U.S. patent application number 11/444754 was filed with the patent office on 2006-09-28 for optical fiber manufacture.
Invention is credited to Robert M. Atkins, James W. Fleming, Paul F. Glodis, Man F. Yan.
Application Number | 20060213231 11/444754 |
Document ID | / |
Family ID | 32849838 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060213231 |
Kind Code |
A1 |
Atkins; Robert M. ; et
al. |
September 28, 2006 |
Optical fiber manufacture
Abstract
The specification describes methods for the manufacture of very
large optical fiber preforms wherein the core material is produced
by MCVD. Previous limitations on preform size inherent in having
the MCVD starting tube as part of the preform process are
eliminated by removing the MCVD starting tube material from the
collapsed MCVD rod by etching or mechanical grinding. Doped
overcladding tubes are used to provide the outer segments of the
refractive index profile thus making most effective use of the MCVD
produced glass and allowing the production of significantly larger
MCVD preforms than previously possible.
Inventors: |
Atkins; Robert M.;
(Millington, NJ) ; Fleming; James W.; (Westfield,
NJ) ; Glodis; Paul F.; (Atlanta, GA) ; Yan;
Man F.; (Berkeley Heights, NJ) |
Correspondence
Address: |
Law Firm of Peter V.D. Wilde
301 East Landing
Williamsburg
VA
23185
US
|
Family ID: |
32849838 |
Appl. No.: |
11/444754 |
Filed: |
May 31, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10366888 |
Feb 14, 2003 |
|
|
|
11444754 |
May 31, 2006 |
|
|
|
Current U.S.
Class: |
65/412 ; 65/417;
65/419; 65/429 |
Current CPC
Class: |
C03B 37/01248 20130101;
C03B 2201/04 20130101; C03B 37/01211 20130101; C03B 2203/22
20130101; C03B 37/01869 20130101; C03B 37/01861 20130101 |
Class at
Publication: |
065/412 ;
065/417; 065/419; 065/429 |
International
Class: |
C03B 37/028 20060101
C03B037/028; C03B 37/018 20060101 C03B037/018 |
Claims
1. Process for the manufacture of optical fiber comprising: (a)
preparing an optical fiber preform, (b) heating the preform to the
softening temperature, and (c) drawing an optical fiber from the
preform, the invention characterized in that the optical fiber
preform is prepared by steps comprising: (i) forming by MCVD a
first glass layer on the inside of a MCVD starting tube and a
second glass layer on the first glass layer, the MCVD starting tube
comprising a first glass material, and where the first glass layer
is a core layer having a first refractive index and the second
glass layer is a first cladding layer having a second refractive
index lower than the first refractive index, (ii) collapsing the
MCVD tube to produce a first solid glass cylindrical body, (iii)
removing at least a portion of the first glass material leaving a
second solid glass cylindrical body of the MCVD glass material, and
(iv) applying a cladding to the second solid glass cylindrical body
of MCVD glass material by inserting the second solid glass
cylindrical body of MCVD glass into a cladding tube and collapsing
the cladding tube around the second solid glass cylindrical body of
MCVD glass material.
2. The process of claim 1 wherein the MCVD starting tube has an
outside diameter OD, and an inside diameter ID and the portion of
the first glass material of the MCVD starting tube cross sectional
area that is removed is:
((OD.sub.2).sup.2-ID.sub.2)/((OD.sub.1).sup.2-ID.sup.2)<0-0.25
where OD.sub.2 is the outside diameter of the second solid glass
cylindrical body.
3. The process of claim 2 wherein all of the first glass material
is removed.
4. The process of claim 1 wherein the step of applying a cladding
involves mounting the solid glass cylindrical body within the
overclad tube leaving an ambient space between the solid glass
cylindrical body and the overlad tube, and controlling the ambient
space by flowing gas through the ambient space.
5. The process of claim 1 wherein the overclad tube comprises glass
with a hydroxyl ion content less than 50 ppB by weight.
6. The process of claim 1 wherein said overclad tube is up-doped
with germanium.
7. The process of claim 1 wherein said overclad tube is downdoped
with fluorine.
8. The process of claim 1 where the solid glass cylindrical body
has a diameter of at least 12 mm.
9. The process of claim 1 wherein the first glass material is
removed by mechanical grinding.
10. The process of claim 1 wherein the first glass material is
removed by plasma etching.
11. The process of claim 1 wherein the first glass material is
removed by chemical etching.
12. The process of claim 1 wherein the first glass material is
removed by a combination of methods including mechanical grinding,
plasma etching and chemical etching.
13. Process for the manufacture of an optical fiber preform
comprising: (a) forming by MCVD a first glass layer on the inside
of a MCVD starting tube and a second glass layer on the first glass
layer, the MCVD starting tube comprising a first glass material,
and where the first glass layer is a core layer having a first
refractive index and the second glass layer is a first cladding
layer having a second refractive index lower than the first
refractive index, (b) collapsing the MCVD tube to produce a first
solid glass cylindrical body, (c) removing at least a portion of
the first glass material leaving a second solid glass cylindrical
body of the MCVD glass material, and (d) applying a cladding to the
second solid glass cylindrical body of MCVD glass material by
inserting the second solid glass cylindrical body of MCVD glass
into a cladding tube and collapsing the cladding tube around the
second solid glass cylindrical body.
14. The process of claim 13 wherein the MCVD starting tube has an
outside diameter OD, and an inside diameter ID and the portion of
the first glass material of the MCVD starting tube cross sectional
area that is removed is:
((OD.sub.2).sup.2-ID.sub.2)/((OD.sub.1).sup.2-ID.sup.2)<0-0.25
where OD.sub.2 is the outside diameter of the second solid glass
cylindrical body.
15. The process of claim 13 wherein all of the first glass material
is removed.
16. The process of claim 13 wherein the step of applying a cladding
involves mounting the second solid glass cylindrical body within
the overclad tube leaving an ambient space between the solid glass
cylindrical body and the overclad tube, and controlling the ambient
space by flowing gas through the ambient space.
17. The process of claim 16 wherein the overclad tube comprises
glass with a hydroxyl ion content less than 50 ppB by weight.
18. The process of claim 13 wherein the first glass material is
removed by a combination of methods including mechanical grinding,
plasma etching and chemical etching.
Description
RELATED APPLICATION
[0001] This application is a continuation of application Ser. No.
10/366,888, filed Feb. 14, 2003.
FIELD OF THE INVENTION
[0002] This invention relates to optical fiber manufacture, and
more specifically to improved optical fiber preform fabrication
techniques.
BACKGROUND OF THE INVENTION
[0003] Manufacture of optical fiber performs, the glass blanks from
which optical fibers are drawn, typically involves a rotating
lathe, where pure glass or glass soot is formed on a rotating
member by chemical vapor deposition or a modification thereof. All
successful methods of fiber manufacture should assure that the
optical quality and purity of the preform glass is high. In
particular, the glass making up the central portion or core of the
preform should be of the highest purity since most of the optical
power in the fiber will be carried within this region. A
significant advance in this direction occurred with the
introduction of the so-called Modified Chemical Vapor Deposition
(MCVD) process in which the glass-forming precursors are introduced
into a rotating hollow starting tube, and glass material is
deposited on the inside wall of the hollow tube. The better control
over the reaction environment provided by this inside deposition
process, allowed exceptionally pure glass to be produced in the
critical core region.
[0004] The MCVD technique has evolved to a highly sophisticated
manufacturing technique, and is widely used in commercial practice
today. However, limiting aspects in MCVD and similar inside
deposition processes are the size and quality of the starting tube
and the total amount of glass that can be deposited inside a
starting tube. The limitation on the total amount of deposited
glass necessarily limits the number of distinct doped regions or
segments of a given size that can be accommodated in a preform of
this type.
[0005] Another preform fabrication technique, Vapor Axial
Deposition (VAD), was developed in which the CVD-formed silica soot
deposits and grows axially from a starting mandrel. In a subsequent
manufacturing stage or stages, the soot body is purified, dried and
sintered into pure glass. At some point, the mandrel is separated
from the deposited body and the entire preform, unlike MCVD, may
thus be made of CVD-deposited material. As a general proposition,
VAD methods are effective and widely practiced, but they still do
not match the ability of MCVD to control precisely the radial
deposition profile of index changing dopants such as germanium and
fluorine. Because of this, VAD methods and other soot
deposition/subsequent sintering methods such as Outside Vapor
Deposition (OVD) are limited in the complexity of the fiber designs
that can be efficiently produced.
[0006] Considering that in a single mode optical fiber the core and
inner cladding together carry greater than 95% of the optical power
but typically comprise less than 5% of the fiber mass, all
manufacturing processes focus special attention on the fabrication
of this region. This has resulted in approaches to preform
manufacture, where the core and inner cladding regions of the
preform are produced by a relatively advanced and expensive method,
while the outer cladding, the bulk of the preform, may be produced
by a less demanding, and less expensive process. The integration of
the core rod and the cladding is carried out in an overcladding
process. The overcladding process in general is described for
example in U.S. Pat. No. 6,105,396 (Glodis et al), and
PCT/EPT00/02651 (25 Mar. 2000), which are incorporated herein by
reference for details of the general techniques. The overcladding
process may consist of multiple steps, each adding a distinct
cladding region, if this is required by the complexity of the
desired fiber refractive index profile. The most prevalent process
of this type is the so-called rod-in-tube method, where the core
rod is made by a very high quality dopant-versatile process, and
the cladding tube is often made of less expensive, lower purity or
single composition glass. In the rod-in-tube overcladding process,
the core rod is inserted into the cladding tube, and the tube
collapsed around the rod to form a unitary body. Again, multiple
overcladding steps may be used and in some cases one or more of the
final overcladding processes may be combined with the fiber drawing
operation.
[0007] State of the art manufacture for very large preforms now
makes use of core rods produced by Outside Vapor Deposition or
Vapor Axial Deposition. If a tube overcladding process is used,
suitable cladding tubes may be produced by soot deposition or
extrusion of fused quartz. Making these very large cladding bodies
with a soot based synthetic glass process leads to high quality
glass but requires extensive processing and is relatively
expensive. Large bodies of fused quartz are less expensive but are
generally not of sufficient purity for large preforms. A more
economical approach for making high quality cladding tubes is to
use sol-gel techniques. This well-known procedure is described, for
example, in J. Zarzycki, "The Gel-Glass Process", pp. 203-31 in
Glass: Current Issues, A. F. Wright and J. Dupois, eds., Martinus
Nijoff, Boston, Mass. (1985). Sol-gel techniques are regarded as
potentially less costly than other known preform fabrication
procedures. Options for producing the cladding tubes are addressed
here for completeness, but the focus of this invention is on the
core rod. The term core rod is used for convenience since the core
rod always contains the central core material. However, the rod may
comprise inner cladding, or both inner and outer cladding, as well
as the central core. These options will be described in more detail
below.
[0008] For producing very high quality central core and inner
cladding material, the MCVD process would appear ideal. However,
the MCVD starting tube can be a limiting factor in several ways.
The most direct limitation is when the glass in the MCVD starting
tube is simply not of sufficient quality and low loss for large
state of the art preforms (where some fraction of the optical power
would be carried by the starting tube material). If the initial
tube quality limitation is avoided by the use of ultra pure (and
typically expensive) material to fabricate the starting tube, the
exposure of the tube to the oxy-hydrogen torch typically used in
MCVD as a heat source can compromise the effective starting tube
quality by the addition of hydroxyl ions to a significant depth.
Finally, the desired refractive index profile may require a dopant
level in the region provided by the starting tube glass that is not
compatible with successful MCVD processing (viscosity, tube
stability or heat transfer considerations).
[0009] It should be evident from the discussion above that the
production of very large core rods for rod in tube methods appears
to be most suitably accomplished by VAD or OVD type methods. While
the MCVD process is capable, along with the VAD and OVD processes,
of producing very high quality glass, the MCVD glass is deposited
inside a starting tube which, because of the reasons outlined
above, can disadvantageously limit the application of the rod in
tube method to preforms below a given size.
SUMMARY OF THE INVENTION
[0010] We have developed a technique that allows the use of MCVD
for producing large preform core rods in a rod-in-tube process.
High-quality glass is deposited on the inside of a MCVD starting
tube, and the tube collapsed in the usual manner to form a solid
rod. The starting tube, at this point the outside shell of the rod,
is then removed from the solid rod leaving just MCVD-deposited
material. The rod is then inserted into a cladding tube and the
cladding tube collapsed to form the preform. The preform, following
this method, has a core region consisting entirely of MCVD
deposited material. Optionally, one or more inner cladding segments
may be deposited along with the central core during the MCVD
process, and the preform completed by the application of one or
more cladding layers over the MCVD central core and inner cladding
layers.
[0011] In a preferred embodiment, the overcladding operation is
accomplished by controlling the atmosphere in the gap between the
MCVD rod and the overcladding tube in much the same way that the
original MCVD process carries out the glass forming reaction inside
a tube to isolate the glass forming reaction from the
environment.
BRIEF DESCRIPTION OF THE DRAWING
[0012] FIGS. 1 and 2 are schematic representations of a rod-in-tube
process for the manufacture of optical fiber preforms;
[0013] FIG. 3 is a schematic representation of an MCVD process
showing deposition of high purity glass on the interior walls of
the MCVD starting tube;
[0014] FIGS. 4 and 5 are schematic diagrams of preform profiles
that are among those adapted to be produced by the method of the
invention;
[0015] FIG. 6 represents the collapse step in the MCVD process;
[0016] FIG. 7 represents the step in the inventive process of
removing the MCVD starting tube, leaving a core rod adapted for a
rod-in-tube method;
[0017] FIG. 8 represents the step of collapsing an overclad tube
onto the MCVD glass rod, using a modified MCVD apparatus or a
dedicated overcladding lathe, which provides isolation from the
ambient atmosphere and where a suitable drying/etching gas or gases
can be made to flow in the inner gap during the process;
[0018] FIG. 9 is a schematic representation of a fiber drawing
apparatus useful for drawing preforms made by the invention into
continuous lengths of optical fiber;
[0019] FIG. 10 is a refractive index profile for a typical optical
fiber made according to the invention; and
[0020] FIG. 11 is a plot of optical loss (dB/km) vs wavelength (nm)
for the optical fiber used for the data of FIG. 10.
DETAILED DESCRIPTION
[0021] Typical rod-in-tube methods are described in conjunction
with FIGS. 1 and 2. It should be understood that the figures
referred to are not necessarily drawn to scale. A cladding tube
representative of dimensions actually used commercially has a
typical length to diameter ratio of 10-15. The core rod 12 is shown
being inserted into the cladding tube 11. There exist several
common options for the composition of the core rod, It may be pure
silica, adapted to be inserted into a down-doped cladding tube. It
may have a pure silica center region with a down-doped inner
cladding region. It may have an up-doped, e.g. germania doped, core
region surrounded by a pure silica cladding region. It may have an
up-doped center core region surrounded by a down-doped inner
cladding region. All of these options, and many variations and
elaborations, are well known in the art and require no further
exposition here. However, the preferred embodiment of the invention
is aimed at the case where the core rod has either a central core
region (only) or a central core region and one cladding region with
the remaining cladding regions or segments provided by the
overcladding process. This takes full advantage of the MCVD process
and allows a substantial increase in preform size relative to prior
art. A typical profile has a central core of up-doped material,
typically germanium doped, and at least one un-doped,
neutrally-doped or down-doped region adjacent to the
germanium-doped central core region. Although such a profile
utilizes the advantages of the MCVD process, it should be
understood that the invention is not so limited. It may be applied
to the production of simple un-doped core rods. Or it may be
applied to making just a germanium up-doped core. In most cases,
other profile regions of differing refractive index will be formed
by one or more doped cladding tubes. Cladding tubes made with very
high quality glass-forming techniques may be used for most or even
all of the cladding layers. In the latter case, the MCVD process
needs to supply only the central core region and standard single
mode preforms equivalent to several thousand kilometers of fiber
per preform meter can be achieved.
[0022] After assembly of the rod 12 and tube 11, the tube is
collapsed onto the rod to produce the final preform 13, shown in
FIG. 2, with the core rod 14 indistinguishable from the cladding
tube except for a small refractive index difference.
[0023] FIG. 3 represents a typical MCVD method. The starting tube
is shown at 31. An oxy-hydrogen torch 32 traverses the length of
the outside of the tube while the tube is rotating. Glass precursor
materials, typically SiCl.sub.4 and dopants such as GeCl.sub.4, are
introduced into the interior tube at 33. When the glass precursors
reach the hot zone, they form a soot deposit 34 downstream of the
torch on the tube wall as shown. This soot layer is sintered into
glass as the torch traverse moves the hot zone downstream. Multiple
passes form thicker glass deposits, and allow the composition of
the glass deposit to vary radially from the tube center. The MCVD
process is well known, and details of the process need no
exposition here. See for example, J. B. MacChesney et al,
"Preparation of Low Loss Optical Fibers Using Simultaneous Vapor
Phase Deposition and Fusion", Xth Int. Congress on Glass, Kyoto,
Japan (1973) 6-40.
[0024] According to the invention, most, and preferably all, of the
central core material is deposited inside the MCVD tube. The ratio
of the central core diameter to the preform diameter for a typical
single mode fiber preform is in the range 1/10 to 1/20. As the
desired preform size increases, the required central core size will
necessarily increase. In conventional MCVD, the MCVD starting tube
limits the amount of deposited central core material to a
relatively small fraction of the total amount of MCVD material.
This is often expressed as the clad to core ratio or D/d ratio
where D refers to the diameter of the total MCVD deposition
(central core plus deposited cladding) and d refers to the diameter
of the central core structure. Typical values of the D/d ratio for
a simple single mode fiber design made by conventional MCVD with a
commercial quality starting tube and designed to match a
conventional overcladding process are in the range of 2.0 to 4.0.
In the method of the invention, there is no limit to the proportion
of MCVD material that can be used for the core since the MCVD tube
is not intended to be used in the final preform structure. That is
to say, the invention allows the attainment of the optimum D/d=1.0
ratio for a large preform. In this case, 100% of the MCVD material
is used to form the central core. The cladding will be applied
later in the overcladding process, after removal of the starting
tube, and each cladding can be any desired thickness where the
final preform diameter is in proportion to the central core
diameter. This allows multiple doped (or undoped) layers, of
essentially any desired thickness and sequence, to be incorporated
in the preform design. Since the preform size scales as the inverse
square of the D/d ratio, a clad to core ratio of 1.0 would provide
a factor of 4 increase in preform size compared to a conventional
MCVD single mode fiber process with a D/d ratio of 2.0. While a D/d
ratio of 1.0 corresponds to the largest possible preform size for a
given amount of MCVD deposition, in some cases it may be
advantageous to deposit the central core and an inner cladding
region or regions by MCVD. In that case the clad to core ratio will
be greater than 1.0 but can still provide a significant advantage
in comparison with standard MCVD practice. For example, if a
central core and adjacent inner cladding with equivalent amounts of
deposition are produced by MCVD, the clad to core ratio would be 2.
Such a preform would still be twice as large as a conventional MCVD
preform with a clad to core ratio of 2.0, both preforms having the
same total amount of MCVD deposit.
[0025] Two typical preform profiles are shown in FIGS. 4 and 5.
These are schematic plots and are general, since actual preform
profiles are not part of the invention. The Y-axis is the
refractive index variation from that of un-doped silica (zero),
with up-doping on the + part of the scale and down-doping on
the--side of the scale. The x-axis is the position along the radius
of the preform with a typical amount of MCVD deposition indicated
schematically. In commercial practice, deposited glass diameters in
the collapsed preform in the range of 12 to 14 mm are readily
achieved by MCVD. As is well known, the profile will be essentially
replicated in the drawn fiber, but the plots shown are for the
preform. FIG. 4 is intended to show generally a profile with a
relatively heavy up-doped core region 41, an un-doped (or neutrally
doped) inner cladding region 42, and an up-doped outer cladding
region 43. Regions 41 and 43 are typically doped with germanium.
There are several options available for producing this profile
using the method of the invention. The core rod may be made with
just the core material (region 41) extending to the limit of MCVD
deposition. At the other extreme, the entire profile may be
encompassed within the MCVD deposition. However, as mentioned
above, the latter results in preform size limitations. Thus the
invention is mainly directed to forming some, but fewer than all,
of the core/cladding segments with MCVD. It is also possible to
design the process for partial formation of a given cladding
segment using MCVD, with the remainder of that segment comprising a
cladding tube. In a preferred embodiment, the core and the first
cladding segment are made by MCVD, with the remaining segments
formed using one or more cladding tubes. For some complex profiles,
as many as five cladding tubes may be used.
[0026] For example, if the core 41 and the inner cladding 42 are
made by MCVD, that would involve 12-14 mm (diameter) of MCVD
material.
[0027] The profile in FIG. 4 is well known and has been widely
used. Other variations in profile design are represented generally
by the profile in FIG. 5. Here the center core region 51 is
relatively lightly doped with germanium, yielding a high quality
low loss core region. The inner cladding layer, 52, is down-doped
with F. The outer cladding layer 53 is either un-doped, or up-doped
slightly. Again, by substituting appropriately doped overclad tubes
for the outer cladding segment or the inner and outer cladding
segments, the amount of MCVD core material will be sufficient for a
very large preform.
[0028] It will be understood by those skilled in the art that the
two profiles shown in FIGS. 4 and 5 are just typical of a large
number of profile variations now known, or to be developed. These
may have more than three core/cladding segments. It will also be
appreciated that the ability to form core material to an increased
thickness, allows wide versatility in core design.
[0029] When deposition of the profile regions that are to be
provided by MCVD is complete the tube is collapsed by known
techniques, i.e. heating the tube to above the glass softening
temperature, i.e. >2000-2400.degree. C. to allow the surface
tension of the glass tube to slowly shrink the tube diameter,
finally resulting, after multiple passes of the torch, in the
desired solid rod. The collapsed rod is shown in FIG. 6, with the
MCVD starting tube shown at 61, and the MCVD deposited core (or
core/cladding) at 62.
[0030] Next, in accordance with a principle step of the invention,
the MCVD tube is removed. This may be accomplished by mechanical
grinding, by plasma etching, by chemical etching or by a
combination of these techniques. In certain cases, depending on the
quality of the starting tube material, it may be permissible to
leave a residual amount of starting tube material surrounding the
MCVD deposited glass but in a preferred embodiment, all the
starting tube glass is removed. The end point of the etching
process can be determined from a refractive index profile of the
collapsed rod. If the desired profile is similar to that of FIG. 5,
with the first deposited layer 53 undoped, there is some margin for
error in the case where complete removal of the starting tube is
required. The etched preform may be measured after grinding or
etching is complete to determine the amount of overetching, which
is then factored into the selection of the cladding tube. It will
be evident that overetching is preferable to underetching in this
case. Accordingly, the MCVD deposition and the etch time may be
designed for limited but finite etching of MCVD deposited
material.
[0031] In general, at least 75% of the starting tube cross
sectional area will be removed in practicing the invention. This
may be expressed as:
((OD.sub.2).sup.2-ID.sup.2)/((OD.sub.1).sup.2-ID.sup.2)<0.25
[0032] where OD.sub.1 and OD.sub.2 are the outside diameters of the
collapsed rod before and after removal, and ID is the inside
diameter of the starting tube after collapse. Preferably, according
to the invention, more than 90% of the tube is removed, and more
typically, all of the tube is removed.
[0033] After removing the MCVD starting tube, the MCVD deposited
glass core remains, as shown in FIG. 7. Before overclad, the
surface of the core rod may be cleaned by plasma treatment or by
chemical etching to remove any residual OH.sup.-, and other
contaminants. The finished rod may then be inserted in a cladding
tube as described above.
[0034] As noted earlier, the MCVD process is limited in the total
amount of glass that can be deposited inside a starting tube.
Typical commercial practice, if directed towards single mode
preforms, would result in less than 15 mm of total MCVD deposit
(expressed as MCVD glass diameter in the collapsed rod) although
somewhat larger amounts can be achieved with special effort. If the
intended size of the final preform is large enough, a substantial
fraction of that total MCVD deposited material is utilized to form
the central core. In that case, significant optical power can
extend in the drawn fiber outside the MCVD deposited region and it
will be advantageous to perform the overcladding process, or at
least the first overclad if a multiple overclad process is used, in
a way that assures the optical quality of the interface between the
MCVD material and the overclad tube and avoids the generation of a
layer of high loss glass. This can be accomplished by controlling
the atmosphere in the gap between the MCVD rod and the overcladding
tube in much the same way that the original MCVD process carries
out the glass forming reaction inside a tube to isolate the glass
forming reaction from the environment. In the overcladding case,
the overcladding tube with the MCVD derived core rod inside can be
attached to an MCVD lathe or similar apparatus and said overclad
tube, coupled to the lathe gas delivery system, then provides the
isolation form the ambient environment.
[0035] A suitable apparatus for conducting this step is illustrated
in FIG. 8. where a cladding tube 71 is shown supported by lathe
elements 72 and 73. Element 71 represents the tube support and
exhaust structure, and element 73 is a rotating seal supporting the
tube on the inlet side. The core rod is shown at 74 with spacers 75
supporting the core rod within the cladding tube. The spacers are
designed to allow gas flow along the tube. The standard MCVD type
torch is shown schematically at 76. The atmosphere 77, between the
core rod 74 and the cladding tube 71 is controlled in this
arrangement and the composition of the controlled atmosphere is
determined by flowing gas from gas inlet 78.
[0036] A gas composition that is effective in removing hydrogen is
introduced into the gap 77 and one or several passes of the heat
source 76 along the length of the tube can be made while the gas is
flowing. Effective gas ambients would include a drying agent such
as chlorine or a drying agent such as chlorine with an etching
agent such as a chlorofluocarbon or sulphur hexafluoride to remove
a very thin surface layer. A hydrogen/oxygen torch (as
schematically shown in FIG. 8) can serve as the heat source but a
preferred method is to use a hydrogen free heat source such as a
plasma torch or an electrically heated furnace. During these
traverses the heat source maintains the tube at a temperature
sufficient for effective drying of the interior surfaces but below
the temperature at which the tube would collapse onto the rod.
After a sufficient number of drying/etching traverses of the heat
source, the temperature is increased and the collapse is carried
out in one or more passes. After the overclad process is completed,
it may be advantageous to remove any residual surface OH on the
outside of the overclad preform with either a plasma etch process,
a chemical etch process or a combination of the two.
[0037] In addition to assuring the quality of at least the first
overclad interface, it may be useful to use an ultra-high-purity
tube for the first overclad. Ultra-high-purity may, in a preferred
case, be defined as having less than 50 ppB hydroxyl ion by weight.
This is especially beneficial in the case where significant optical
power is carried in the fiber in the region corresponding to this
tube, and where a tube of typical quality would introduce
noticeable excess loss. As noted above, this tube, since it is only
used in an overcladding process, may be of a size or dopant
composition that typically would not be used as an MCVD starting
tube. Examples include very thin walled tubes, tubes downdoped in
refractive index with high levels of fluorine, or tubes updoped in
refractive index with germanium. A more elaborate example can be
envisioned where an overcladding tube is fabricated with several
distinct regions and each region has a different dopant profile to
produce a large preform version of a fiber design with multiple
cladding structures. Alternatively, the same goal could be
approached by a multiple overclad process.
[0038] In some cases, it will be advantageous to introduce an
elongation step at an intermediate stage of the multiple
overcladding sequence. In this elongation step, the glass body,
after the completion of one or more overclad steps, is stretched in
length and reduced in diameter. Additional overcladding steps may
be carried out after the stretching to produce a final preform
ready for draw. Inclusion of an elongation or stretching step does
not change the fiber kilometer equivalent of the original core rod
but can allow a smaller diameter (and longer length) final preform
which may be more suited to a particular fiber draw facility.
[0039] As just described, the use of an ultra-high-purity first
overclad tube is also preferred if the fiber design has enough
optical power propagating outside the MCVD region to otherwise
adversely affect the fiber loss. The MCVD core glass will typically
have a diameter of 12 to 15 mm. The central core region that
contains most of the guided light may have a diameter of 6 mm to 15
mm. The overall preform, after applying one or more cladding tubes,
may have a diameter of 200 mm or more. Thus the method of the
invention provides for a very large preform, with a complex profile
structure, in which the core is formed entirely by MCVD. For the
purpose of definition, a large preform is considered as one with a
diameter of at least 120 mm.
[0040] Although in the description so far the MCVD core rod is
intended as part of a rod-in-tube method, alternatively the MCVD
core rod may be used as a substrate for soot deposition. In this
way, cladding layers or partial cladding layers may be deposited
using soot techniques.
[0041] Although the MCVD process as described above uses a flame
torch and a fuel of mixed oxygen and hydrogen, plasma torches or
electrically heated furnaces may also be used in these kinds of
processes. Also, gas torches other than oxy-hydrogen torches can be
used.
[0042] The optical fiber preform, as described above, is then used
for drawing optical fiber. FIG. 9 shows an optical fiber drawing
apparatus with preform 81, and susceptor 82 representing the
furnace (not shown) used to soften the glass preform and initiate
fiber draw. The drawn fiber is shown at 83. The nascent fiber
surface is then passed through a coating cup, indicated generally
at 84, which has chamber 85 containing a coating prepolymer 86. The
liquid coated fiber from the coating chamber exits through die 91.
The combination of die 91 and the fluid dynamics of the prepolymer,
controls the coating thickness. The prepolymer coated fiber 94 is
then exposed to UV lamps 95 to cure the prepolymer and complete the
coating process. Other curing radiation may be used where
appropriate. The fiber, with the coating cured, is then taken up by
take-up reel 97. The take-up reel controls the draw speed of the
fiber. Draw speeds in the range typically of 1-20 m/sec. can be
used. It is important that the fiber be centered within the coating
cup, and particularly within the exit die 91, to maintain
concentricity of the fiber and coating. A commercial apparatus
typically has pulleys that control the alignment of the fiber.
Hydrodynamic pressure in the die itself aids in centering the
fiber. A stepper motor, controlled by a micro-step indexer (not
shown), controls the take-up reel.
[0043] Coating materials for optical fibers are typically
urethanes, acrylates, or urethane-acrylates, with a UV
photoinitiator added. The apparatus is FIG. 9 is shown with a
single coating cup, but dual coating apparatus with dual coating
cups are commonly used. In dual coated fibers, typical primary or
inner coating materials are soft, low modulus materials such as
silicone, hot melt wax, or any of a number of polymer materials
having a relatively low modulus. The usual materials for the second
or outer coating are high modulus polymers, typically urethanes or
acrylics. In commercial practice both materials may be low and high
modulus acrylates. The coating thickness typically ranges from
150-300 .mu.m in diameter, with approximately 240 .mu.m
standard.
[0044] As an example of the described process, we have fabricated a
preform sized to yield 1500 kilometers of fiber per meter of core
rod length. A refractive index profile obtained from a sample of
the drawn fiber is shown in Figure X. The graded central core
region and the adjacent downdoped inner cladding region were
fabricated with MCVD inside an undoped silica starting tube. As
described above, after the MCVD collapse step the starting tube was
completely removed by a process combining mechanical grinding,
plasma etching and chemical etching leaving a core rod of MCVD
material approximately 13 mm in diameter. This MCVD glass rod was
then overclad with an ultra-pure germanium doped silica overclad
tube which provided the updoped cladding region seen in the
refractive index profile. This first overclad operation was carried
out as described earlier with care to preserve the optical quality
of the glass in the interface region. Three more overclad steps,
one of which made use of a downdoped overclad tube, along with an
intermediate stretching step to size the final preform diameter for
a conventional fiber drawing furnace, were used to complete the
preform. The optical properties of the fiber drawn from this
preform were equivalent to those of fibers with similar refractive
index profiles drawn from significantly smaller preforms made with
a conventional, prior art MCVD and overclad process. In particular,
the optical loss of the fiber drawn from the example preform was
equivalent or lower than was typical of fiber drawn from such prior
art preforms. To illustrate this point, FIG. 11 shows a spectral
loss curve (fiber attenuation vs. wavelength) of a fiber from the
example preform.
[0045] Various additional modifications of this invention will
occur to those skilled in the art. All deviations from the specific
teachings of this specification that basically rely on the
principles and their equivalents through which the art has been
advanced are properly considered within the scope of the invention
as described and claimed.
* * * * *