Note: Descriptions are shown in the official language in which they were submitted.
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TANTALA DOPED WAVEGUIDE AND METHOD OF MANUFACTURE
FIELD OF THE INVENTION
The present invention relates generally to optical waveguide glass
having a high index of refraction and a method for manufacturing such optical
waveguide glass, and more particularly to a method of doping optical
waveguide glass with Ta205 to produce essentially crystalline free optical
waveguide fiber.
While the invention is capable of being carried out using a number of
soot collection and doping techniques, it is particularly well suited for use
in
conjunction with the outside vapor deposition (OVD) process, and will be
particularly described in that regard.
BACKGROUND OF THE INVENTION
In the rapidly expanding field of telecommunications, there is an ever-
increasing demand for systems that transfer greater amounts of data in shorter
periods of time. Accordingly, in the opto-electronics field, there is a
continuing
need for new optical waveguide systems, and consequently new optical
waveguides and new optical waveguide components for meeting the demands
of those systems.
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Generally speaking, optical waveguide fibers include a core surrounded
by a cladding material having a refractive index lower than that of the core.
Such optical waveguide fibers are generally composed of silica that is
selectively doped with a dopant such a germanium. Although germanium is
the principal and most widely used dopant, other dopants such as
phosphorous, fluorine, boron and erbium, to name a few, are often used.
Germania, however, is most commonly used due to its low melting point and
high refractive index in relation to silica.
All dopants, including germania have shortcomings that limit their
usefulness to certain applications. Accordingly, as technology improves and
the requirements for new applications increases, the requirement for new
optical waveguide fiber capable of meeting the demands of these applications
increases as well. Such needs provide the incentive to consider the
application
of new dopants and new methods of doping optical waveguide fibers to meet
these demands. In addition, competition is continually driving researchers to
develop optical waveguide fibers at lower cost. Because germania costs
approximately $1,000 per kilogram, a less expensive dopant capable of
providing a higher index of refraction than germania with less of that
alternative
dopant would be ideal.
One such dopant known to have a high refractive index is tantala. In
fact, Ta205 thin films are widely used in thin-film waveguide lenses and anti-
reflective coatings for silicon wafer solar cells. Because of the
attractiveness of
Ta205, thin films for integrated optical devices, many researchers have been
active in this area. Thin films for integrated optical devices containing
Ta205
are typically fabricated using sputtering techniques and result in measurable
losses of about 0.4 dB/cm. In the field of thin-films it is believed that a
contributing factor to such high losses is the subsequent heat treatment of
thin-
films following sputtering. It was found that the heat treatment caused the
film
to change from amorphous to crystalline. Such a defect, if formed in an
optical
waveguide fiber, would adversely affect that optical waveguide fiber operating
properties and would render the fiber non-functional in an optical waveguide
fiber system.
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Planar devices have also been fabricated using Ta205. Ta205-Si02 core
glass for such devices is laid down using an electron beam vapor deposition
technique. However, the lowest loss observed for such devices has been
approximately 0.15 dB/cm or 15,000 dB/km. For optical waveguide fiber,
losses of less than approximately 1 dB/km is the target. Thus, neither the
thin-
films nor the planar optical devices suggest the usefulness of tantala doped
silica for optical waveguide fibers.
In view of the foregoing, there is a need for a dopant that, in limited
quantities, is capable of providing a high core index of refraction to an
optical
waveguide fiber. In addition, there exists a need for a dopant that has good
non-linear properties, does not adversely impact the mechanical properties of
the optical waveguide fiber in which is resides, and exhibits beneficial
amplification characteristics. Moreover, there is a need for a method of
providing the dopant to an optical waveguide fiber with minimal deviation from
present optical fiber manufacturing techniques, thus making it economically
feasible and desirable. The low cost of tantala compared to germania, as well
as tantala's high index of refraction makes it a promising candidate for such
a
dopant.
SUMMARY OF THE INVENTION
One aspect of the present invention relates to a method of
manufacturing a low loss optical waveguide having a high refractive index core
by forming a soot blank which includes Ta205 and Si02, consolidating the soot
blank to form a cane under conditions suitable to prevent crystallization of
the
Ta205- Si02 containing glass and drawing the cane into an optical fiber.
In another aspect, the invention relates to an optical fiber that is
manufactured by preparing a soot blank which includes at least Ta205 and
Si02, consolidating the soot blank to form a cane under conditions suitable to
prevent crystallization of the Ta205- Si02 containing glass and drawing the
cane into an optical fiber.
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A further aspect of the invention relates to an optical fiber having a high
purity glass cladding, and a high refractive index glass core bounded by the
cladding. The glass core includes between about 2 to 5 wt% Ta205, so that
light attenuation in the optical fiber is less than about 1.8 dB/km at 1550
nm.
Yet another aspect of the invention relates to a glass for use in the core
of the optical waveguide that includes Si02 and, by weight on an oxide basis,
between about 2% non-crystallized Ta205, to 5% non-crystallized Ta205 after
consolidation.
The glass and method of the present invention results in a number of
advantages over other glasses and methods known in the art. One of the most
attractive features of using tantala in the glass for the present invention is
its
high index of refraction, which is reported to be 2.2 at 632.8 nm.
Accordingly,
in the glass of the present invention, the same refractive index change can be
achieved with a much lower addition of Ta205 than can be achieved with Ge02.
Moreover, because tantala is far less expensive than germania, there is a
significant cost savings resulting from the selection of tantala as a dopant.
Another advantage is the high viscosity of Ta205-Si02 glass, which is a
function of the high melting point of tantala. Ta205 has a melting point of
1887°C while Si02 and Ge02 have melting points of 1715°C and
1116°C,
respectively. Accordingly, the high viscosity of tantala silicate glass makes
the
glass of the present invention a likely candidate for viscosity matching.
Additional advantages of the present invention are that tantalum oxide is
chemically stable and insoluble in water, the thermal expansion of glass
containing tantala is lower than that of glass containing germania, and the
method of the present invention essentially eliminates crystallization within
the
Ta205- Si02 containing glass during the manufacture of optical waveguides.
The latter advantage results in improved optical characteristics.
Additional features and advantages of the invention will be set forth in
the detailed description which follows, and in part will be readily apparent
to
those skilled in the art from the description or recognized by practicing the
invention as described in the written description and claims hereof, as well
as
the appended drawings.
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It is to be understood that both the foregoing general description and the
following detailed description are merely exemplary of the invention and are
intended to provide an overview or framework to understanding the nature and
character of the invention as it is claimed.
5 The accompanying drawings are included to provide a further
understanding of the invention and are incorporated in and constitute a part
of
this specification. The drawings illustrate one or more embodiments of the
invention, and together with the description serve to explain the principles
and
operation of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a perspective view of an optical fiber manufactured in
accordance with the present invention.
Fig. 2 is a cross-section view of the optical fiber of Fig. 1 taken through
line 2-2 in Fig. 1.
Fig. 3 is a cross-section view of a C12 reactor of the present invention.
Fig. 4 is a schematic view of a vapor delivery system shown forming a
soot blank in accordance with the present invention.
Fig. 5 is a schematic view of a first preferred embodiment of a
consolidation furnace of the present invention taken in cross-section.
Fig. 6 is a schematic view of a second preferred embodiment of a
consolidation furnace of the present invention taken in cross-section.
Fig. 7 is a photomicrograph of a Ta205 doped core glass consolidated at
1450°C in a helium atmosphere.
Fig. 8 is a photomicrograph of a Ta205 doped core glass consolidated at
1450°C in a helium atmosphere.
Fig. 9 is a photomicrograph showing the core-clad interface of Ta205
doped glass consolidated at 1450°C in a helium atmosphere.
Fig. 10 is a photomicrograph of a Ta205 doped core glass consolidated
at 1550°C in a helium atmosphere.
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Fig. 11 is a photomicrograph of a Ta205 doped core glass consolidated
at 1550°C in a helium atmosphere.
Fig. 12 is a photomicrograph of a Ta205 doped core glass consolidated
at 1550°C in a helium atmosphere.
Fig. 13 is a photomicrograph of a Ta205 doped core glass consolidated
at 1450°C in a vacuum atmosphere.
Fig. 14 is a photomicrograph of a Ta205 doped core glass consolidated
at 1550°C in a vacuum atmosphere.
Fig. 15 is a photomicrograph of a Ta205 doped core glass consolidated
at 1650°C in a vacuum atmosphere.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention expressly contemplates the manufacture of
single-mode optical waveguide fibers, multimode optical waveguide fibers, and
planar waveguides regardless of any specific description, drawings, or
examples set out herein. In addition, it is anticipated that the present
invention
can be practiced in conjunction with any of the known optical waveguide
processing techniques, including, but not limited to, the outside vapor
deposition (OVD) technique, the modified chemical vapor deposition (MCVD)
technique, the vertical axial deposition (VAD) technique, the plasma chemical
vapor deposition (PCVD) technique, and sol-gel techniques, to name a few.
However, for the purposes of this specification, the tantala silicate soot and
blanks described herein and shown in the accompanying drawing figures are
described as being manufactured using the OVD technique.
Reference will now be made in detail to the present preferred
embodiments of the invention, examples of which are illustrated in the
accompanying drawings. Wherever possible, the same reference characters
will be used throughout the drawings to refer to the same or like parts. An
exemplary embodiment of the optical waveguide of the present invention is
shown in Fig. 1, and is designated generally throughout by reference character
20.
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In accordance with the invention, the present invention for an optical
waveguide fiber 20 includes a high purity glass cladding 22 and a high
refractive index glass core 24 bonded by the cladding 22. As embodied herein,
and depicted in Figs. 1 and 2, high purity glass cladding 22 is predominantly
silica, and core 24 includes silica doped with tantala in the desired
proportions.
Optical waveguide fiber 20 having between about 2 to 5 wt% non-crystalline
Ta205 after consolidation has been demonstrated to exhibit a loss of less than
about 1.8 dB/km at 1550 nm. In a preferred embodiment, light attenuation in
optical waveguide fiber 20 is less than 0.25 dB/km at 1550 nm.
A preferred embodiment of the method of manufacturing a low-loss
optical waveguide having a high refractive index core includes the steps of
forming a soot blank which includes Ta205 and Si02, consolidating the soot
blank to form a cane under conditions suitable to prevent crystallization of
the
Ta205, and drawing the cane into an optical fiber. The Ta205 can be delivered
using chemical vapor deposition techniques known in the art or via liquid
delivery. The Si02 can similarly be delivered using known chemical vapor
deposition techniques or liquid delivery.
An exemplary embodiment of a reactor for use with the chemical vapor
deposition technique is shown in Fig. 3. Reactor 26 includes a diffuser 28, a
preheat zone 30, and a reaction zone 32. In operation, fragments of tantalum
34 are packed within the preheat zone 30 of reactor 26 and chlorine (C12) gas
is flowed through diffuser 28 and over the fragments of tantalum 34 within
reactor 26. Reactor 26 includes two separate heater coils (not shown) for the
for the preheat zone 30 and reaction zone 32. When the heat in the reaction
zone is 350°C or greater, a sufficient quantity of TaClS gas is formed
in reactor
26 to provide a desired amount of Ta205 in the soot.
As shown schematically in Fig. 4, TaCl5 gas is delivered from vapor
delivery system 36 to a burner assembly 38. The TaCl5 is converted to Ta205
in the burner flame 40 according to the following reaction:
4 TaClS (g) + 5 02 (g) = 2 Ta205 + 10 C12 (g)
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Finely divided amorphous Ta205 containing soot 42 is thereafter projected from
the flame for capture and further processing. In a preferred embodiment, soot
42 is captured on a rotating mandrel 46 to form a soot blank 44. The amount
of Ta205 captured on soot blank 44 is determined by the number of lateral
passes made by burner assembly 38 along the length of soot blank 44, as well
as the flow rate of C12 through reactor 26.
The consolidation furnaces used for consolidating germania silicate
blanks manufactured using OVD techniques typically provide temperatures of
between 1000°C and 1450°C. Through experimentation, it has been
found
that such furnaces do not provide the heat necessary to perform the
consolidation step without crystallization in the Ta205-Si02 containing glass
as
required for the present invention. Accordingly, improved consolidation
furnaces capable of achieving temperatures in excess of 1450°C are
needed
for the present invention. The preferred embodiments of such consolidation
furnaces are shown schematically in Figs. 5 and 6.
Fig. 5 depicts a first preferred embodiment of the consolidation step of
the method of manufacturing a low loss optical waveguide having a high
refractory index core. Soot blank 44 is held within consolidation furnace 48
where it is exposed to a gas 50. Gases such as, but not limited to, chlorine,
helium, and oxygen, or combinations thereof, are delivered into consolidation
furnace 48 to form the atmosphere 52 therein. Presently, the preferred gas,
helium, is flowed across soot blank 44 while temperatures within consolidation
furnace 48 are preferably elevated to 1600°C or greater. These
conditions are
maintained within consolidation furnace 48 until the Ta205-Si02 core glass
temperatures are maintained at 1600°C or higher for a suitable time to
sinter
and vitrify the glass. After taking the additional processing steps commonly
known to those skilled in the art in optical fiber manufacture, the resulting
cane
is drawn into an optical fiber. It is anticipated that an optical fiber
manufactured
from a Si02 soot blank containing 2 to 5 wt% Ta205, and heat treated to a
temperature of 1600°C or higher in a flowing helium atmosphere will
have an
attenuation of less than about 0.25 dB/km at 1550 nm. In a preferred
embodiment, the temperature range is approximately 1600°C to
1700°C.
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Fig. 6 depicts a second preferred embodiment of consolidation furnace
48 shown supporting soot blank 44. In this embodiment of the present
invention, soot blank 44 is heated within a vacuum atmosphere. As used
herein, the phrase "vacuum atmosphere" means an atmosphere less than
atmospheric pressure. As depicted in Fig. 6, a pump 56 or other pressure-
reducing device, removes the air from within consolidation furnace 48, thereby
decreasing the pressure therein. As a result, soot blank 44 can be heat
treated
at temperatures lower than 1600°C to sinter and vitrify soot blank 44.
Typically, soot blank 44 is heated to a temperature between 1500°C
and
1600°C in a vacuum atmosphere so that the Ta205-Si02 core glass
temperatures reach between 1500°C and 1600°C for a sufficient
time to result
in clear glass which is substantially free of crystals. In a preferred
embodiment, the vacuum atmosphere 54 within consolidation furnace 48,
exhibits a pressure of less than about 10-4 torr. Following the additional
processing steps commonly known to those skilled in the art of optical fiber
manufacture, the resulting cane is drawn into an optical fiber. An optical
fiber
manufactured from a soot blank 44 containing Si02 and about 2 to 5 wt%
Ta205, and heat treated at temperatures ranging between 1500°C and
1600°C
in a vacuum atmosphere having a pressure of less then 104 torr is expected to
exhibit attenuation of less than about 0.25 dB/km at 1550 nm.
A significant advantage of the method of the present invention is the
crystalline free consolidation of Ta205 containing soot blanks. The following
examples illustrate the effectiveness of the method of the present invention.
Example 1
A core blank was made by depositing 100 passes of Ta205-Si02 at an
analyzed chemical wt% of 5.55 Ta205, followed by 177 passes of Si02. The
resulting soot preform specimen was cut into cross-sectional slices
approximately 25 millimeters long and approximately 50 to 60 millimeters in
diameter. Samples were then fired at a temperature of 1450°C in flowing
helium as shown in Figs. 7-9. The scanning electron micrographs (SEMs) of
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the core material (Figs. 7 and 8) and the core material below the core-clad
interface (Fig. 9) show that crystallization is prevalent in the Ta205-Si02
containing glass. As shown clearly in the fiber section 60 of FIG. 9, the
silica
cladding 62 is easily distinguished from the Ta205-Si02 containing core 64 as
5 the cladding 62 has consolidated to a clear, amorphous glass. A core-clad
interface region 66 is clearly visible between the cladding 62 and core 64.
Example 2
10 Additional slices of the soot preform specimen described above with
respect to Example 1 were heated to 1550°C under a flowing helium
atmosphere. The results of this experiment are shown in Figs. 10 and 11. The
SEM's again show that the Ta205 containing core glass depicted in Figs. 10
and 11 contained numerous crystals. In fact, crystallization is so prevalent
that
increasing the temperature by approximately 100°C does not appear to
reduce
crystallization as compared to Example 1.
Example 3
An additional slice from the soot preform specimen described in
Example 1 above was heat treated in a flowing helium atmosphere to a
temperature of 1650°C. As shown in the SEM of Fig. 12, the core sample
consolidated to a clear glass having no apparent crystallization.
Example 4
Additional slices of the soot preform specimen described in Example 1
were also fired at temperatures of 1450°C, 1550°C and
1650°C in a vacuum
atmosphere of 1 x 10-4 torr. As seen in Fig. 13, the SEM shows that
crystallization is present in the Ta205 containing core glass after heat
treatment
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at 1450°C. However, at treatment temperatures of 1550°C and
1650°C, as
shown in the SEM's of Figs. 14 and 15, respectively, no crystallization occurs
in
the Ta205-Si02 core glass.
To permit other testing, single-mode step index optical fibers were
drawn from other core blanks prepared in a manner substantially similar to
that
described above with respect to examples 1 - 4. The % 0, and attenuation for
fibers containing different amounts of Ta205 by weight percent are shown
below in Table 1.
Table I
Results for Single Mode Fibers with Tantala Silicate Core
Wt% AttenuationAttenuationAttenuation
Sample Ta205 Delta % ~a7. 1310 ~ 1380 Cc~ 1550
# nm nm nm
1 2.0 0.25 15.6 29.5 4.3
2 2.0 0.25 33.3 40.6 12.4
3 2.0 0.25 26.7 38.8 11.3
4 2.9 0.31 3.6 16.4 2.25
5 2.9 0.30 2.89 7.26 1.73
6 3.1 0.34 4.3 21.5 2.21
7 4.5 0.50 212.7 175.2 82.4
The consolidation furnace used to heat treat the fibers listed in Table I
were standard furnaces commonly used to consolidate Ge02-Si02 optical fiber
preforms. Accordingly, the maximum temperature available for consolidation
was 1450°C. Thus, the maximum temperature of 1450°C was used to
consolidate each of the core blanks listed in Table I above. The lowest loss
attained was for the core blank having 2.9 wt% Ta205. At 1550 nm the
attenuation was 1.73 dB/km. These results confirm the importance of using
consolidation temperatures higher than 1450°C for Ta205-Si02 containing
optical fibers. Based upon this information and the experiments described
above in Examples 1 through 4, it is anticipated that Ta205-Si02 containing
optical fibers will exhibit losses of less than about 0.25 dB/km at 1550 nm
when
the soot blanks corresponding to these fibers are consolidated in
consolidation
furnaces capable of sustaining temperatures greater than 1500°C.
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It will be apparent to those skilled in the art that modifications and
variations can be made to the present invention without departing from the
spirit or scope of the invention. Thus it is intended that the present
invention
cover the modifications and variations of this invention provided they come
within the scope of the appended claims and their equivalents.
