U.S. patent application number 10/610127 was filed with the patent office on 2005-04-21 for preform for producing an optical fiber and method therefor.
Invention is credited to Kliner, Dahv A. V., Koplow, Jeffery P..
Application Number | 20050084222 10/610127 |
Document ID | / |
Family ID | 30001149 |
Filed Date | 2005-04-21 |
United States Patent
Application |
20050084222 |
Kind Code |
A1 |
Kliner, Dahv A. V. ; et
al. |
April 21, 2005 |
PREFORM FOR PRODUCING AN OPTICAL FIBER AND METHOD THEREFOR
Abstract
The present invention provides a simple method for fabricating
fiber-optic glass preforms having complex refractive index
configurations and/or dopant distributions in a radial direction
with a high degree of accuracy and precision. The method teaches
bundling together a plurality of glass rods of specific physical,
chemical, or optical properties and wherein the rod bundle is fused
in a manner that maintains the cross-sectional composition and
refractive-index profiles established by the position of the
rods.
Inventors: |
Kliner, Dahv A. V.; (San
Ramon, CA) ; Koplow, Jeffery P.; (Washington,
DC) |
Correspondence
Address: |
Timothy Evans
Sandia National Laboratories
MS 9031
7011 East Avenue
Livermore
CA
94550
US
|
Family ID: |
30001149 |
Appl. No.: |
10/610127 |
Filed: |
June 30, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10610127 |
Jun 30, 2003 |
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09778329 |
Feb 6, 2001 |
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6711918 |
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Current U.S.
Class: |
385/115 |
Current CPC
Class: |
C03B 37/01297 20130101;
C03B 37/01274 20130101; C03B 37/01225 20130101; C03B 2203/30
20130101; C03B 37/01217 20130101; H01S 3/06716 20130101; C03B
37/0122 20130101; C03B 2203/42 20130101; C03B 2201/31 20130101;
C03B 2201/34 20130101; C03B 19/106 20130101; C03B 37/0279
20130101 |
Class at
Publication: |
385/115 |
International
Class: |
G02B 006/04 |
Goverment Interests
[0001] The United States Government has rights in this invention
pursuant to Contract No. DE-AC04-94AL85000 between the United
States Department of Energy and Sandia Corporation for the
operation of Sandia National Laboratories.
Claims
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52. A glass preform for providing a drawn optical fiber,
comprising: a plurality of first and second glass rods, having a
chemical composition comprising silica, said first glass rods
further comprising a plurality of groups of glass rods each
comprising a rare-earth dopant, wherein each of said groups of
glass rods comprise a concentration of said rare-earth dopant which
is different from every other group, and wherein each of said
groups are concentrically disposed with respect to each other to
provide a core region having an rare-earth-dopant concentration
increasing radially outward from said longitudinal axis, said
second glass rods further comprising one of two or more different
refractive indices, wherein said first glass rods are bundled
together to form a substantially contiguous core bundle around a
longitudinal axis, and said second glass rods uniformly grouped
around said core bundle in the form of a substantially cylindrical
annulus, wherein said cylindrical annulus has an refractive index
falling within a range spanned by said two or more refractive
indices substantially equal to a predetermined average refractive
index, and wherein said core bundle and said cylindrical annulus
form a composite preform bundle; and a glass tube surrounding and
containing said composite preform bundle, said glass tube and said
composite preform bundle fused at a glass fusion temperature to
form a solid glass preform, wherein said chemical compositions of
each of said glass rods is maintained in a location proximate or
about coincident with a position of each said glass rods within
said glass preform bundle.
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61. The glass preform of claim 52, wherein said contiguous core
bundle further comprises a successive series of nested groups of
glass rods, wherein each said group of glass rods comprises a
quantity of second glass rods distributed in a quantity of first
glass rods, and wherein the fraction of second glass rods to said
first glass rods in each group increases radially from said
longitudinal axis.
62. The glass preform of claim 52, wherein said second glass rods
further comprise a coefficient of thermal expansion different than
said first glass rods.
63. The glass preform of claim 62, wherein said plurality of groups
of glass rods are disposed with respect to each other to form an
elliptical bundle of first glass rods.
64. The glass preform of claim 62, wherein said plurality of groups
of glass rods are disposed with respect to each other to form two
equal and opposing radial sections of first glass rods.
65. The glass preform of claim 64, wherein said radial sections are
sectors of an annulus.
66. The glass preform of claim 64, wherein said radial sections are
circular sections.
67. The glass preform of claim 52, wherein said second glass rods
further comprise a co-dopant species for increasing the solubility
of said one or more rare-earth dopant elements and for adjusting a
refractive index.
68. The glass preform of claim 52, wherein said second glass rods
further comprise a means for eliminating or substantially reducing
propagation of amplified spontaneous emission.
69. The glass preform of claim 68, wherein said means for
eliminating or substantially reducing propagation of amplified
spontaneous emission comprises one or more dopant compounds.
70. The glass preform of claim 68, wherein said means for
eliminating or substantially reducing propagation of amplified
spontaneous emission is substantially restricted to an outer
portion of an inner cladding.
71. The glass preform of claim 69, wherein said means for
eliminating or substantially reducing propagation of amplified
spontaneous emission comprises a metal dopant.
72. The glass preform of claim 71 comprises a metal dopant selected
from the list consisting of terbium, titanium, and zirconium or
combinations thereof.
Description
BACKGROUND OF THE INVENTION
[0002] The present invention is a method for fabricating
fiber-optic preforms with complex refractive-index and/or dopant
distributions to a high degree of accuracy and precision. In
particular, the present invention focuses on the fabrication of
performs for providing rare-earth-doped optical fibers such as
those widely used in fiber amplifiers and lasers.
[0003] The simplest method of preform fabrication is the so-called
"rod-in-tube" method such as is disclosed and described in Pat.
Nos. 4,668,263 and 4,264,347. A rod of glass that will form the
core of the fiber is inserted into a thick-walled tube that will
become the cladding, and the two are fused together at high
temperature. The relative dimensions of the core and cladding in
the drawn fiber are identical to that of the original preform. The
main advantage of the rod-in-tube technique is its simplicity and
as such it was used almost exclusively during the earliest years of
fiber development. However, while simple this technique was also
quite limited in its ability to implement optical fiber designs
having any but the most rudimentary characteristics, and newer
methods capable of producing ultra-low-loss fibers, such as are
required for optical telecommunications, have essentially replaced
the rod-in-tube technique.
[0004] In order to practice the rod-in-tube method bulk glass is
usually synthesized by mixing together the various ingredients in
powder form and melting the mixture in a high-temperature furnace.
All modern preform fabrication methods, however, are based instead
on vapor-deposition techniques. The core and cladding materials are
formed by reacting various gas-phase precursors at high temperature
to form a glass "soot" that is subsequently sintered into a solid
material. A principle advantage of the vapor-deposition process is
its inherent capacity for providing a built-in purification step
that immediately precedes the synthesis step. Starting reagents
(liquids or solids) are heated and delivered to a reaction zone as
a vapor phase. This distillation-like process leaves behind the
vast majority of contaminating species typically present as trace
constituents in the starting reagent materials, most notably
transition metals.
[0005] Three types of vapor-deposition processes have been
developed for fabrication of fiber-optic preforms. By far the most
widely used method in the manufacture of rare-earth-doped fibers is
the so-called "Modified Chemical Vapor Deposition" (MCVD) process.
In this technique, volatile compounds, usually halides or chelated
complexes, containing the desired dopant species 1, as a gas phase,
are reacted with oxygen within an inside portion 2 of a
thick-walled silica reaction tube 3, as shown in FIG. 1. As
reactants 1 are delivered, silica reaction tube 3 is rotated while
its outside surface is heated with an oxygen-hydrogen flame 4. The
flame is translated back and forth along the axis of the tube.
Combustion of gas-phase reactants 1 is confined to heated zone 2a,
inside the tube, and deposition of the products of combustion
("soot" 5) occurs on the inner surface 2a of silica reaction tube
3. Following the combustion/deposition step, the temperature in the
tube is increased to .about.1500.degree. C., which sinters the
deposited soot 5 into a solid layer of material. The deposition and
sintering cycle is then repeated to build up additional layers of
glass, after which the temperature of the tube is raised to
>2000.degree. C., at which point surface tension causes the tube
to slowly collapse inward to form a solid rod serving as the
finished preform.
[0006] In the simplest version of MCVD, silica tube 3 forms the
"cladding" of the preform (i.e., the region surrounding the core),
and vapor-deposited material 5 forms the "core". One of the main
advantages of MCVD, however, is that the chemical composition of
the glass can be varied as a function of its radial position in the
preform. That is, by adjusting the mixture of dopant species as
each successive layer is deposited, the composition of the core
and, if desired, of the portion of the cladding formed by the
deposition process can be customized for specific applications.
This procedure can thereby be used to achieve a structured or
graded dopant profile in the preform and thus a corresponding
structured or graded refractive-index profile in the subsequently
fabricated optical fiber.
[0007] An important variant of the standard MCVD process is a
technique called "solution doping", which provides an alternative
method for introducing a dopant-oxide species into the preform. In
this method variation, a soluble salt of one or more dopant species
is dissolved in a suitable solvent, such as alcohol. The partially
sintered glass soot is soaked in the salt solution, and the solvent
is subsequently removed by evaporation. The sintering process then
proceeds as before, consolidating the dopant species and host
material into a solid glass preform.
[0008] Related to MCVD are two other vapor deposition processes,
referred to as "Outside Vapor Deposition" (OVD) and "Vapor Axial
Deposition" (VAD). In both techniques, a chloride of the desired
dopant species I is introduced and reacted with H.sub.2O generated
in an oxygen/hydrogen flame. Flame 4 is directed against solid
substrate 6 where soot 5 is deposited. The substrate in the OVD
process is a rotating silica rod, as shown in FIG. 2. When enough
material has been deposited, the partially sintered boule of glass
is removed from the silica rod and fully sintered. The sintered
mass is then collapsed, as before, at high temperature to form the
solid glass preform. In the VAD technique, torch flame 4 is
directed onto the end of a rotating silica pedestal 7 as shown in
FIG. 3. As with MCVD, solution doping can be used with the OVD and
VAD processes to incorporate additional dopant species into the
pre-sintered glass preform before the final sintering step is
carried out. The main differences between the OVD and.VAD
techniques are:
[0009] 1) The radial profiles of the dopant species (including
rare-earth constituents and other species such as B, Al, P, Ge, and
F), and therefore the refractive index, can be controlled more
easily in the OVD process.
[0010] 2) The VAD process eliminates the sometimes difficult step
of removing the pre-sintered soot boule from the silica rod.
[0011] 3) The VAD process does not require the preform-collapse
step.
[0012] A characteristic common to all vapor-deposition techniques
is poor process control. Delivering known and stable concentrations
of dopant precursor species is particularly difficult. The
rare-earth chlorides, for example, must be delivered as vapor
through heated delivery lines to avoid recondensation. In addition,
these species are very reactive, making it difficult to use
mass-flow controllers or similar devices to regulate reactant flow
rates and therefore rates of species addition. Furthermore,
fluctuations in the temperature distribution of the reaction zone
affect the composition of the preform by changing the relative
rates of the various oxidation reactions and by changing the soot
deposition efficiency. Similarly, with the solution doping
technique, the distribution of dopant species incorporated into the
host material is often non-uniform and unpredictable (the density
and pore size of the partially sintered glass network can vary
substantially). In practice, it is usually necessary to adjust the
various process parameters by trial and error, fabricating several
preforms until one of acceptable quality is obtained. Where
tolerances on refractive index and/or dopant concentration are
important, or where the shapes of the required dopant and/or
refractive-index profiles are complex, the probability of producing
a preform having an acceptable level of quality decreases
dramatically. As a result, the range of fiber designs that can be
fabricated is quite limited. This limitation persists despite large
investments of time and resources in the development of optical
fibers for a wide variety of commercially significant applications
{see S. E. Miller and A. G. Chynoweth eds., Optical Fiber
Telecommunications (Academic Press, San Diego, Calif., 1979); P. C.
Becker, N. A. Olsson, and J. R. Simpson, Erbium-Doped Fiber
Amplifiers (Academic Press, San Diego, Calif., 1999)}.
[0013] The present invention is directed toward solving these
problems by providing a technique wherein a plurality of rods is
bundled and fused into a glass preform, which is subsequently drawn
into an optical fiber. Related art includes the development of
multicore optical fibers (Pat. Nos. 6,041,154; 5,706,825;
4,613,205; and 4,011,007), in which several cores share a common
cladding, e.g., for passive image-transfer applications. Although
the present technique provides the flexibility to fabricate similar
structures (and many others), such "multiple fibers" are not the
emphasis of this invention, nor do they have the novel properties
of the fibers discussed below.
SUMMARY OF THE INVENTION
[0014] It is an object of this invention to provide a practical
method for fabricating a glass preform to provide drawn optical
fibers having highly controlled and controllable compositions, both
perpendicular to, and parallel with, the drawn glass fiber axis,
and therefore providing optical fibers having highly controlled and
controllable physical, chemical, and optical properties.
[0015] It is another object of the invention to provide a method
for providing a glass preform for use in fabricating an optical
fiber having a complex cross sectional structure.
[0016] It is yet another object of the invention to provide a
method for providing a glass preform for use in fabricating an
optical fiber incorporating internal structures having physical,
chemical, and optical properties that can be simply and easily
contained within a predefined, fixed location.
[0017] Still another object of this invention is to provide a
method for providing a glass preform for use in fabricating a
single-mode optical fiber having a large mode-field area.
[0018] Yet another object of this invention is to provide a glass
preform for use in fabricating a single-mode or multimode optical
fiber with a core numerical aperture below 0.1.
[0019] Another object of this invention is to provide a glass
preform for use in fabricating a multimode optical fiber with
properties that facilitate suppression of light propagation in the
LP.sub.11 and higher-order modes.
[0020] Another object of this invention is to provide a glass
preform for use in fabricating a multimode optical fiber with
properties that provide preferential gain for light propagating in
the fundamental mode (LP.sub.01).
[0021] A further another object of this invention is to provide a
glass preform for use in fabricating a multimode optical fiber
having a non-uniform dopant distribution within a central core
region.
[0022] Still another object of the invention is to provide a glass
preform for use in fabricating a polarization-maintaining optical
fiber, and for providing such a fiber exhibiting any or all of the
forgoing characteristics
[0023] It is still another object of the invention to provide a
glass preform for use in fabricating a double-clad optical fiber
and such a fiber wherein an
amplified-spontaneous-emission-absorbing dopant is incorporated in
an inner clad region of said optical fiber.
[0024] Yet another object of the invention is to provide a glass
preform for fabricating optical fibers having any combination of
the forgoing properties and characteristics.
[0025] The foregoing objects are meant as illustrative of the
invention only and not as an exhaustive list. These and other
objects will become apparent to those having ordinary skill in
these arts as the invention is described in detail.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 shows a schematic representation of the Modified
Chemical Vapor Deposition (MCVD) technique for providing a sintered
glass preform.
[0027] FIG. 2 shows a variant of the MCVD technique known as
Outside Vapor Deposition (OVD) wherein the sintered glass preform
is formed on the outside of a substrate rod.
[0028] FIG. 3 shows a second variant of the MCVD technique known as
Vapor Axial Deposition (VAD) wherein the sintered glass preform is
formed on an outside end of a substrate rod.
[0029] FIG. 4 illustrates a preform bundle of the present invention
for providing a step-index optical fiber, wherein a cladding
portion comprises a random distribution of glass rods each having
either a higher or lower refractive index than a target refractive
index such that the average cladding index equals the target
index.
[0030] FIG. 5 shows the elemental and refractive-index profiles of
a rare-earth-doped preform fabricated by the MCVD process and
illustrating the effects of "burnout."
[0031] FIGS. 6A-C show the effect of spatial averaging (bundling a
plurality of doped rods or fibers) that mitigates the problem of
"burnout."
[0032] FIG. 7 illustrates a preform bundle of the present invention
for providing a step-index optical fiber, wherein a cladding
portion comprises a random distribution of glass rods each having
either a higher or lower refractive index than a target refractive
index, and additionally including a central core region containing
a confined rare-earth-doped region.
[0033] FIGS. 8A-D shows four designs used in commercially available
polarization-maintaining optical fibers (for passive light
transmission).
[0034] FIG. 9 illustrates a preform bundle of the present invention
for providing a step-index optical fiber similar to that shown in
FIG. 7 and additionally including two sectors confined within
opposing segments of the cladding region which contain rods having
thermal expansion properties designed to impart an internal stress
to a finished optical fiber.
[0035] FIG. 10 illustrates a simple assembly technique for
preparing a rod bundle.
[0036] FIG. 11 illustrates the assembled rod bundle contained in a
silica glass tube suitable for cleaning and drying of the
bundle.
[0037] FIG. 12 shows the finished and sealed glass ampule
containing the rod bundle of the present invention.
[0038] FIG. 13 illustrates a schematic of the furnace apparatus for
processing the sealed preform ampule.
[0039] FIG. 14 illustrates how the reactant gas-delivery system of
a conventional MCVD setup would be modified to provide a method for
preparing the glass soot as a powder.
[0040] FIG. 15 illustrates how the collection vessel used to
collect the glass soot of the MCVD process as a powder is modified
to serve as a crucible for melting the collected powder and then
drawing the glass melt as a rod or fiber.
DETAILED DESCRIPTION OF THE INVENTION
[0041] FIG. 4 is exemplary of the present invention, wherein
preform 40 is fabricated from a large number of glass rods bundled
together and then fused at high temperature. FIG. 4 shows such a
preform, looking down the axis of the preform, prior to fusion. The
preform shown in this example is for a rare-earth-doped fiber with
a stepped refractive index core 42 of uniform dopant density (a
"step-index" profile).
[0042] Core region 42 of the preform contains rods 42a fabricated
from rare-earth-doped glass with a refractive index of n.sub.core.
Cladding region 41 of the preform contains two different types of
glass rods, 41a and 41b, neither of which contains rare-earth
dopants. One type of cladding rod has a refractive index slightly
greater than the desired cladding refractive index, n.sub.clad. The
other type of cladding rod has a refractive index slightly less
than clad. The ratio of low-index/high-index cladding rods is
chosen so that the average index of refraction in the cladding
region is equal to n.sub.clad; in the example shown in FIG. 4, the
placement of low-index/high-index cladding rods is random. This
bundle is then fused to make a solid preform. As with
conventionally fabricated preforms, the relative sizes of the core
and cladding in the preform will be retained in the fiber.
[0043] When the preform is drawn into fiber, the "granularity" of
the refractive-index distribution in the cladding region will be
largely preserved. If this granularity is made fine enough,
however, such a fiber will behave as though the cladding were made
of a single, homogeneous material with index n.sub.clad.
Small-scale variations in the index of refraction are, in effect,
averaged over by the mode field of the light propagating in the
fiber. Similarly, it is clear that the core of the fiber is not
truly circular in shape. The circular region in the center of FIG.
4 is made up of a finite number of pixels (rods). However, the
mode-field distribution for light traveling down the fiber is known
to be insensitive to the fine-scale features of the
refractive-index distribution. If the number of pixels (number of
rods in the bundle) is made large enough, the mode "sees" a
circular core.
[0044] Fortunately, the number of pixels required to obtain the
desired refractive-index-averaging effect is not prohibitively
large. The present invention therefore allows the properties of the
preform to be engineered to almost arbitrary specifications. For
example, to change the numerical aperture NA; where:
NA=(n.sub.core.sup.2-n.sub.clad.sup.2).sup.1/2,
[0045] of the preform shown in FIG. 4, one would simply change the
ratio of low-index to high-index rods in the cladding region to
change n.sub.clad. Such adjustments can be made with excellent
accuracy (i.e., control of the absolute refractive-index value) and
precision (reproducibility). In addition, this technique is very
versatile and practical in its implementation in that a wide
variety of preforms can be fabricated using only two different
types of cladding rods.
[0046] Representative Applications to Fiber Lasers and
Amplifiers
[0047] The utility of the present invention is best illustrated by
example. In the following sections several potential applications
to fiber lasers and amplifiers that are of current interest are
outlined.
[0048] Large-Mode-Area, Single-Mode Fibers
[0049] One area of considerable recent activity is the development
of single-mode fibers with large mode-field areas. Such fibers are
of interest for lasers and amplifiers capable of generating very
high peak power pulses and for narrow-linewidth fiber sources
capable of generating high average powers. One known approach to
increasing the mode-field area while preserving single-mode
operation (required to maintain diffraction-limited beam quality)
is to lower the NA of the fiber. The NA for a typical
telecommunications fiber is in the range of 0.15 to 0.20. For
pulsed fiber amplifiers, NA's of 0.1 and lower are of interest,
with the lower limit ultimately determined by fiber bend-loss
considerations. The fabrication of preforms for ultra-low NA
fibers, however, is neither straightforward nor trivial.
[0050] The refractive-index difference (.DELTA.n) between the fiber
core and cladding regions for conventional telecommunications
fibers is typically 0.01 to 0.02. In comparison, the .DELTA.n value
for a 0.05 NA step-index fiber is smaller by a factor of 10 to 20.
With conventional preform-fabrication techniques it is very
difficult to achieve the level of precision and accuracy in the
refractive-index distribution required for ultra-low-NA fibers.
Furthermore, poor accuracy in the refractive-index distribution
will result in fibers that have too large an NA, or in fibers that
are weakly or altogether non-guiding. Even if the average .DELTA.n
in the preform is very close to the target value, poor precision
results in variations in NA along the length of the fiber that
greatly increase sensitivity to bend loss. These considerations are
of special concern for rare-earth-doped fibers since the
fabrication process typically utilizes a multiplicity of dopant
species, all of which must be carefully and simultaneously
controlled. Typical dopants include one or more rare-earth-ions
taken from the Lanthanide Series of elements, as well as
refractive-index raising/lowering dopants and dopants used to
enhance the solubility of the rare-earth ions (e.g., compounds
containing species taken from elements on the Periodic Table of
Elements designated as new IUPAC Groups 13-17, such as boron,
aluminum, silicon, phosphorous, and germanium, certain members of
the Halide Group, e.g. fluorine, and various members of the
Transition metals listed in new IUPAC Groups 3-12, such as
zirconium, titanium, and zinc).
[0051] This situation is further complicated by the need to
fabricate rare-earth-doped fibers intended for high-peak-power
operation. Fibers of this type require as high a rare-earth-dopant
density as possible. However, this requirement conflicts with the
low-NA requirement because high dopant densities in the core
typically leads to large .DELTA.n since, as discussed earlier, in
the widely practiced MCVD technique, the cladding glass is usually
undoped silica whose index of refraction is substantially less than
that of the rare-earth-doped glass.
[0052] The present invention allows the problems of poor process
control and incompatibility between core composition and .DELTA.n
to be circumvented. Referring back to FIG. 4, the requirement for
high rare-earth concentration in the core can be met by fabricating
core rods with as high a dopant density as possible, without
concern for the refractive index. The cladding is then constructed
using the appropriate mixture of high- and low-index cladding rods
to achieve the desired target .DELTA.n. The problem of
refractive-index incompatibility is eliminated because the
refractive index of the cladding can be tailored to that of the
core. Furthermore, because the cladding refractive index is very
well controlled, the requirement for a small (and uniform)
core/cladding index difference does not present a problem.
[0053] Multimode Fibers
[0054] As noted earlier, there is a limit to how low the NA can be
made in a practical fiber amplifier. Further increases in
mode-field area can be realized by using a multimode gain fiber
that is constrained to operate on only the lowest-order transverse
mode (LP.sub.01). One way to obtain such single-mode operation in a
multimode amplifier is to carefully control the launch conditions
of the signal being amplified; the signal injected into the
multimode amplifier should ideally excite only the LP.sub.01 mode.
Another technique that can be used to obtain preferential
amplification of signals in the LP.sub.01 mode is to use bend loss
to discriminate against higher-order modes. In both approaches, the
second lowest order mode (LP.sub.11) is the most difficult to
suppress.
[0055] Conventional preform fabrication techniques (with the
exception of VAD) entail a final step in which the cladding tube,
with an inner coating of material formed during the
vapor-deposition process, is collapsed to form a solid rod (the
preform). The highest temperatures are reached during this step in
the fabrication process, and it is at these elevated temperatures
that a phenomenon known as "burnout" occurs, wherein some of the
co-dopant species, most notably Ge and P compounds, undergo thermal
decomposition. Thermal decomposition occurs preferentially at the
inner surface of the preform (which will become the central region
of the core following preform collapse), where gas-phase products
are able to escape as they are evolved. The effects of burnout in
the finished preform are shown graphically in FIG. 5 where the
concentration of dopant species and the refractive index of the
affected preform is measured across a diameter of the preform. As
seen in FIG. 5, in the central or core region of the preform, the
refractive index is reduced and the rare-earth-dopant concentration
is substantially depleted, resulting in donut-shaped
refractive-index and dopant distributions. This characteristic is
preserved when the preform is drawn into a fiber and is ubiquitous
in preforms fabricated by either the MCVD or the OVD process.
[0056] Because the LP.sub.11 mode also has a donut-shaped intensity
distribution, it is heavily favored over the LP.sub.01 mode in a
multimode fiber that has sustained the effects of burnout because:
i) the donut-shaped refractive-index profile makes it difficult to
propagate light in the LP.sub.01 mode since light injected into the
central portion of the core is instead guided into the
higher-refractive-index annular region at the perimeter; and ii)
the small-signal gain depends exponentially on the overlap integral
of the dopant and mode-field distributions. The intensity maximum
of the LP.sub.01 mode coincides with the "hole" in the dopant
distribution. Conversely, the LP.sub.11 mode and the donut-shaped
dopant distribution are well matched to each other. For these
reasons, burnout results in refractive-index and dopant profiles
that are exactly the opposite of what is required for operation on
the lowest-order mode.
[0057] In the present invention, the problems associated with
preform burnout can be eliminated, for the following reasons:
[0058] 1) Because adjustments to the core and cladding indices are
decoupled, the core glass can be fabricated using co-dopants that
are not subject to thermal decomposition at high temperature
without any constraints related to fiber NA.
[0059] 2) To the extent that thermal decomposition does occur
during high-temperature processing, the shape of the
refractive-index and dopant profiles are not altered in a manner
that disfavors LP.sub.01, as shown in FIG. 6.
[0060] 3) Finally, as will be described later, the forces on the
preform bundle during the final fusion step are considerably
greater than the aforementioned surface tension relied upon in the
MCVD process and as such the temperature required for preform
bundle fusion can be made low enough to prevent thermal
decomposition from occurring.
[0061] The goal, therefore, of achieving a true step-index profile
and a similar dopant profile, or a variety of other profiles
described below, can be realized.
[0062] Fibers with Non-Uniform Dopant Distributions
[0063] The above discussion of burnout suggests how the design of a
multimode fiber laser/amplifier might be further improved to favor
amplification of the lowest-order mode. Because the present
invention allows direct control over the refractive-index and
dopant distributions, more complicated preform designs intended to
optimize discrimination between the LP.sub.01 and LP.sub.11 modes
are feasible. The simplest form of such optimization would be to
restrict the rare-earth dopant to the central portion of the core
since in this embodiment amplification coincides with the intensity
maximum of the LP.sub.01 mode in the central region of the core and
with the intensity minimum of the LP.sub.11 mode. With the present
invention, the design and fabrication of such customized preforms
becomes realistic. FIG. 7 shows a representative preform for a
step-index fiber with a cladding-to-core diameter ratio of about
10:3 and wherein the rare-earth dopant is confined to a central
region of the core having a diameter about one-half (1/2) that of
the core region. (Typical, representative dimensions of these
regions would be a 200 .mu.m .O slashed. cladding and a 60 .mu.m .O
slashed. core region, comprising a 15 .mu.m thick annular ring
surrounding a 30 .mu.m .O slashed. central, rare-earth-doped core
zone. Each of these dimension may be varied, however, to suit the
requirements of the application.)
[0064] In the annular, undoped region of the core (the "core
annulus"), of FIG. 7, the ratio of high/low refractive index rods
is adjusted to match the refractive index of the rare-earth-doped
rods. As shown in FIG. 7, placement of the high/low refractive
index rods is random, or nearly so.
[0065] It is likely that even better suppression of the LP.sub.11
mode could be obtained with more complicated dopant and/or
refractive-index distributions (e.g., radially graded profiles,
with the rare-earth-dopant concentration and/or the refractive
index decreasing monotonically with distance from the center of the
core). The present invention makes such preform designs
straightforward to implement in a systematic and controlled
manner.
[0066] Polarization-Maintaining Fiber
[0067] In many applications, the output polarization state of a
fiber laser/amplifier is important. Because of fiber birefringence,
the output polarization of conventional rare-earth-doped fiber
amplifiers is in general elliptical and time-varying. The best
solution to the problem of fiber birefringence is the use of
Polarization Maintaining (PM) fiber. In a PM fiber, the propagation
constants (indices of refraction) are made sufficiently different
for two orthogonal axes (e.g., horizontal and vertical) that light
polarized along one axis is not strongly coupled to the other axis.
Linearly polarized light launched along one of the polarization
axes of a PM fiber therefore remains linearly polarized, with
negligible power transferred to the other polarization state. One
way to make the indices of refraction different for the two
orthogonal linear polarization states is to place the fiber in a
stress field that is cylindrically asymmetric. The most common
approach to generating the required stress field is the
incorporation of stress members into the cladding of the preform.
The stress members are made from a glass whose coefficient of
thermal expansion is substantially different (usually larger) than
that of the cladding glass, resulting in a stress field that is
permanently frozen into the fiber once fabricated.
[0068] FIG. 8 shows the various designs for stress elements used in
commercially available PM fibers. Note that none of the PM fibers
shown in FIG. 8 contain any rare-earth dopant (i.e., they are used
for passive transmission of polarized light, but not for
amplification). There is currently only one rare-earth-doped PM
fiber commercially available, a single-clad Er-doped fiber
manufactured by FiberCore in the UK. Double-clad, rare-earth-doped,
PM fibers have recently been reported, but they are not widely
available. (Note: in a single-clad, rare-earth-doped fiber, the
pump and signal beams are confined to the core of the fiber. In a
double-clad fiber, the cladding region is converted into a high-NA
multimode waveguide, referred to as the "inner cladding," by adding
a low-index polymer coating to the outside of the fiber. The
advantage of a double-clad fiber is that much larger pump powers
can be coupled into the fiber using multimode pump sources by
launching the pump light into the inner cladding rather than into
the core. The pump light is still absorbed in the core, and the
signal light still propagates in the core.)
[0069] The fabrication of a cylindrically asymmetric structure is
difficult using traditional methods for preform manufacture. In
contrast, in the present invention, the incorporation of stress
rods is straightforward. FIG. 9 depicts a PM version of the preform
shown in FIG. 7. As in a conventional (passive) PM fiber, the
material used for the stress rods (e.g., borosilicate) would have a
coefficient of thermal expansion substantially different from that
of the cladding glass.
[0070] By fabricating the stress rods from a glass whose index of
refraction is less than that of the cladding glass, the problem of
helical rays (rays that are confined to the inner cladding but do
not intersect the core of the fiber) can be eliminated. In a
conventional double-clad fiber, the trajectories of helical rays
are scrambled by making the cross-section of the inner cladding
non-circular (e.g., a rectangle or hexagon). Alternatively, the
problem of helical rays can be circumvented by off-setting the core
from the center of the inner cladding. In both cases, the preform
must be carefully ground and polished, and possibly re-sleeved, to
achieve the desired shape before drawing. With the present
invention, it is straightforward to construct a preform in which
the stress rods provide the required mode scrambling effect. This
approach makes it possible to use a preform of circular
cross-section; in addition to simplifying the preform fabrication
process, a double-clad fiber of circular cross-section is
advantageous from the standpoint of fiber cleaving and fusion
splicing. Furthermore, for applications in which an off-set core or
a non-circular inner cladding is desirable, the present invention
allows fabrication of the required preform without machining or
re-sleeving (see below).
[0071] Double-Clad Fibers with Very High-NA Inner Cladding
[0072] As mentioned above, the advantage of a double-clad
(cladding-pumped) fiber is that much more pump light can be
launched into the fiber (at much lower cost) than with a
single-clad (core-pumped) fiber. This advantage results from two
effects: 1) the cross-sectional area of the cladding is much larger
than that of the core, and 2) the input acceptance angle is much
greater for the high-NA inner cladding than for the lower-NA core.
The NA of the inner cladding is determined by the difference in
refractive index between the low-index polymer coating and the
silica cladding glass. An NA of 0.35 is obtained with a silicone
coating, and NA's as high as 0.47 can be achieved with more
recently developed fluoropolymers. As described earlier, the
present invention makes it possible to use cladding materials other
than pure silica. As a result, the NA of the inner cladding can be
increased significantly by increasing n.sub.clad. For example, one
material that is promising for the construction of rare-earth-doped
fibers is a mixed alkali-zinc-silicate glass manufactured by Schott
Glass Technologies Inc., and identified as IOG-10. The index of
refraction of IOG-10 is 1.530, allowing the NA of
fluoropolymer-clad fibers to be increased from 0.47 to 0.66. This
NA corresponds to greater than a two-fold increase in the amount of
pump light that can be coupled into the double-clad fiber, for a
pump source of a given brightness.
[0073] Double-clad fibers with ASE-absorbing dopants in the inner
cladding In any fiber amplifier, an upper limit to the population
inversion (i.e., to the stored energy and the gain) is determined
by a process known as "Amplified Spontaneous Emission" (ASE).
Although most ASE propagates in the core, in a double-clad fiber, a
significant amount of power can be lost to ASE propagating in the
high-NA inner cladding. In addition to reducing the population
inversion, cladding ASE can degrade the output beam quality and can
cause parasitic "lasing" when the gain is not lowered by another
process (e.g., by seeding the amplifier with sufficient power).
Approaches to reducing cladding ASE including angle-polishing the
fiber (although very large angles are required to suppress lasing
in the high-NA inner cladding) and mode-stripping the ends of the
fiber (if the fiber is end-pumped, only one end can be
mode-stripped). Both these techniques can only suppress cladding
ASE at the ends of the fiber: they allow.
[0074] ASE to propagate in the inner cladding, but they prevent it
from emerging from the amplifier or from being recirculated by
back-reflections from the fiber ends.
[0075] A superior approach for suppressing cladding ASE would be to
dope the inner cladding with a material that absorbs ASE but does
not absorb pump light (which is to be absorbed only in the core).
For example, where erbium has been used as a core dopant, the
rare-earth metal terbium could serve as the ASE-absorbing species.
This approach would have the advantage of providing distributed
suppression of cladding ASE, i.e., it would prevent ASE from
experiencing gain along the entire fiber. This approach has not
been employed in double-clad fibers fabricated by conventional
techniques, perhaps because of the danger of introducing a
contaminant into the MCVD apparatus that, if present in the fiber
core, would cause unacceptably high losses for the signal beam.
This risk is eliminated by the present invention, in which the
cladding rods can be fabricated in a different apparatus than are
the core rods, ensuring that contamination of the core will not
occur.
[0076] The ASE-absorbing dopant would likely be contained in only
part of the inner cladding (e.g., in a ring well outside the core).
In a single-mode fiber, the electric field of the light propagating
in the core has significant amplitude in the cladding; should this
field interact with the ASE-absorbing dopant, the fiber would
experience excessive signal loss. Restricting the ASE-absorbing
dopant to the outer portion of the inner cladding would minimize or
eliminate this loss. (In the multimode fibers discussed above, less
of the core light propagates in the cladding, reducing the
importance of this consideration.) Another advantage of placing the
ASE-absorbing dopant in the outer portion of the inner cladding is
10 that the refractive index of these rods would not have to be
well-matched to that of the cladding; the core NA will depend only
on the refractive indices of the core rods and the regular cladding
rods adjacent to the core (i.e., those not doped to absorb ASE).
Moreover, because the core light does not interact strongly with
the ASE-absorbing rods, they can be relatively lossy and can be
fabricated using standard, bulk-glass techniques (ultra-high purity
is not required).
[0077] Double-Clad Fiber with an Off-Set Core or a Non-Circular
Inner Cladding
[0078] As mentioned above in the context of PM fibers, two
approaches to circumventing the problem of helical rays in
double-clad fibers (i.e., rays propagating in the inner cladding
that do not intersect the fiber core) are: (1) to off-set the core
from the center of the fiber, typically by grinding and possibly
re-sleeving of the preform; and (2) to make the inner cladding
non-circular. The present invention allows preforms with either or
both of these features to be fabricated directly. For achieving an
off-set core, the core rods would be located non-centrally in the
bundle. For obtaining a non-circular inner cladding, the outer tube
used during the construction of the bundle would have the desired
non-circular shape; alternatively, the bundle would be cylindrical,
but it would include etchable glass rods that would provide the
desired non-circular shape after etching. Of course, these features
can be combined with any of the other features discussed above
(i.e., non-uniform dopant distribution, low NA, etc.).
BEST MODE FOR IMPLEMENTING THE INVENTION
[0079] In the following sections methods are described that may be
used to implement the present invention for the fabrication of
fiber preforms. This description is not meant to be exhaustive;
rather, it outlines some of the considerations involved in reducing
the invention to practice, and it demonstrates that the invention
is practical for the fabrication of useful preforms with unique and
hitherto unattainable characteristics.
[0080] The discussion will focus on silica-based fibers, which are
by far the most common. This emphasis, however, does not and
therefore should not be interpreted to imply that the invention is
applicable only to silica-based fibers. It is, in fact, applicable
to a wide variety of glass compositions, including halide-based
glasses (e.g., fluoride or "ZBLAN" glass), chalcagonide glasses
(e.g. sulfide, selenide, and telluride glasses), and various
multi-component glasses (e.g.,
SiO.sub.2--Al.sub.2O.sub.3--NaO.sub.2--CaO) comprising compounds of
boron, silicon, aluminum, phosphorous, germanium, zinc, titanium,
zirconium, any of the alkali and alkaline-earth elements and/or any
of the various alloys thereof. The term "glass," therefore, is
intended by the Applicants to be interpreted broadly to mean any
material that is or has been found to have utility as an optical
fiber that is comprised of and prepared from the above list of
materials.
[0081] Similarly, the discussion will focus on fibers with a
circular cladding and with a circular core located in the center of
the cladding. Preform bundles of the present invention, as seen in
FIGS. 4 and 7-9, take on a generally cylindrical shape when
assembled, especially as the size of the underlying rods decreases
and their numbers greatly increase. This emphasis, however, does
not and therefore should not be interpreted to imply that the
invention is applicable only to circular fibers with circular,
centrally located cores. It is, in fact, applicable to any cladding
with a closed cross-sectional shape (including elliptical, square,
rectangular, hexagonal, octagonal, and rhombic), to core bundles
with a comparable variety of shapes, and to any core position. All
that is required is that the preform bundle be contiguous and
constrained.
[0082] Assembly of Preform Bundles
[0083] In each of the preform bundles described thus far, random
placement of the high/low-index rods in the cladding and
core-annulus regions has been assumed. In such a scheme, the
low/high index rods could be counted (individually or by weight)
and mixed together thoroughly before being incorporated into the
preform bundle. If the number of rods in the preform bundle is
large, the possibility of obtaining an "uneven" refractive-index
distribution that exerts any significant effect on the mode-field
distribution is remote. For bundles consisting of a smaller number
of rods, semi-random or non-random placement of rods are both
options. Semi-random placement largely preserves the main advantage
of random placement, i.e., there is no need to place each rod
individually. In this approach, the bundle is constructed using
random placement, but the low/high-index rods are color coded or
otherwise marked so that they can be identified when viewed end-on;
any "clumps" of high/low-index rods that result from poor mixing or
statistical variation can then be visually identified and
redistributed if necessary. Non-random placement entails the
distribution of rods in a predetermined and regular pattern, most
likely by an automated device.
[0084] A different approach is to fabricate the cladding from a
collection of identical rods: rods composed of a composite material
whose average refractive index is equal to n.sub.clad. These
composite cladding rods would themselves be fabricated from a
preform bundle containing a mixture of high/low-index rods in the
appropriate ratio. In this two-step process, the effective pixel
density in the cladding of the finished preform would be equal to
the product of the pixel densities for each step. The length scale
for random variations in refractive index would therefore be
constrained to be less than or equal to the diameter of the
composite cladding rods.
[0085] FIG. 10 shows how the assembly process might be accomplished
for preparing a preform bundle 1000. The preform bundle shown in
this example is for a rare-earth-doped fiber with a step-index core
of uniform dopant density, similar to that of FIG. 4. In a first
step, the entire cladding tube 1001 (a thin-walled tube whose
purpose is to contain the bundle) is packed with a mixture of
low-refractive-index rods 1002a and high-refractive-index rods
1002b in the appropriate ratio. In a next step, cladding rods from
the middle portion 1003 of the bundle are removed and replaced with
a corresponding volume of core rods (not shown).
[0086] As shown in FIG. 10, a preform template 1004 delineates the
core/cladding boundary, showing directly which cladding rods should
be removed. Perform template 1004 can, of course, be modified to
improve the ease with which the transfer of rods is accomplished.
In particular, the "stepped" central portion of the template can be
replaced with a removable plug 1005 that allows the user to
partially displace the desired rods, as shown. Plug 1005 then would
be removed, and the displaced volume in the perform bundle would be
"back-filled" through the hole left behind by the plug with new
glass rods having the desired property (e.g., core rods). This
procedure, therefore, prevents the cladding rods from inadvertently
moving during the replacement process because the bare region of
the preform always contain rods contained within tube 1001 would be
placed on its side before
[0087] Finally, those skilled in the art will appreciate that
preform template 1004 can comprise any number of distinct regions,
or plugs, having a variety of shapes, sizes, and locations (e.g.,
for the stress elements described in the context of PM fibers).
This approach thus provides a simple method for assembling a
preform bundle, with wide flexibility in the range and complexity
of physical structures and chemical properties imparted to the
finished perform.
[0088] Consolidation of Preform Bundles
[0089] FIG. 11 shows the next stage of processing. Bundle 1000 is
transferred into a second cladding tube 1100 in which it is
suspended and immobilized between two plugs, e.g., of fiberglass
wool 1105 (ultra-high purity silica, available commercially).
Fiberglass packing 1105 prevents the bundle from sliding in
cladding tube 1100 and ensures that there is no relative movement
of rods within bundle 1000. This second cladding tube 1100 is
fabricated with an inner lip or waist 1101 (formed by partial
collapse of the cladding tube under vacuum) to provide mechanical
support of the above assembly. Because the fiberglass plug is
porous, the entire assembly can be cleaned and dried in place,
without any need to handle the bundle directly, thereby preventing
contamination. The cleaning and drying steps would likely involve
both liquid-phase and gas-phase processes, similar to those used
with the MCVD method. The cleaned and dried assembly is then
evacuated and the cladding tube sealed off at both ends to form an
ampule 1200 as shown in FIG. 12.
[0090] The cladding tube can be fabricated from either of the
materials used for the cladding rods. Alternatively, if
hydrofluoric acid is used to remove the cladding tube from the
finished preform, any glass with similar thermal properties can be
used.
[0091] FIG. 13 shows how the evacuated ampule is processed at high
temperature (typically <2000.degree. C., with the exact value
depending on the glass composition) to yield the finished preform.
The apparatus shown in FIG. 13 is designed to fuse the bundle while
preventing the formation of trapped bubbles. The rod bundle does
not "melt" in the usual sense but instead softens substantially,
enough so as to fuse the bundle into a monolithic preform. By
gradually inserting the ampule into the heated zone of a tube
furnace 1301, fusion begins at one end of the bundle and progresses
slowly, "zipping up" the bundle so that gas bubbles are excluded.
Alternatively, a "ring-burner" system (not shown) could be
substituted as a means for fusing the ampule to form the preform.
The ampule is processed inside silica tube 1302 that is carefully
cleaned before insertion of the ampule. The silica tube 1302 (which
does not substantially soften when inserted into the
furnace/ring-burner) is mounted on a stepper-motor-driven
translation stage that controls the rate at which the ampule is fed
into the furnace/ring-burner. In addition, silica tube 1302 is
continuously rotated, which prevents slumping of the softened glass
and adhesion of the ampule to the inner surface of tube 1302.
Order-of-magnitude estimates for the translation rate 1303 and
rotation rate 1304 are 1 inch per hour and 20 revolutions per
minute, respectively. As shown in FIG. 13, the entire assembly is
mounted at a slight angle 1305 to ensure that the rolling ampule
does not wander along the axis 1306 of silica tube 1302. Annealing
of the preform occurs as it gradually exits the heated zone. If
necessary, the finished preform can then be treated with
hydrofluoric acid to remove any surface contamination from the
inner surface of silica tube 1302.
[0092] In addition to maintaining a controlled environment in which
contamination of the bundle (and the inside of the cladding tube)
is substantially eliminated, the evacuated ampule serves another
important function. The one-atmosphere pressure differential
between the inside and outside of the ampule greatly accelerates
the collapse/fusion process when the ampule is softened at high
temperature. In the MCVD and OVD processes, the force responsible
for collapse of the cladding tube is surface tension. The
collapsing force exerted on an evacuated tube is several hundred
times larger than the force generated by surface tension. For this
reason, the temperature required for the preform collapse step can
be lowered by about 500.degree. C. This large reduction in
temperature makes processing of the preform more straightforward
and substantially reduces or eliminates the problem of dopant
burnout. In addition, the furnace or ring-burner could be placed in
a chamber that is pressurized to more than one atmosphere, which
would provide an even greater collapsing force on the ampule.
[0093] Fabrication of Core and Cladding Rods
[0094] The materials required for the core and cladding rods can be
synthesized in powder form using a conventional MCVD setup. In this
approach, the sintering process is omitted and as much of the soot
as possible is collected. During a typical MCVD fabrication run,
only a fraction of the soot that is generated in the reaction zone
is deposited on the inner wall of the tubing. Most of the soot
remains suspended in the exhaust gas and is typically discarded.
The transport process that governs the deposition efficiency is
thermophoresis. In thermophoresis, suspended particles are
transported down a temperature gradient because momentum transfer
from colliding gas molecules is unequal on the "hot" and "cold"
sides of the particle. In the reaction zone, the radial temperature
gradient is such that particles generated are transported away from
the walls of the tube into the center of the flow. Further down the
tube, the direction of this gradient reverses as a result of
cooling of the tube by ambient air. Under these conditions,
thermophoresis causes particles to migrate towards the wall of the
tube, where deposition occurs. A number of techniques have been
suggested for improving the deposition efficiency.
[0095] The apparatus 1400, shown in FIG. 14, was designed with
these considerations in mind. Reactant vapor stream 1401 is
generated by a conventional MCVD gas-delivery system. A small-tube
furnace 1403 is used to heat a reaction zone 1404. Again, a
"ring-burner" could be substituted as a means for heating the
reaction zone. The flow path and temperature distribution are such
that the efficiency of the deposition into collection tube 1402 is
maximized. Collection tube 1402 is fabricated from high-purity
fused-silica (such glassware is commercially available and is used
routinely in the semiconductor industry for synthesis of ultra-pure
starting materials). Because final product 1405 is stored in the
collection tube, there is no need for any further handling. In this
way, the complexity of a full-blown MCVD setup (glass lathe, rotary
seals, H.sub.2/O.sub.2 torch, motorized translation stage, etc.) is
avoided.
[0096] Such a process could greatly ease the control requirements
necessary to assure that proper.compositional ranges are maintained
during conventional MCVD fabrication. As noted earlier,
vapor-deposition techniques are difficult to control. Many require
delivery of multiple species by vapor transpiration techniques: the
rare-earth chlorides, for example, must be delivered through heated
delivery lines to avoid recondensation. Furthermore, these species
tend to be chemically aggressive and use of flow regulating devices
to control rates of species addition to the reaction zone is
problematic due to the potential for equipment failure. Finally,
temperature fluctuations in the reaction zone effect the
composition of the final product by changing the relative rates of
the various oxidation reactions and by changing the soot deposition
efficiency.
[0097] However, by simply collecting the oxide soots of individual
reactant species generated in separate reaction processes in the
glass ampule by weight it is far more likely that a final target
glass composition can be achieved accurately and reproducibly. This
result would be achieved by combusting a single reactant gas stream
and determining the incremental weight gain of the ampule as the
oxide soot collects on its interior walls until a target weight is
achieved. The process would be repeated with each subsequent
reactant specie until each had been combusted and the desired
quantity of its oxide collected. The collected powders would be
mixed (e.g. by tumbling them within the ampule), and the ampule
would be sealed.
[0098] The glass in powdered form is then zone sintered (similar to
the procedure used with OVD and VAD soot preforms) and drawn into
rod or fiber using a single-crucible method. The need for a
separate crucible can be eliminated by incorporating a "break-off"
fixture 1501 at the base of the collection tube, similar: to a
conventional glass ampule (see FIG. 15), which provides a hole
through which the rods can be drawn. Furthermore, rather that
actually breaking off this "tail", the extension may be used as a
self-contained appendage which would be grasped by the fiber
drawing mechanism in order to initiate the drawing process.
[0099] This approach greatly simplifies the fabrication of high
purity rods and further reduces the possibility of contamination.
Zone sintering is carried out at the beginning of the fiber
draw.
[0100] Measurement of the Refractive Index
[0101] Once the core and cladding rods have been fabricated, a
precise measurement of the refractive index must be performed. As
discussed earlier, the difference in refractive index between the
core and cladding rods can be extremely small, and a precise
measurement of these differences is required (although an accurate,
absolute refractive-index measurement is not necessary). The
following procedure provides the requisite precision. A
representative rod from each group (e.g., a core rod, a high-index
cladding rod, and a low-index cladding rod) is bent into the shape
of a "U", and the bottom of each "U" is immersed in a
temperature-controlled bath of refractive-index-matching fluid.
Each rod is placed between a light source and a detector (i.e.,
light is launched into each rod at one end and is detected at the
other end). The refractive index of the fluid can be precisely and
reproducibly adjusted by changing the temperature of the bath. As
the temperature is increased, the refractive index of the fluid
decreases. As the refractive index of the fluid approaches that of
a given rod, the transmitted power drops abruptly. In a plot of
transmitted power vs. temperature, a v-shaped notch is observed,
with a minimum at the temperature corresponding to a perfect
refractive-index match. By recording the refractive-index-match
temperature for each rod, the refractive-index difference between
the various rods can be calculated, provided the temperature
coefficient (dn/dT) for the refractive-index-matching fluid is
known. For most refractive-index-matching fluids, dn/dT is
approximately 450 ppm/.degree. C. (where ppm denotes "parts per
million"), and the precise value of dn/dT can be measured with a
standard refractometer. The temperature coefficient for the
refractive index of silica is 18 ppm/.degree. C. and can thus be
ignored. For an ultra-low NA fiber (NA=0.05), the refractive-index
difference between the core and cladding is .about.600 ppm. The
temperature of the bath can easily be measured to within
.+-.0.1.degree. C., which corresponds to an refractive-index
uncertainty of .+-.45 ppm. One may conclude, therefore, that the
proposed refractive-index measurement will have the required high
degree of precision necessary to determine the differences among
the various rods and thus between the core and clad regions of the
fiber.
* * * * *