 5 micron upper bound of the MIR band, it is important to
exclude any passive glass constituents whose inherent absorption
wavelength is lower than 15.4 microns. Our research has
demonstrated that the higher order phonon energy terms for any
glass constituent whose inherent absorption wavelength is less than
that of Tellurium (IV) Oxide can introduce significant absorption
features in the MIR band. This has shown to be important for both
passive fibers in which the length extends to meters or tens of
meters and active fibers to avoid concentration quenching.
Consequently, the glass formulation uses glass constituents for the
intermediate and modifier whose inherent absorption wavelength is
longer than that of Tellurium (IV) Oxide. Second, optimum reduction
of OH-- is not achieved by using very high concentrations of
halides (e.g. ZnF.sub.2) during the melt that leave a large
residual amount of halide (F) in the final glass composition. Such
large concentrations can in fact promote the retention of moderate
amounts of OH by breaking the tellurium oxide bonds creating more
non-bridging oxygen and producing other unwanted absorbers in
addition to the fact that high concentrations of Fluoride tend to
weaken the glass and reduce its chemical durability.
[0044] Our glass formulation only includes glass constituents that
provide the intermediate, modifiers and any halides (for OH--
reduction) whose inherent absorption wavelength is longer than that
of Tellurium (IV) oxide. The glass formulation is substantially
free of Sodium Oxide and any other passive glass constituent
including hydroxyl whose inherent absorption wavelength is shorter
than that of Tellurium (IV) oxide. The glass formulation preferably
includes only a small residual amount of halide. The pre-melt glass
formulation suitably includes only a sufficient amount of halide to
react with and carry away the hydroxyl during the melt. A non-zero
residual amount of halide in the glass indicates that substantially
all of the hydroxyl is removed. A small residual amount does not
degrade the chemical durability of the glass nor does it lead to
breakage of tellurium oxygen bonds that would increase absorption
in the MIR band and weaken the glass.
[0045] FIG. 1 is a table 10 of the peak phonon energy, the
corresponding inherent absorption wavelength and the absorption
wavelengths corresponding to higher order phonon energies for a
number of glass constituents including the common glass formers as
well as certain second or third component materials commonly used
in producing stable glass formulations. Tellurium (IV) oxide has a
peak phonon energy of 650 cm.sup.-1 and a corresponding inherent
absorption wavelength of 15.4 microns. The glass constituents
Germanium (Ge), Tungsten (W) and Niobium (Nb) that are commonly
used in tellurite glass for MIR applications each have an inherent
absorption wavelength lower than that of Tellurium (IV) oxide,
hence they are not viable candidates for our oxide glass
formulation. Sodium (Na) and Lead (Pb) do have absorption
wavelengths above that of Tellurium (IV) oxide but are excluded for
other reasons; Sodium inhibits the reduction of OH, severely lowers
Tg and weakens the glass and Lead is toxic. In some cases lead may
be used.
[0046] Our challenge was to find glass formers and other glass
constituents to produce a stable, strong and chemically durable
glass formulation using only materials whose inherent absorption
wavelength is longer than that of Tellurium (IV) oxide.
[0047] Lanthanum oxide (La.sub.2O.sub.3) in glass can impart higher
glass transition temperatures, strength, and durability. Lanthanum
oxide is a glass intermediate only forming glass when combined with
other constituents such as one of the common glass formers Silica
dioxide (SiO.sub.2), Phosphorus pentoxide (P.sub.2O.sub.5),
Germanium dioxide (GeO.sub.2) or Boron oxide (B.sub.2O.sub.3).
Glasses containing lanthanum oxide however are known to be only
moderately stable over a limited range of composition. Lanthanum
oxide in glass is therefore generally limited to a role as a minor
constituent comprising less than a few mole percentage of a given
composition. Lanthanum oxide possesses very low phonon energy and
even after four phonons only absorbs at wavelengths over 5 microns.
Lanthanum oxide therefore possesses very desirable characteristics
of high Tg, high corrosion resistance and low UV, Vis and MIR
absorption.
[0048] To form fiber by the common rod-in-tube technique, a glass
preform consisting of a core rod with a polished outer surface is
placed inside a tube with polished inner and outer surface and
heated to a temperature at which the glass rod and tube soften and
fuse to be subsequently drawn or pulled to a fiber. A common
benchmark for glass stability, .DELTA.T, is defined as the
difference of the peak crystallization temperature Tx and glass
transition temperature Tg or .DELTA.T=Tx-Tg as determined by the
common calorimetric technique Differential Scanning Calorimetry
(DSC). In general a larger .DELTA.T is desirable with a value
greater than 100 C often cited as reasonable for fiber drawing.
Glasses containing only tellurium and lanthanum oxides have been
made but exhibit very low values of .DELTA.T in the range of 40 C
(Mallawany J. Mater. Sci. (2010) 45: 871-887).
[0049] Typically tungsten, niobium or germanium is used to form
more stable compositions with a larger value of .DELTA.T. Again by
careful analysis of Table 10 one realizes tungsten, niobium or
germanium containing glasses all have phonon energies larger than
TeO.sub.2 and will produce unwanted absorptions within only as few
as three multiples of the peak phonon energy. The third phonon edge
is relevant for passive fibers meters to tens-of-meters in length
or for much shorter active fibers. These materials must then be
eliminated from any composition for which exceptionally low loss at
wavelengths shorter than that of tellurium itself is desired.
[0050] Consequently other glass constituents must be found which
can stabilize the Te--La glass compositions. Oxides formed from
Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), Zinc
(Zn), Lead (Pb) and Cadmium (Cd) may be used to stabilize the
Te--La glass composition. Each of these oxides has an inherent
absorption wavelength above that of tellurium. These oxides in
combination with tellurium and lanthanum oxides form a glass over
an extended compositional region. Addition of these oxides to
tellurium lanthanum glass compositions leads to increased .DELTA.T
values that are greater than 100 C. A preferred embodiment may use
ZnO as it has been demonstrated to form highly stable TeLaZnO
glass.
[0051] The glass formulation may include various other glass
constituents for other glass forming purposes such as to overcome
the tendency towards crystallization and to adjust other physical,
thermal and optical properties such as Tg, thermal expansion
coefficient, or refractive index. Any such constituent that is
present in the finished glass in other than a de minimus amount
must not have an absorption feature with an inherent absorption
wavelength shorter than that of Tellurium (IV) oxide. For example,
the base glass can be modified by the addition of other alkali,
alkaline earth, transition metal, or heavy metal oxides or halides
including Li, K, Rb, Cs, Mg, Ba, Ca, Sr, Y, Ti, Zr, Hf, Nb, Ta, Mo,
Ga, In, Tl, Pb, As, Ge, Sb and Bi.
[0052] Additionally when doped with certain transition metals
and/or rare earth metal oxides and/or halides singly or in
combination, the described fiber is useful for laser light
generation with greatly reduced concentration-quenching effects and
enhanced efficiency. Possible dopants capable of supporting optical
gain are Er, Ho, Tm, Dy, Sm, Fe and Cr for stimulated emission at a
wavelength between 2 microns to 4.5 microns. Other dopants may be
used either singly or in combination. Alternately, active dopants
selected from Ce, Pr, Eu, Tb, Lu, Bi, Ti, Er, Yb may be used for
stimulated emission at wavelengths less than 2 microns. Also, the
dopant materials used in the MIR IR bands may also have emission
lines less than 2 microns. Because the concentration-effects are
greatly reduced, the dopant concentrations may be much higher than
normal, up to and exceeding 2.5 wt % and 5 wt %.
[0053] Our next challenge was to provide a final glass formulation
substantially free of hydroxyl (OH) without degrading the strength,
stability and chemical durability of the glass and without creating
new absorbers in the IR band. As with earlier techniques used by
Liao and Massera, we introduce a halide, typically fluoride, into
the glass composition that reacts with the hydroxyl during the melt
to form volatile HF that is carried away in the furnace gas purge.
Other halides such as Chloride, Bromide or Iodides may be used. The
fluoride and chloride are typically introduced as a metal halide
such as ZnF.sub.2 or PbF.sub.2.
[0054] However, unlike Liao and Massera who use large amounts of
ZnF.sub.2 in the initial glass leaving significant amounts of
F.sub.2 in the final glass, our preferred approach is to limit the
initial fluoride content so that only a small non-zero residual
amount of F.sub.2 is present in the finished glass, typically less
than 1 mol % and preferably less than 0.5 mol %. To ensure
effective OH reduction, it is useful that the finished glass does
contain a non-zero residual amount of F.sub.2. Our research has
shown that a large amount of ZnF.sub.2 in the initial glass does
not improve OH reduction. In fact, substantial residual fluorine
content in the finished glass leads to a breakage of the tellurium
oxygen bonds and increased absorption due to TeO.sup.3+1/TeO.sub.3
units. This breaking of bonds also weakens the glass structure
leading to increased potential for corrosion and reduced physical
strength. By limiting the initial fluoride content so that only a
small residual amount of F.sub.2 is present in the finished glass,
the hydroxyl is removed without breaking the tellurium oxygen bonds
or otherwise weakening the glass structure. Even absent efforts to
minimize the residual F.sub.2, the content is still at most 5 mol
%, which is less than Liao and Massera.
[0055] Referring now to FIG. 2, an embodiment of an oxide glass
formulation 20 for a finished glass comprises:
[0056] a glass network former of Tellurium (IV) oxide TeO.sub.2 22
of 65 to 90 mole percent:
[0057] a glass intermediate of Lanthanum (III) oxide
La.sub.2O.sub.3 24 of 0.2 to 15 mole percent;
[0058] a glass modifier MO 26 of 0.2 to 35 mole percent where M is
selected from Mg, Ca, Sr, Ba, Zn, Pb and Cd or combinations
thereof;
[0059] and a halide 28 of 0 to 5 percent,
[0060] wherein the oxide glass composition is substantially free of
each of Sodium Oxide Na.sub.2O, Hydroxide OH-- and any other
passive glass constituent having an absorption feature with an
inherent absorption wavelength less than that of Tellurium (IV)
oxide.
[0061] Once melted into a glass the various constituents become
associated with other atoms in a complex way. So TeO.sub.2 becomes
a "chain" of Te atoms with four O atoms associated with it
"TeO.sub.4" groups or three oxygen "TeO.sub.3" groups or something
in between TeO.sub.3+1 groups. Zinc fills in spaces between Te
groups without becoming part of the "chain" and becomes only
loosely associated with the oxygen or fluorine. Zn changes the
shape of neighboring TeO.sub.4 groups forming TeO.sub.3+1 groups
with a distorted electronic cloud. F or O from the Zn move to a
position that compensates for the distorted electronic cloud to
maintain electrical balance.
[0062] In an embodiment, the concentration of Lanthanum (III) oxide
La.sub.2O.sub.3 24 is 5-15 mole percent. In another embodiment, the
concentration is 8-10%. In general, we have found that it is
advantageous for the concentration of Lanthanum (III) oxide to be
as high as possible consistent with forming a stable glass.
Lanthanum (III) oxide increases the glass transition temperature
and improves glass mechanical properties.
[0063] In an embodiment, the glass modifier MO 26 is ZnO. In an
embodiment, the concentration of ZnO is a combination of ZnO
provided in the initial glass pre-melt and ZnO that is formed by
converting ZnF.sub.2 or ZnCl during hydroxyl reduction. In an
embodiment, the concentration of ZnO is at least 10 mole percent.
Of the possible modifiers, ZnO has been found to produce the most
stable glass with Lanthanum (III) oxide, which is particularly
important for fiber drawing using the rod-in-tube technique.
[0064] In an embodiment, the halide 28 is present as a non-zero
amount less than 1 mole percent, and preferably less than 0.5%. The
presence of a non-zero amount of the halide ensures that the
hydroxyl reduction process during the melt was not starved for a
halide to react with the hydroxyl. A small amount of halide ensures
that the tellurium oxide bonds are not broken and that the glass is
not otherwise weakened by the presence of a large concentration of
halide. In an embodiment, the halide 28 is Fluoride. Fluoride
provides the most effective drying of any of the halides.
[0065] In an embodiment, the oxide glass is lead free.
[0066] In an embodiment, the oxide glass is a passive glass free of
any active dopants.
[0067] In an embodiment, the active glass includes active dopants
selected from Erbium (Er), Holmium (Ho), Thulium (Tm), Dysprosium
(Dy), Samarium (Sm), Iron (Fe) and Chromium (Cr) for stimulated
emission at a wavelength between 2 microns to 4.5 microns. The
oxide glass exhibits solubility to support high concentrations of
active dopants up to and exceeding 2.5 and even 5 weight percent.
In other embodiments, lower dopant concentrations may be used.
[0068] FIG. 3 plots the absorption coefficient versus wavelength
for three different glasses Tellurium Tungsten 30, Tellurium
Tungsten w/ OH-- reduction 32 and TeLaZnO w/ OH-- reduction 34
illustrating first the improvement in optical performance of
effective OH-- reduction and then elimination of any passive
constituents whose inherent absorption wavelength is shorter than
that of Tellurium (IV) oxide. The absorption coefficient is a
measure of the ratio of transmitted power to incident power.
Tungsten is a standard glass former for Tellurium (IV) Oxide that
is well known to form strong, stable and chemically durable
glasses. However, as shown in FIG. 1 Tungsten has an inherent
absorption wavelength shorter than that of Tellurium (IV)
Oxide.
[0069] As shown, the Tellurium Tungsten glass 30 starts showing
some absorption around 2.7 microns that becomes significant at
about 3.0 microns. This is due to the absorption features of
hydroxyl (OH--). The Tellurium Tungsten glass w/ OH-- reduction 32
starts showing some absorption around 3.0 microns that becomes
significant at about 3.7 microns. The effective reduction of OH--
(without breaking the TeO.sub.2 bonds) does significantly extend
the bandwidth over which the absorption coefficient remains low.
However, the presence of Tungsten (W) introduces absorbers at 2.7
microns and 3.6 microns due to the 4.sup.th and 3.sup.rd phonon
energies, respectively. These higher order terms result in
significant absorption. The TeLaZnO w/ OH-- reduction glass 34
starts showing some absorption around 4.0 microns that becomes
significant at about 4.4 microns, thus extending the low loss
bandwidth well into the MIR. Because of the "log" scale, these
changes in the absorption coefficient are dramatic and have
significant effects on loss in passive fibers having lengths in the
meters to tens of meters and concentration-quenching in active
fibers.
[0070] In an embodiment, the oxide glass has an optical loss of
less than 2 dB/m over the entire spectral range from 0.6 microns to
4.5 microns. The glass exhibits an optical loss of less than 0.5
dB/m over a spectral sub-band from 0.65 to 4.2 microns. The glass
exhibits an optical loss of less than 0.3 dB/m over a sub-band from
2 microns to 4 microns including an optical loss of less than 0.3
dB/m due to OH-- at approximately 3 microns corresponding to the
peak absorption of OH--.
[0071] An embodiment of a process for making a bulk finished glass
and using a rod-in-tube technique to draw fiber from the finished
glass is described in FIG. 4. A base glass composition is defined
(step 40). In this example, the base glass composition includes
TeO.sub.2 (70%), La.sub.2O.sub.3 (5%) and ZnO (25%). Hydroxyl
(OH--) is present as an impurity in the constituent powders,
typically at a level of a fraction of a percent. Next a certain
percentage, here 5%, of the ZnO is substituted with a halide, here
ZnF.sub.2, (step 42) that acts as the drying agent to remove the
OH-- during the melt. In general, the percentage of ZnF.sub.2 is
preferably selected such that the finished glass includes a small
non-zero residual amount of the Fluoride, enough to ensure that the
drying process was not starved of Fluoride and not so much as to
break Tellurium Oxide bonds or to otherwise weaken the glass. In
different embodiments, the halide might be as low as 0.2% and as
high as 7% in the initial glass. If the glass is to be a passive
glass (step 44) the formulation is complete. However, if the glass
is to be an active glass (step 46), dopant is added (step 48). By
convention rare earth dopant is typically substituted for a certain
percentage of the Lanthanum Oxide due to their physical and
electronic similarity. Because the process is so effective at
removing OH-- and thereby avoiding concentration-quenching,
relatively very high dopant percentages can be supported. Dopant
concentrations up to and exceeding 2.5 and even 5 weight percent
have been demonstrated. Dopant concentrations in other known
tellurite glasses typically cannot exceed 2 wt % due to
concentration-quenching from residual hydroxyl.
[0072] Once the glass formulation is set, the oxides and halides
are provided, typically in the form of powders, mixed (step 50) and
melted (step 52). The time, temperature and purified gas atmosphere
for the glass melt are controlled (step 54). In an embodiment, the
mixed powders are melted in a gold crucible in a furnace at
approximately 750-950 degrees C. The atmosphere surrounding the
crucible may be controlled to consist of dry inert gas, oxygen or
mixtures of inert gas and oxygen. The materials within the crucible
are maintained in a liquid state for sufficient time to obtain a
homogenous melt. Melts can be mechanically stirred or bubbled with
flowing gas to aid in homogenization and/or drying.
[0073] During the melting and mixing process the fluoride present
in the melt will chemically react with residual hydrogen present in
the melt (step 56). The fluoride chemical byproducts HF are purged
in the exhaust gas (step 58) leaving the glass with no or a minimum
amount of hydrogen. Some or all of the initially present zinc
fluoride is converted to zinc oxide and the resultant glass has a
lower concentration of zinc fluoride than was present in the
starting materials. It is desirable to minimize the zinc fluoride
concentration in the glass, since the zinc fluoride reduces the
glass transition temperature and makes the glass less mechanically
robust. In this example, the finished glass composition includes
TeO.sub.2 (70%), La.sub.2O.sub.3 (5%), ZnO (24.5%) and ZnF.sub.2
(0.5%) (step 60).
[0074] Glasses obtained may be cast into preheated molds and slowly
cooled (annealed) to room temperature in order to relieve stresses
(step 62). The mechanical robustness of the resultant glass allows
casting of ingots of arbitrary size and shape. An ingot size of
approximately 1''.times.1''.times.5'' is sufficiently large to be
used in the fabrication of bulk optical components or preforms
suitable for drawing optical fiber. Alternative methods may be used
to form a glass, such as the sol-gel method.
[0075] An optical fiber can be fabricated from the tellurite glass.
Optical fiber fabricated from the tellurite glass can preserve the
glass intrinsic material transmission if the fiber is properly
drawn. One method to form optical fiber is the rod-in-tube method.
The preform consists of an inner rod surrounded by an outer tube.
These shapes may be readily fabricated (step 64) from the glass
ingot by conventional glass fabrication techniques, such as
mechanical cutting, grinding, and polishing. An important property
of the glass is its resistance to chipping and fracturing, which
enables fabrication of the preforms. The preform is made by placing
a rod with polished outer surfaces inside a tube with polished
inner and outer surfaces. The inner rod forms the fiber core and
the outer tube forms the fiber cladding. The refractive index of
the core and clad glasses can be manipulated by changing the ratio
of glass constituents such that any value of numerical aperture
from 0 to greater than 0.4 can be obtained. One method to change
the refractive index is to vary the TeO.sub.2 to ZnO ratio while
leaving the La.sub.2O.sub.3 level approximately constant. Optical,
thermal and mechanical properties of the base glass can similarly
be adjusted by manipulation of constituent ratios or by the
addition of other constituents that have no unwanted absorptions,
such as those listed previously.
[0076] To draw the fiber (step 66), the preform can then be placed
into a furnace with a controlled atmosphere. The controlled
atmosphere may be an inert gas such as nitrogen, argon or helium,
or an atmosphere of oxygen or a combination of gases. The preform
may then be heated to a temperature sufficient to soften the glass,
causing the core rod and surrounding tube to fuse together with or
without the aid of vacuum. The preform pulling temperature is above
the glass transition temperature of approximately 300 C. The
softened and fused glass is subsequently pulled and drawn into
fiber. Care must be taken during the drawing to avoid
recrystallization, which can result in the formation of scattering
sites at grain boundaries. A protective coating may be applied to
the outer cladding surface and the fiber spooled for convenient
storage and transport. The protective coating may be a polymer,
metal, carbon, or other coating material. The protective coating
helps to preserve the intrinsic strength of the tellurite glass
fiber. Fibers drawn from the telluride glass have demonstrated a
tensile strength exceeding 50 KPSI. This high tensile strength
facilitates forming a mechanically robust fiber that can be readily
handled, spooled, and routed through bulkheads without
breakage.
[0077] Many types of fiber structures may be made in addition to
the basic single core/single clad geometry. For example, a double
clad fiber may be made by using two tubes surrounding the core. A
fiber with multiple cores may be made. The fiber may include a
photonic crystal structure consisting on an array of holes within
the fiber. The fiber core may be designed to support either single
mode or multimode operation. For single mode operation the core
dimensions are typically small, no more than several times the
wavelength of the light being transmitted. For a multimode fiber,
the core dimensions are typically many times larger than the
wavelength of the transmitted light. A fiber core diameter of 100
microns may be used although larger and smaller cores may be used
depending on the application.
[0078] Alternative methods of fabricating a fiber may be used in
addition to the rod-in-tube method described above. Preforms may be
fabricated by vapor deposition of material on the inner surface of
a polished tube. Alternatively, glass in molten form may fill the
cavity of a tube. The molten glass may then be cooled and the
resultant assembly used as a preform. Instead of forming the tube
by mechanical methods, a tube may be formed by cooling and
solidifying the outer surfaces of a glass ingot and allowing the
inner molten regions to drain away.
[0079] FIGS. 5a and 5b show the transmission and propagation loss
versus wavelength for an exemplary tellurite glass. In this
example, the finished glass formulation comprises TeO.sub.2 (70%),
La.sub.2O.sub.3 (5%), ZnO (24.5%) and ZnF.sub.2 (0.5%). The bulk
glass and fiber were formed using the processes described
above.
[0080] FIG. 5a shows the transmission spectrum 70 of a 1.5 mm thick
tellurite glass sample prior to fiber draw. The measurement
includes the Fresnel reflective losses, so the maximum transmission
value is only about 80%. In a broad spectral region from
approximately 600 nm to 4000 nm the glass has very low absorption
losses. In the short wavelength region the glass maintains some
level transmission well into the UV. For example, the sample
transmission does not drop to 5% until 353 nm. In the long
wavelength region the glass maintains some level of transmission
well into the mid-infrared. The absorption is negligible until
approximately 4000 nm. The region from approximately 4000 nm to
5500 nm shows a gradual transmission drop. Above 5500 nm the
transmission roll off is steeper, but the sample transmission does
not drop to 5% until 6380 nm. While in some applications the glass
may be useful at shorter or longer wavelengths, most applications
for the glass will be in the wavelength range of approximately 350
nm to 6400 nm. For applications that require high transmittance or
long propagation lengths in the glass, the useful operating range
is narrower approximately 600 nm to 4500 nm. The spectral region
outside this low loss band may be useful where transmittances can
be low or propagation lengths short or moderate.
[0081] FIG. 5b illustrates that low loss optical fiber can be drawn
from the tellurite glass. The fiber used in this measurement had a
core diameter of 70 microns and a cladding outer diameter of 170
microns. The propagation loss 72 is less than 0.5 dB/m for IR
wavelengths shorter than approximately 4 microns. Since the thin
sample transmission measurements show no absorption features across
the near IR region, the fiber drawn from the glass maintains
continuous high transmission over this entire spectral region.
Propagation loss begins to rise for wavelengths longer than
approximately 4 microns, consistent with the thin sample
transmission measurements shown in FIG. 5a. However, the
propagation loss is still less than 1.5 dB/m for wavelengths longer
than 4.5 microns. Fiber with propagation losses much higher than
1.5 dB/m is still useful in applications where the fiber lengths
are short or high transmission is not a requirement.
[0082] By way of comparison, Berger claims a "surprisingly low"
absorption of 3.5 dB/cm (350 dB/m) at 3.2 microns. Ebendorff
reported an OH absorption peak of 40-50 dB/m at 3.3 microns. Our
glass has an optical loss of 0.5 dB/m at the same wavelength. Liao
and Massera report their data as absorption coefficient as a
function of wavenumber preventing a direct comparison. However,
FIG. 3 of Liao and FIG. 3a of Massera clearly show significant
absorption features between 2.5 and 3 microns. As a result both
glasses will have much higher absorption in the MIR band than ours.
Massera FIG. 4 shows results for fiber with .about.10 dB/m loss
(10% transmitted light in 1 meter) at 1310 nm. By comparison, our
glass exhibits <0.3 dB/m (93% transmitted) at 1310 nm. Massera
does not show loss at 3 micron where hydroxyl has its peak
absorption. But his fiber transmits only 1% of the light at 2
meters, which is useless in many applications, whereas our fiber
transmits 86% of the light.
[0083] The glass and fiber described here are useful in a variety
of applications, such as a simple transport fiber with a broad
transmission range, a source for the generation of broadband
supercontinuum light, an amplifier or a laser. As a transport
fiber, where the fiber guides light originating at a first location
to a second location remote from the first location, the invention
can be used in spectroscopy, chemical sensing, medical
applications, military infrared countermeasures and guiding
laser-generated light. The ability to transmit light in the
ultraviolet, visible and mid infrared regions simultaneously is a
key advantage. For active devices, the fiber includes an active
constituent for light generation at a wavelength in the MIR band
between 2-4.5 microns and a source (e.g. a pump and signal) to
properly energize the active constituent. "Active constituent"
could be rare-earth or metal dopants (for lasers and amplifiers),
or Raman activity in core glass (for Raman amplifier/laser), or
nonlinear (self phase modulation, n2, Raman) activity in core glass
(for SC generation). In a laser, the pump signal is absorbed and
generates stimulated emission for lasing action in the fiber. In an
amplifier, the pump generates gain for signal light propagating
through the active fiber. In a supercontinuum source, the pump
generates the supercontinuum light in the fiber by a nonlinear
mechanism.
[0084] Referring now to FIG. 6, a passive fiber 80 formed from the
Te/La/ZnO glass is used to transport light spanning the UV, Visible
and MIR bands with an optical loss of less than 1.5 dB/m. A UV
source 82 generates UV light that is coupled into passive fiber 80
via an optical combiner 84. A visible source 86 generates visible
light that is coupled into passive fiber 80 via an optical combiner
88. A MIR source 90 generates UV light that is coupled into passive
fiber 80 via an optical combiner 92. The sources may be active
simultaneously in which case they generate a combined UV/Vis/MIR
output 94 from passive fiber 80. Alternately, passive fiber 80 may
form a transport network to selectively transport one or more of
the different sources at one time to another location. In either
case, low optical loss from the UV to approximately 4.5 microns in
the MIR is desired.
[0085] In alternate embodiments, the passive fiber 80 may be used
with only a single wavelength source or a plurality of optical
sources over any spectral region within the fiber transmission
band. In some applications, the energy of one propagating
wavelength may be used to generate a different propagating
wavelength. An example of such an application is in chemical
sensing where a visible wavelength signal may propagate to the
distal end of a fiber where it causes emission of a MIR wavelength,
which in turn is propagated back through the fiber where it may be
detected.
[0086] Referring now to FIG. 7, an active fiber 100 formed from the
Te/La/ZnO glass and doped with Erbium provides the gain media for a
MIR laser 102.
[0087] Alternately, the Te/La/ZnO glass may be doped with other
dopants to provide lasers in other bands, or un-doped in the case
of a Raman-based laser. MIR laser 102 includes a pair of fiber
gratings 104 and 106 which provide optical feedback on opposite
sides of active fiber 100 to form a resonate cavity. A resonant
cavity may alternatively be formed by using the mirror-like end
face of an optical fiber having the appropriate reflectivity in
place of one or both of the gratings. A diode pump laser 108
provides a pump signal that is coupled into the resonant cavity to
stimulate emission of the dopant to produce a narrowband laser
output 110 through a passive fiber 112, suitably formed of the same
Te/La/ZnO glass.
[0088] Supercontinuum generation is based on optical nonlinearities
excited by illumination of a sample with optical pulses having high
peak power. The high peak power of the optical pulse mixes with
itself via self phase modulation, Raman conversion, four wave
mixing, and other nonlinear effects, transforming the initially
narrow spectral output of the laser source into a broad spectrum.
Optical fiber is a particularly useful structure for generating
supercontinuum emission, since all the generated optical fields
remain guided by the fiber, significantly reducing the power
required to efficiently generate the supercontinuum. The required
power level for efficient supercontinuum generation is proportional
to the .chi..sup.3 nonlinear coefficients, which drives the
spectral broadening. The tellurite glass fiber described herein has
a nonlinear coefficient 40 to 60 times greater than that of silica
glass fiber. This allows the supercontinuum to be generated with
lower power optical pulses. Alternatively, efficient generation may
be obtained with equal optical powers and a shorter interaction
length. For tellurite glass fibers efficient supercontinuum
emission can be generated with fiber lengths shorter than 10 cm. In
some cases the fiber length may be less than 5 cm. The width of the
supercontinuum spectra is limited by the transparency window of the
fiber where the supercontinuum is generated. For conventional
silica fiber the spectra can extend continuously from about 2300 nm
to 370 nm, the transmission window of the silica fiber. The
tellurite glass fiber can advantageously have much broader spectral
output, since the fiber is capable of transmitting light in the
ultraviolet, visible and mid infrared regions simultaneously. The
fiber may generate a supercontinuum extending from approximately
370 to 6000 nm.
[0089] In an embodiment, a tellurite glass fiber is used to
generate a broadband supercontinuum spectrum. The output radiation
is confined to an output fiber, which delivers the radiation to a
fiber splitter. The output of the splitter is a plurality of
transport fibers that direct the supercontinuum light to various
points. All the fiber and components illustrated in the fiber may
be based on tellurite glass. This architecture allows a single
laser source to deliver broad-spectrum radiation at multiple
points. The laser source and delivery system are constructed
entirely from fiber and fused fiber connections, eliminating the
possibility of mechanical misalignment. In other embodiments, the
supercontinuum source may be coupled to a single transport
fiber.
[0090] While several illustrative embodiments of the invention have
been shown and described, numerous variations and alternate
embodiments will occur to those skilled in the art. Such variations
and alternate embodiments are contemplated, and can be made without
departing from the spirit and scope of the invention as defined in
the appended claims.
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