Note: Descriptions are shown in the official language in which they were submitted.
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METHODS AND APPARATUS FOR PRODUCING OPTICAL FIBER
FIELD OF THE INVENTION
The present invention relates generally to improvements in optical fiber
waveguides and their manufacture. More particularly, the present invention
relates to novel methods and apparatus for forming optical fiber waveguides
via
filament in tube and stick in tube methods of fiberization.
BACKGROUND OF THE INVENTION
Optical fiber waveguides have come to play an increasingly important
role in communications. A range of optical fiber types with regard to size,
index profiles, operating wavelengths, materials, etc., must be available in
order to fulfill many different system applications. Further, there is an
increasing need for active devices, such as amplifiers, lasers, switches and
dispersion compensators. Additionally, optical fiber cables must be spliced
together without excessive practical difficulties. It is important that these
splicing techniques may be applied with ease in field locations where cable
connection takes place. It is particularly important in many applications that
a
new fiber may be readily spliced to an existing fiber already in place. Put
otherwise, removing all the existing fiber and replacing it with new fiber
having
different characteristics is often not an option.
Various techniquEa are used to make optical fibers. In one procedure
(see U.S. Patent No. 3,659,915), a rod of core material is placed within a
tube
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2
of lower refractive index cladding material, forming a tight, concentric fit.
The
core material must be uniform in cross section and have a smooth surface.
The temperature is then raised, and the rod and tube are drawn to the desired
cross-sectional area. The resultant optical fiber by this process might not be
ideal for communication because of excessive losses and dispersion.
Another method (see U.S Patent No. 5,651,083) involves the insertion of
a core material into a molten cladding material to create a preform. The core
insertion is performed rapidly so that the core does not soften or dissolve
during the procedure. The resultant preform is then drawn into optical fiber.
Flouride glasses, such as ZBLAN, manufactured in this fashion are not fusion
spliceable to silica fibers., are prone to devitrification and have poor
durability.
One of the more important methods employed in making soot used in
the manufacture of low loss optical fiber is the chemical vapor deposition
(CVD)
process. In one embodiment of the CVD process, relatively pure chemicals
(such as silicon tetrachloride), are passed into a manifold with oxygen. They
are then mixed and fed into a burner which is moved beneath a rapidly rotating
bait rod or high purity fused silica tube. The result is that the silicon is
oxidized
to silica on the bait rod or silica tube. The deposit may be doped with a
variety
of materials. A resulting preform is typically consolidated and then drawn
into
optical fiber. This process is an outside CVD or OVD process. An inside or
MCVD process is also a known CVD process.
Current CVD methods for optical fiber fabrication are limited to
compositions consisting almost entirely of silica. Only modest amounts of rare
earth elements can be incorporated without clustering or crystallization.
Volatile components such as alkalis and halogens cannot be readily introduced
because of their tendency to vaporize during lay down. Other important glass
modifiers such as alkaline earths cannot be incorporated due to lack of high
vapor pressure CVD precursors. Even if glass soot can be deposited by CVD it
must subsequently be consolidated which can lead to crystallization or to loss
of glass components with high vapor pressures.
Another fabrication technique known as the cutlet in tube method has
recently been developed. That approach is described in U.S. Patent
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3
Application Serial No. 08/944,932 filed October 2, 1997 which is assigned to
the assignee of the presE:nt invention, and incorporated by reference herein
in
its entirety. In the culiet in tube process, a core Gullet feedstock (having a
particle size typically in the range of 100 - 5,000 um) is introduced into a
cladding structure. The End of the core/cladding structure is heated in a
furnace to near the softening temperature of the cladding and drawn into
optical
fiber. This method overcomes some of the disadvantages of typical CVD
processes, allowing the cladding composition to consist of pure Si02 and the
core composition to consist of multicomponent glasses.
A need, however, exists for methods and apparatus for making optical
fiber from a variety of glass and glass-ceramic compositions that overcomes
the disadvantages of the known methods, and that is more practical, efficient,
and economical than conventional methods.
By way of example, disadvantages of various prior art CVD techniques
include the very limited compositions which can be fabricated using current
CVD methods. Only modest amounts of rare earth elements can be
incorporated without clustering. Volatile components, such as alkalis and
halogens, cannot be introduced in significant quantities because of their
tendency to vaporize during lay down. Other important glass modifiers such as
the alkaline earths are difficult to incorporate due to lack of high vapor
pressure
CVD precursors. Even if a glass soot can be deposited by CVD, it must
subsequently be consolidated which can also lead to a loss of glass
components with high vapor pressures or crystallization.
Disadvantages of typical rod-in-tube techniques include the requirement
that the core and clad be highly similar. Both the coefficient of expansion
and
viscosity temperature profiles need to be similar, otherwise, the end product
will
be subject to cracking or breakage upon cooling.
The present invention also relates to an optical fiber having a numerical
aperture greater than about .35. Such fibers which comprise a core and clad
region which are glass, arid are doped with a rare earth element which is
selected from the group consisting of ytterbium, neodymium, and erbium, can
be used to fiber lasers. Such fiber lasers can be made by coupling a pump
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4
source coupled to the high NA fiber. Using the methods disclosed herein,
numerical apertures of about .40 or greater (e.g. .45) have been achieved.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatus for producing a
wide variety of optical fibers via filament in tube or stick in tube methods
of
fiberization. In one aspect, the present invention comprises the steps of
filling
a glass tube with a glass filament or stick of the desired material and
subsequent drawing or elongation of the glass tube at elevated temperatures.
The material within the tube melts at the draw temperature and fills the tube
to
form a continuous core. 'The loose fitting feedstock can be automatically fed
or
melted down by gravity to maintain a constant depth of molten feedstock,
yielding a homogeneous and reproducible product. The feedstock can be
comprised of a core material or a core/clad material. Likewise, the tube can
be
comprised of additional core material (e.g., which could be used to form the
outer core region), core/clad material, or a cladding material. The present
invention can be used todraw optical fiber directly (filament in tube) or can
be
used to make a core canE: or a core clad cane which is then overcladded with
additional material before being drawn into optical fiber (stick in tube).
The present invention allows almost any glass that can be produced by
chemical (sol-gel, vapor deposition, etc.) or physical (batch and melt)
techniques to be economically fabricated in the form of a continuous clad
filament. The rapid quenching permitted by this technique allows for
previously
unstable glasses and gla;>s-ceramics to be formed as stable fibers.
A more complete understanding of the present invention, as well as
further features and advantages, will be apparent from the following Detailed
Description and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
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FIG 1 is a cross sectional drawing of suitable apparatus for performing
the filament in tube method of drawing optical fiber in accordance with the
present invention;
FIG. 2 is a cross sectional drawing of suitable apparatus for performing
5 the stick in tube method of drawing optical fiber in accordance with the
present
invention;
FIG. 3 illustrates suitable apparatus for overcladding an optical cane
formed in accordance with the present invention which may then be drawn into
optical fiber in accordance with the present invention;
FIG. 4 is a graph showing loss as a function of wavelength for a 5 meter
span of optical fiber produced in accordance with a filament-in-tube method of
the present invention;
FIG. 5 is a graph showing the refractive index profile of a core clad cane
produced in accordance with the present invention;
FIG. 6 is a graph showing loss as a function of wavelength for a 5 meter
span of optical fiber produced in accordance with a stick-in-tube-method of
the
present invention; and
FIG. 7 is a graph showing the loss and mode field diameter as a function
of fiber length for an optical fiber produced in accordance with the present
invention.
DETAILED DESCRIPTION
The present invention provides methods and apparatus for producing a
wide variety of optical fibers via filament in tube and stick in tube methods
of
fiberization as more fully discussed below. Before addressing the present
invention in detail in connection with the drawings, various general aspects
and
advantages will first be generally addressed. First, a glass or crystalline
stick of
the desired core composition should be obtained. It does not matter if the
stick
has a round, square, or triangular, or some other different cross section, it
need
only fit within a cladding tube with which it is to be utilized. Unlike the
well
known rod-in-tube method, the inventive method does not require the core to
fit
tightly and concentrically within the cladding tube, since the core filament
melts
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6
to conform to the cladding walls. Likewise, the tube bore need not be
circular,
but can be of a rectangular, elliptical, or other non-circular shape to enable
formation of a fiber having a non-circular core. Because, in the present
invention, the core glass or stick conforms to the shapes of the internal
diameter of the tube, whE;n a rectangular shaped tube bore is employed, the
core glass will deform to the shape of the tube, thereby forming a core region
upon solidification which is rectangular in shape. A fiber having a generally
rectangular core can be made by employing a tube having a rectangular
shaped inside diameter. After drawing of the fiber, the core remains of a
substantially rectangular shape in cross-section (with some rounding of the
corners of the rectangle). Fibers having a rectangular shaped core have been
made, using the methods. of the present invention disclosed herein, which
exhibited a numerical aperture (NA) of greater than about .35, in particular
we
have achieved numerical apertures of about .45. Such high NA fibers cannot
generally be made using CVD techniques, because the large amount of
modifier needed for such refractive index changes causes crystallization,
cracking, or sagging of the core material during manufacture. Such a fiber
having rectangular shaped core can be used for efficiently coupling the light
from a stripe laser diode. For instance, a typical high powered stripe laser
diode emits a beam having essentially a 100 Nm x 1 pm rectangular geometry.
Accordingly, a beam having this geometry is more efficiently captured by a
fiber
formed in accordance with the present invention to have a substantially
similar
core geometry to that of the laser beam.
Other non-circular shaped tube bores could also be employed, including
elliptical, which could be employed to form polarization maintaining fibers.
Likewise, tubes having an outer periphery that is non-circular in cross-
section
can also be utilized, to manufacture fibers whose outer periphery is non-
circular. In each of these embodiments in which a non-circular ID or OD is to
be maintained, high drawing viscosity is preferable to facilitate the fiber or
core
cane at least substantially retaining the shape of the ID or OD, or both. In
particular, in one preferred embodiment used to make a fiber or core cane
having either non-circular core or a non-circular outer fiber diameter, or
both,
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7
the draw rate and temperature of the preform is maintained so that the
resultant
fiber or core cane at least substantially retains the shape of the inside and
outside of the tube. Such a result is facilitated by maintaining the draw or
redraw (in the case of making a core cane) temperature so that the tube
viscosity is maintained at greater than about 10' Poise.
The inventive method also does not require the core stick to be uniform
in cross section and have a smooth surtace, unlike prior rod-in-tube
technology.
The core stick can be fabricated by conventional crucible melting and
casting, drawing, sol-gel, or some other technique. The stick is then loaded
into the cladding tube. The composition of the tube which becomes the fiber
cladding is not limited and can range from pure Si02 to multicomponent
glasses. The only requirE;ment is that the core glass melt at or below the
softening point of the cladding tube and that the thermal expansion difference
between the core and clad not be so large as to shatter the resultant fiber
upon
cooling as addressed in greater detail below.
After the cladding tube is filled, it can be drawn down into fiber or canes
for overcladding. The filled tube is heated to soften the cladding glass for
elongation. As the cladding tube softens during the draw, the core stick will
melt, fine (removal of bubbles), and conform to the walls of the cladding tube
forming an interface determined by the inner surface of the cladding tube. The
ratio of the outer diameter {OD) to the inner diameter (ID) of the tube will
be
roughly the same as the fiber or cane OD/1D ratio although it can be
controlled
by the pressure (positive or negative) applied over the molten core relative
to
outside the cladding tube. The draw temperature can also be used to control
the core diameter as higher draw temperatures will lead to smaller core
diameters for the same given fiber OD. This control represents a substantial
advantage over conventional preforms where this ratio is fixed once the blank
is fabricated. The higher temperatures used to draw the cladding tube
{2000°C
for the case of pure Si02 cladding) serve to homogenize the core melt and
drive off detrimental water in the glass. in addition, a vacuum can be applied
to
a centerline to enhance water removal and fining.
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Utilizing an open centerline during the first draw step allows for
atmospheric control of the melt at the drawing temperature. Oxidizing as well
as reducing atmosphere:. can be introduced above the melt to control the redox
state of the core material or maintain reduced metallic cores or
superconductors in a dielectric cladding. Multiple concentric or parallel
cores
may also be made by this method where one core may carry optical information
and the other electrical information. For example, stress rods (e.g. B203
doped
Si02 glass rods) can be placed within bores that are drilled in the walls of
the
tube. Such stress rods could be made via CVD and consolidated into glass,
then placed within the bores drilled into the side walls of the tube.
Alternatively,
the stress rods could be comprised of a non-CVD formed glass (e.g. a melt
glass) and placed within i:he bores of the side walls of the tube. In either
case,
after the glass stress rods were positioned within the bores of the sidewalls
of
the tube on opposite sides of the tube, the core glass feedstock could be fed
into the inside diameter of the tube as described further herein. The
resultant
preform could be subsequently drawn to make a polarization maintaining fiber.
Alternatively, two electrically conductive wires could be used in place of the
stress rods (and likewise inserted into holes bored into the sidewalls of the
tube} to make a fiber which may be used as an electro-optic switch (e.g.,
enabling application a voltage between the two wires to change the refractive
index of the core).
The pressure within the tube above the core can also be controlled to
regulate the core diameter. This type of process control is not available with
any of the current preform fiberization methods, and in the present invention,
such control greatly facilitates the formation of certain combinations of tube
and
core glasses. For example, with thicker walled tubes, such vacuum assistance
might not be important. i-iowever, with some thinner walled tubes, or in cases
where the core glass has a larger melt depth within the tube, such vacuum
application within the tube from above the core glass can be used to
compensate for the forces of the melt glass pushing outwardly on the tube
walls, thereby helping to maintain the internal shape of the tube walls.
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Controlled glass composition and thermal history can also be used to
generate graded index profiles. Since the core is molten and the cladding is
softening, diffusional processes are relatively fast, so graded index profiles
can
be created in situ. With appropriate choice of cladding material, the fibers
produced can be fusion spliced to conventional fibers making them quite
practical in existing fiber networks and easing device manufacture.
The stick-in-tube method allows for complicated index profiles. For
example, the first cladding tube could have a refractive index in between that
of
the core and the overclacl tube to control the numerical aperture of the fiber
or it
could contain refractive index moats and rings inserted to engineer the
dispersion and mode field diameter of the fiber. The first draw reduces the
radial dimensions of the index profile by a factor of 6-8, and the second
draw,
reduces them down again by a factor of 400-500, so very fine structures can be
achieved.
The present invention will now be described more fully below with
reference to the accompanying drawings, in which several presently preferred
embodiments of the invention are shown. This invention may, however, be
embodied in various forms and should not be construed as limited to the
presently preferred embodiments set forth herein. Rather, applicants provide
these embodiments so that this disclosure will be thorough and complete, and
will fully convey the scopE; of the invention to those skilled in the art who
will be
readily able to adapt these teachings to a wide range of embodiments and
applications.
For example, though the present invention will be principally described in
terms of waveguiding optical fiber, those skilled in the art will appreciate
that
the optical articles contemplated by the invention may also include, but are
not
limited to, planar amplifiers, couplers, fiber lasers, Faraday rotators,
filters,
optical isolators, and nonlinear waveguiding fibers. Moreover, the fabrication
of
continuous clad filaments for conductive conduit is contemplated, resulting in
superconducting wire. Electro-optical and photonic crystal composites are also
envisaged.
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FIG. 1 is a cross sectional drawing of an apparatus 100 which may
suitably be used for implementing the filament in tube method of drawing
optical fiber in accordance with the present invention. First, a cladding tube
112, having a 57 mm OD and a 2 mm ID in a preferred embodiment, with an
5 inner wall 118 is purged with a drying gas, for example chlorine (CI2) or
chlorine mixed with an inert gas, to remove unwanted moisture. A core
feedstock or filament 110, having a 1.5 mm diameter in a preferred
embodiment, is disposed within the cladding tube 112. This feedstock or
filament 110 is preferably an elongated monolithic rod of material, however, a
10 plurality of elongated rods can be stacked one atop the other within the
cladding tube 112 to form the feedstock. Using a plurality of rods is
particularly
well suited for the production of dispersion managed fiber. The cladding tube
112 and the core filament 110 form a filled cladding tube with an open
centerline 122 which is heated by a furnace 114, as described further below.
The furnace 114 is operated at a draw temperature which is at or above the
melting temperature of the core filament 110, but only causes the cladding
tube
112 to soften. As the cladding tube 112 softens, the core filament 110 will
melt
at the draw temperature forming a core melt 120 contained within the cladding
tube 112. It is presently preferred that the draw temperature is at or above
the
liquidus temperature of the core filament to eliminate crystals in core melt
120.
As used herein, melt means that the core filament 110 flows and fills or
deforms to the interior of the cladding tube 112 so that a filled cladding
structure results.
In accordance with one preferred embodiment of the present invention,
during the step in which i:he core melts and deforms to the interior of the
tube,
the core preferably exhibits a viscosity of less than 106 poise, more
preferably
104 poise, most preferably 1000 poise or less, and the cladding structure
maintains a viscosity sufficient for the cladding to substantially retain its
internal
shape. Most preferably, the cladding tube 112 exhibits a viscosity greater
than
10''6 poise at this temperature. This distinguishes the present invention from
the more conventional methods (e.g. rod and tube or CVD) of the prior art, in
which the viscosity of the core and clad were typically matched so that both
the
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rod and tube had a viscosity that differed by less than a factor of about 10
at
the temperature at which the fiber is drawn or cane is drawn. An optical fiber
116 is then drawn. While melting, the core filament 110 will preferably fine
and
conform to the interior wall 118 of the cladding tube 112, forming an
interface
determined by the inner surface 118, and completely filling the interior of
the
cladding tube 112.
At its softening paint, a glass cladding material 112 has a viscosity of
about 10''6 poise. For some types of Si02, this occurs at a temperature of
about 2000°C. The cladding material should be selected so that at the
temperature at which the core material is filling the interior of the cladding
tube
it has a viscosity greater than 10' poise, and preferably greater than 10''6,
and
most preferably greater than 108. However, at this same temperature, a core
120, such as 69.86 mole % silica (Si02), 18.63 mole % aluminum oxide (A1203),
4.66 mole % sodium oxide (Na20), and 6.85 mole % lanthanum fluoride (La2F6)
will have a viscosity of approximately 10 poise, seven orders of magnitude
less
than the cladding 112. The core 120 might suitably have a viscosity less than
or equal to about 104'5 poise. In contrast, a typical rod in tube process
would
typically employ both core and cladding material having substantially the same
viscosity.
One significant advantage of the present invention is that the core
filament 110 can be produced in any shape (round, square, triangular, etc.)
and
via any method (conventional crucible melting and casting, drawing, sol-gel,
etc.). The only physical requirement is that the core filament 110 fit within
the
inner walls of the cladding tube 112. Thus, less rigid process controls are
required during the manufacture of the core filament 110. Moreover, the loose
fitting core filament 110 nnay be automatically fed down or dropped down as
its
bottom is melted to maintain a constant depth of molten core 120, yielding a
homogeneous and reproducible optical fiber 116. The core filament 110 has a
melting temperature, as defined above, which is below the softening
temperature of the cladding 112, and the thermal expansion difference between
the core filament 110 and the cladding 112 is not so large as to shatter the
fiber
116 when it is cooled. The composition of the cladding 112 is preferably
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72
silicate glass, but it will be appreciated by those skilled in the art, that
the
composition of cladding 112 is essentially not limited and can range from pure
Si02 to multicomponent glasses.
FIG. 2 is a cross sectional drawing of an apparatus 200 which may
suitably be employed for performing the stick in tube method of drawing
optical
fiber in accordance with the present invention. A one meter long Si02 cladding
tube 212 (55 mm in outE:r diameter and 6 mm in inner diameter) is purged with
drying gas to remove unwanted moisture. A 5 mm diameter core stick 210 is
disposed or placed within the cladding tube 212 to form a filled cladding
tube.
The filled cladding tube is heated by a furnace 214 to 1700° C to
soften the
cladding tube 212 in preparation for elongation. As the cladding tube 212
softens, the core stick 2110 melts, and a 6 mm outer diameter optical core
cane
216 is then drawn in a standard manner. By "cane" or "core cane" as used
herein, we mean an optical fiber precursor element comprising a core glass
material, to which additional cladding must be added to the core cane prior to
its being drawn into an optical fiber. Such additional cladding may be
applied,
for example, by inserting the cane into a glass cladding sleeve, or depositing
additional core and/or cladding glass via outside vapor deposition or other
methods. While melting,, the core stick 210 will fine and conform to the
interior
walls of the cladding tube 212, forming an interface determined by the inner
surface of the cladding tube 212.
In this embodiment, wherein the core stock and tube are first redrawn
into a core cane, the cladding material 212 preferably has a viscosity of
approximately 10$ poise at the draw temperature (e.g. about 1700°C for
some
forms of silica) and the core stick 210 will have a viscosity of approximately
104
poise or less at the draw temperature.
In an embodiment of the present invention shown in FIG. 2, the resulting
core cane 216 is then placed within an overcladding tube 220. The filled
overcladding tube 220 is heated by a furnace 222 to soften the overcladding
tube 220 in preparation for elongation. As the overcladding tube 220 softens,
the cane 216 will soften, and an optical fiber 224 is drawn.
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In an exemplary method for forming fiber utilizing the stick in tube
process of the present invention, a core glass of molar composition 70.0 Si02 -
11.25 AI203 - 7.5 Ta205 ~- 10 Ca0 - 2 CaF2 - .05 Er203 was hatched from high
purity powders, mixed, calcined at 400°C for 12 hours to dry the batch,
and
then melted in a covered high purity silica crucible at 1650°C for 4
hours. The
melt was stirred with a fused silica rod to promote homogeneity, then cooled
to
1500°C and drawn up into a 4-5 mm diameter stick from the melt.
The 5 mm diameter stick of core glass was then inserted into a meter
long 55 mm outer diameter (OD), consolidated, Si02 blank previously
manufactured using the outside vapor deposition process with a 6 mm inner
diameter (ID). The tube was purged with dry He gas to remove unwanted
moisture and heated to 1800°C to soften the Si02 blank and drawn down
into a
6 mm diameter core/clad cane which was flame cut into 1 meter long pieces. A
1 meter long piece was then mounted on a CVD lathe and overclad with Si02
soot to obtain the desired clad diameter/core diameter ratio of 32:1. The
overclad cane was then consolidated between 1440 and 1500°C to form a
monolithic Si02 blank with a core. This blank was then heated to 1950-
2000°C
in a graphite resistance furnace and drawn into standard 125 micron diameter
fiber at a rate of 2 m/s. The resultant fiber having an Er-doped core is
suitable
for use as an optical amplifier.
FIG. 3 is a drawing of an apparatus 300 used for overcladding an optical
cane via an alternative CVD process and then drawing an optical fiber in
accordance with the present invention. A cane 216, produced by the
embodiment of the present invention shown in FIG. 2, is cut into 1 meter long
pieces. The cut cane 216 is then mounted on a CVD lathe 332 and overclad
with Si02 to obtain the desired ratio between the clad diameter and the core
diameter, forming an ove~rclad optical cane 330. The overclad cane 330 is then
consolidated at a temperature between 1400° C and 1500° C to
form a
monolithic Si02 blank 335. An end of the monolithic blank 336 is then heated
in a furnace 338 to a draw temperature of 1950-2000° C and drawn into
standard 125 micron diameter optical fiber 340.
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FIG. 4 is a graph 400 showing loss as a function of wavelength for a 5
meter span of an optical fiber produced in accordance with the present
invention. The low loss optical fiber (0.07 dBlm at 1310 nm) exhibits the same
loss per meter, beginning to end, over a 2000 meter span. The core of the
optical fiber was successfully doped with erbium ions, Er3+, as evidenced by
the
adsorption bands at 980 and 1500 nm. Additionally, Er3+ fluorescence was
observed from the optical fiber when 980 nm laser light was pumped into the
fiber. Fig. 4 illustrates that, using the methods of the present invention,
fiber
can be made having a rare earth dopant therein which exhibit a background
attenuation of less than 2 dB/m.
FIG. 5 is a graph ;>UO showing the refractive index profile of a core cane
produced in accordance 'with the present invention using the core glass
composition described above with respect to Fig. 2. The core cane has a
drawn diameter of 2.74 mm, the core of the cane having a diameter of .21 mm,
for a core/clad thickness ratio of about .077. As can be seen in Fig. 5, the
core
exhibited a refractive index delta of about .11 (with respect to the silica
core), or
a delta percent of about fi.76 percent (again with respect to the undoped
silica
cladding). The observed maximum delta of fi.76 percent is significantly higher
than that seen for typical CVD produced fiber. The cane was subsequently
overclad and drawn into '10,000 m of homogeneous optical fiber. The total core
diameter variance over the 10,000 m span was ~ 0.25 p.m, as compared to the
cutlet in tube method which would yield a variance of ~ 4 Vim. Thus, fiber
manufactured in accordance with the present invention shows an improvement
of at least an order of magnitude. Additionally, utilizing Si02 as the
cladding
material allowed the resultant optical fiber to be fusion spliced using
conventional fusion splicers. Splice losses of less than 0.5 dB have been
achieved when splicing the fibers made in accordance with the invention to
SMF-28 optical fiber, and splice losses of less than 0.2 dB have been made
when fibers in accordance with the invention were spliced to CS-980 optical
fiber.
FIG. 6 is a graph Ei00 showing loss as a function of wavelength for a 5
meter span of optical fiber produced in accordance with the present invention.
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A tube was made by depositing pure silica, followed by germania doped silica,
followed by a pure silica caadding region. The resultant soot preform was
consolidated to form a tube. Then, the same type of core glass rod described
above with respect to Fig.. 2 was inserted into the glass tube and drawn into
a
5 fiber. The resultant multi-rnmponent core in this case was comprised a
central
high index region surrounded by a silica moat, which was in turn surrounded by
a ring of Si02 doped with germanium oxide (Ge02). This demonstrates that
complex index of refraction profiles can be made with less than 0.5 dBlm
background attenuation
10 FIG. 7 is a graph 700 showing the loss and mode field diameter as a
function of fiber length for an optical fiber produced in accordance with the
present invention. Fig. 7 demonstrates that mode held diameters can be
expanded by employing a raised index ring outside the central high index
region of the core, to thereby expand the mode field diameter beyond that of a
15 what would be achieved using a single raised index core. Minimal variations
in
loss and mode field diamEaer for varying lengths are achievable, as
illustrated
by Fig. 7.
The method of the present invention has a variety of advantages. The
method of the invention opens up a large range of compositions for
fiberization
that have not previously been attainable through conventional CVD techniques
which have been employE:d to make optical fiber. New compositions with high
rare earth solubility, improved gain flatness and improved optical properties
can
be readily fabricated into fiber form. The method also accommodates large
differences in thermal expansion between the core filament 110 or core stick
210, and cladding material 112 or cladding material 212, since the core 110,
210 is not rigidly bonded i:o the clad 112, 212 until the core filament 110 or
core
stick 210 is in fiber or cane form when the stress due to thermal expansion
mismatch are much smaller than in a rigid monolithic preform of greater size,
as these stress forces vary inversely with the square of the radius of the
fiber,
preform or the like. Accordingly, very large numerical aperture fibers for use
as
efficient couplers and lasE:rs can be produced by the method of the present
invention.
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16
The method also allows for atmospheric control of the core melt 120,
220 at the drawing temperature. Either oxidizing, reducing or chemically
reactive atmospheres can be introduced utilizing the open centerline to
control
the redox state. The pressure above the core filament 110 or core stick 210
can be controlled to regulate the core diameter, as can the draw temperature.
Higher draw temperatures wilt lead to smaller core diameters for the same
given fiber outer diameter (OD), in contrast to conventional preforms where
this
ratio is fixed once the blank is fabricated. For example, these factors can be
used to modulate core diameter by plus or minus 50% utilizing the present
invention. The ratio of the OD to the inner diameter (ID) of the tube will be
roughly the same as the optical fiber OD to ID ratio although, as stated, it
can
be controlled by positive or negative pressure applied over the molten core
120, 220 relative to outside the cladding tube 112 or cladding tube 212,
respectively. Additionally, the high temperatures used to draw the optical
fiber
116, 216 serve to homogenize the core melt 120, 220 and drive off detrimental
water present in the core melt 120, 220.
While the foregoing description includes detail which will enable those
skilled in the art to practice the invention, it should be recognized that the
description is illustrative in nature and that many modifications and
variations
thereof will be apparent to those skilled in the art having the benefit of
these
teachings. By way of example, while it is presently preferred that a core
feedstock, such as core feedstock 110, be a solid rod, the core feedstock
could
conceivably be hollow, or' be divided into several large blocks. Further, the
term feedstock is intended to encompass a thin filament, a thicker stick, a
plurality of elongated filaments bundled for insertion into the tube, or
elongated
filaments or sticks stacked axially one on top of the other for insertion into
the
tube, or the like, which will properly feed down upon melting. On the other
hand, feedstock as definE:d herein preferably is not powder or cutlet.
Moreover,
the feedstock can be formed from a core material alone or from a core material
having a cladding material disposed thereon. Either of these embodiments can
then be disposed within a tube formed from cladding material. Similarly, the
tube can be formed from core material or cladding material. Thus, it is
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17
conceivable to manufacture a preform having a plurality of concentric rings of
core material and cladding material, each ring having the same or different
optical characteristics as other rings within the preform. In addition, a
preform
might be formed, cooled, stored and then later reheated and drawn although
this is not presently preferred. Further, as appropriate, the term optical
fiber
should be construed as encompassing any fiber or fiber component employed
in applications including lout not limited to optical waveguides, single mode
fibers, multi-mode fibers, amplifiers, electro-optical fibers, couplers,
lasers, or
the like.
It will be apparent 'to those skilled in the art that various modifications
and variations can be made in the present invention without departing from the
spirit or the scope of the invention. Thus, it is intended that the present
invention cover the modification and variations of this invention provided
they
come within the scope of the appended claims and their equivalents.
