U.S. patent application number 12/364045 was filed with the patent office on 2010-08-05 for fiber with airlines.
Invention is credited to Randy Lee Bennett, Scott Robertson Bickham, Jeffrey Coon, Leonard Charles Dabich, II, Daniel Warren Hawtof, Joseph Edward McCarthy.
Application Number | 20100195964 12/364045 |
Document ID | / |
Family ID | 42173759 |
Filed Date | 2010-08-05 |
United States Patent
Application |
20100195964 |
Kind Code |
A1 |
Bennett; Randy Lee ; et
al. |
August 5, 2010 |
FIBER WITH AIRLINES
Abstract
An optical fiber comprising: (i) a core; (ii) a cladding
surrounding the core; wherein the cladding comprises a cladding
ring that: (a) has a width W equal to or less than 10 microns; (b)
includes at least 50 airlines, each airline having a maximum
diameter or a maximum width of not more than 2 microns and more
than 50% of said airlines have a length of more than 20 m; (c) has
an air fill fraction of 0. 1% to 10%, and (d) has an inner radius
R.sub.in and an outer radius R.sub.out, wherein 6
.mu.m.ltoreq.R.sub.in.ltoreq.14 .mu.m, and 8
.mu.m.ltoreq.R.sub.out.ltoreq.14 .mu.m; and (iii) an outer cladding
surrounding said cladding ring.
Inventors: |
Bennett; Randy Lee; (Painted
Post, NY) ; Bickham; Scott Robertson; (Corning,
NY) ; Coon; Jeffrey; (Wilmington, NC) ;
Dabich, II; Leonard Charles; (Painted Post, NY) ;
Hawtof; Daniel Warren; (Corning, NY) ; McCarthy;
Joseph Edward; (Addison, NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
42173759 |
Appl. No.: |
12/364045 |
Filed: |
February 2, 2009 |
Current U.S.
Class: |
385/125 ;
65/393 |
Current CPC
Class: |
G02B 6/0365 20130101;
C03B 37/0124 20130101; G02B 6/02342 20130101; C03B 37/02781
20130101; C03B 37/014 20130101; C03B 2201/31 20130101; C03B 2203/42
20130101; C03B 37/01208 20130101; C03B 2203/14 20130101; C03B
2203/22 20130101 |
Class at
Publication: |
385/125 ;
65/393 |
International
Class: |
G02B 6/032 20060101
G02B006/032; C03B 37/027 20060101 C03B037/027 |
Claims
1. An optical fiber comprising: (i) a core; (ii) a cladding
surrounding the core; wherein the cladding comprises a cladding
ring that: (a) has a width W of less than 10 microns; (b) includes
at least 50 airlines, each airline having a maximum diameter or a
maximum width of not more than 2 microns and more than 50% of said
airlines have a length of more than 20 m; (c) has an air fill
fraction of 0.1% to 10%, and (d) has an inner radius R.sub.in and
an outer radius R.sub.out, wherein 6
.mu.m.ltoreq.R.sub.in.ltoreq.14 .mu.m, and 8
.mu.m.ltoreq.R.sub.out.ltoreq.14 .mu.m; and (iii) an outer cladding
surrounding said cladding ring.
2. An optical fiber comprising: (i) a core; (ii) a cladding
surrounding the core; wherein the cladding comprises a cladding
ring that: (a) has a width W not greater than 10 microns; (b)
includes at least 50 airlines, each airline having a maximum
diameter or a maximum width of not more than 2 microns and more
than 50% of said airlines are continuous along the axis of the
optical fiber; (c) has an air fill fraction of 0.1% to 10%, and (d)
has an inner radius R.sub.in and an outer radius R.sub.out, wherein
6 .mu.m.ltoreq.R.sub.in.ltoreq.14 .mu.m, and 8
.mu.m.ltoreq.R.sub.out.ltoreq.14 .mu.m; and (iii) an outer cladding
surrounding said cladding ring.
3. The optical fiber of claim 2, wherein said cladding comprises: a
first cladding region in contact with and surrounding the core;
said first cladding region being (i) in contact with and (ii)
surrounded by said cladding ring.
4. The optical fiber of claim 3, wherein said cladding ring
comprises at least 200 airlines.
5. The optical fiber of claim 2, wherein said annular cladding ring
comprises at least 200 airlines.
6. The optical fiber of claim 3, wherein said core has a relative
refractive index between 0.2% and 0.5% with respect to said outer
cladding.
7. The optical fiber of claim 2, wherein said core has a relative
refractive index between 0.2% and 0.5% with respect to said outer
cladding.
8. The optical fiber of claim 3, wherein said first cladding region
is comprised essentially of silica.
9. The optical fiber of claim 3, wherein said cladding ring has a
width W of 2 to 10 microns.
10. The optical fiber of claim 2, wherein said cladding ring has a
width W of 3 to 8 microns.
11. The optical fiber of claim 3, wherein the cable cutoff is less
than 1260 nm, the zero dispersion wavelength is between 1302 and
1324 nm, and the mode field diameter at 1310 nm is between 8.0 and
10.0 microns.
12. A method for making an optical fiber, said method comprising
the steps of: (i) wrapping a sintered core blank with a rod ribbon
containing at least 100 glass: (a) rods, (b) tubes, or (c) a
combination of rods and tubes, to produce a wrapped preform; (ii)
heating the wrapped preform to a heating temperature to at least
partially attach each of the rods or tubes to (a) the core blank
and/or (b) to at least one other rod or tube, thereby creating a
core blank assembly; (iii) re-drawing the core blank assembly to
produce a smaller diameter cane.
13. The method of claim 12, wherein said cane is: overclad with
glass soot and consolidated to create a final preform blank.
14. The method of claim 13, further including the step of drawing
an optical fiber from said final preform blank.
15. The method of claim 12, wherein said rod ribbon is wrapped
around said core blank at least 3 times.
16. The method of claim 12 wherein the outer diameter D of said
core blank is at least 10 times the average outer diameter d of
said rods or tubes.
17. The method of claim 16 wherein the outer diameter D of said
core blank is at least 20 times the average outer diameter d of
said rods or tubes.
18. The method of claim 16 wherein the outer diameter D of said
core blank is at least 30 times the average outer diameter d of
said rods or tubes.
19. The method of claim 12 wherein the average outer diameter d of
said rods or tubes is not greater than 2 mm.
20. The method of claim 12 wherein said heating temperature is
between 1900.degree. C. and 2100.degree. C.
21. The method of claim 12 wherein the Torque during said redraw
step is at least 15 Nm.
22. The method of claim 21 wherein the Torque during said redraw
step is greater than 20 Nm and less than 100 Nm.
23. An optical fiber produced by the method of claim 12.
24. The optical fiber of claim 23 wherein optical fiber comprises:
(i) a core; (ii) a cladding surrounding the core; wherein the
cladding comprises a cladding ring that: (a) has a width W of less
than 10 microns; (b) includes at least 50 airlines, each airline
having a maximum diameter or a maximum width of not more than 2
microns and more than 50% of said airlines have a length of more
than 20 m; (c) has an air fill fraction of 0.1% to 10%, and (d) has
an inner radius R.sub.in and an outer radius R.sub.out, wherein 6
.mu.m.ltoreq.R.sub.in.ltoreq.14 .mu.m, and 8
.mu.m.ltoreq.R.sub.out.ltoreq.14 .mu.m; and (iii) an outer cladding
surrounding said cladding ring.
25. The optical fiber of claim 24, wherein said annular cladding
ring comprises at least 200 airlines.
26. The optical fiber of claim 23 wherein optical fiber comprises:
(i) a core; (ii) a cladding surrounding the core; wherein the
cladding comprises a cladding ring that: (a) has a width W not
greater than 10 microns; (b) includes at least 50 airlines, each
airline having a maximum diameter or a maximum width of not more
than 2 microns and more than 50% of said airlines are continuous
along the axis of the optical fiber; (c) has an air fill fraction
of 0.1% to 10%, and (d) has an inner radius R.sub.in and an outer
radius R.sub.out, wherein 6 .mu.m.ltoreq.R.sub.in.ltoreq.14 .mu.m,
and 8 .mu.m.ltoreq.R.sub.out.ltoreq.14 .mu.m; and (iii) an outer
cladding surrounding said cladding ring.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] The present invention relates generally to optical fibers,
and particularly to optical fibers with airline features, and to a
method of manufacturing such fibers.
[0003] 2. Technical Background
[0004] Corning, Inc introduced a new optical fiber, ClearCurve.RTM.
optical fiber, in 2007. This fiber has improved bend performance
due to gas filled voids randomly situated in an annular region
surrounding the core of the fiber. The gas filled voids are
randomly distributed and are created by trapping gas in the soot
during sintering of the preform. The gas filled voids are elongated
as the preform is drawn into fiber. These gas filled voids do not
extend over the entire length of the fiber and are typically less
than 10 m in length.
[0005] Photonic crystal fibers and photonic band gap fibers (PCFs
and PBGFs) are usually created by a "stack and draw" method. The
term "stack and draw" refers to assembling a preform from
constituent parts using, for example, small tubes and/or rods with
round or hexagonal cross-sections, and stacking them together in a
precise orientation. These stacked components are usually situated
inside an overclad tube. The overclad tube with the stacked
components is then either collapsed and pulled into intermediate
preform parts, or directly drawn into an optical fiber. The preform
may also be etched to enlarge the inside diameters of the small
tubes in order to change the ratio of glass to air in a cross
section of the preform, to enhance optical properties of the
fiber.
[0006] PCFs and PBGFs are expensive to make, because in order to
propagate light properly, they require a great amount of precision
in placement of different preform components (e., g., core rod(s),
and the glass tubes surrounding the core rod(s). The preform
components such as the overclad tube and the small glass tubes
surrounding the core rod(s) are typically expensive and contribute
to the cost of making these fibers. In addition, PCFs and PBGFs are
expensive due to the relatively small amount of fiber resulting per
preform assembly (when compared to a standard optical transmission
fiber preform making processes), resulting from the relatively
small size of the optical preforms.
SUMMARY
[0007] One aspect of the invention is an optical fiber comprising:
[0008] (i) a core; (ii) a cladding surrounding the core; and (iii)
an outer cladding. The cladding includes a cladding ring that: (a)
has a width W equal to or less than 10 microns; (b) not greater
than 10 microns; (b) comprises at least 50 airlines, each airline
having a maximum diameter or maximum width of no more than 2
microns; (c) has an air filled fraction of 0. 1% to 10%, and (d)
has an inner radius Rin and an outer radius R.sub.out, wherein 6
.mu.m.ltoreq.R.sub.in.ltoreq.14 .mu.m, and 8
.mu.m.ltoreq.R.sub.out.ltoreq.14 .mu.m. According to some
embodiments, the width W is 3 .mu.m to 6 .mu.m
(W=R.sub.out-R.sub.in). Preferably, the cladding ring includes at
least 75 airlines, and more preferably at least 100 airlines.
Preferably, most (more than 50%), or all of the airlines are
continuous along the axis of the optical fiber.
[0009] One example of the present invention is an optical fiber
comprising: a glass core, a first cladding, a second cladding and
an outer cladding; wherein the second cladding is the cladding ring
that comprises at least 100 airlines. Preferably, the
cross-sectional diameter d of the airlines is 0.5 .mu.m to 2 .mu.m,
for example 0.75 .mu.m to 1.5 .mu.m. Preferably the length of most
or all of the airlines is greater than 20 m, and more preferably
greater than 100 m.
[0010] In another aspect, the present invention includes a method
for making an optical fiber, said method comprising the steps of:
[0011] (i) wrapping a sintered core blank with a rod ribbon
containing at least 100 glass:
[0012] (a) rods, (b) tubes, or (c) a combination of rods and tubes,
to produce a wrapped preform; [0013] (ii) heating the wrapped
preform so as to attach each of the rods or tubes to (a) the core
blank and/or (b) to at least one other rod or tube, thereby
creating a core blank assembly; [0014] (iii) re-drawing the core
blank assembly to produce a cane.
[0015] According to the exemplary embodiments described herein the
cane has a smaller diameter than the wrapped preform or the core
blank assembly.
[0016] According to some preferred embodiments of the present
invention, the fiber cane is overclad with glass soot; and
consolidated to create the final preform blank.
[0017] An advantage of the method of making fiber according to the
present invention is that this method is less expensive than the
"stack and draw" process of making a fiber. The stack and draw
manufacturing process is a high cost process. There are three high
cost elements contributing to the high cost of the stack and draw
process. These are: 1) the cost of tubes in the stack, 2) the
manual labor associated with assembling the structure, and 3) the
overclad tube cost. The exemplary embodiments of the method of
making optical fiber according to the present invention
advantageously: (i) eliminate one or more (and preferably all
three) of these high cost elements, and (ii) are capable of
producing large size preforms, which contribute to low cost through
high equipment utilization that lowers fixed costs of equipment
operators and equipment downtime between set-ups.
[0018] During the photonic band gap (PBG) fiber manufacturing
processes, when assemblies of rods or tubes are redrawn, the
interstitial holes are closed. More specifically, the PBG
manufacturing process typically utilizes a vacuum pull (negative
pressure) during fiber draw (and preform blank redraw) in order to
collapse the interstitial holes between the tubes and rods. A fiber
drawn or preform blank redrawn with a very low torque could also
collapse the interstitial holes, but with the adverse effect of
lowering the air-fill fraction created by the tubes. However,
because it is desired that PBG fibers have a high air-filled
fraction, low torque (T<10 Nm) draw or re-draw steps are not
utilized in manufacturing of PBG fibers. In contrast, according to
some embodiments of the present invention, high torque (e.g.,
T>15 Nm, T>25 Nm, or T=15-10 Nm) is utilized to keep the
interstitial holes open. According to some exemplary embodiments of
the present invention, the core blank assembly and/or preform blank
is capped during redraw and/or the fiber draw process, or is
pressurized (positive pressure is applied) in order to keep the
interstitial holes open. Capping to trap interstitial gasses or
pressurizing interstitial gaps to keep the interstitial holes open
is the opposite of pulling the vacuum (which collapses the
interstitial holes).
[0019] Additional features and advantages of the invention will be
set forth in the detailed description which follows, and in part
will be readily apparent to those skilled in the art from that
description or recognized by practicing the invention as described
herein, including the detailed description which follows, the
claims, as well as the appended drawings.
[0020] It is to be understood that both the foregoing general
description and the following detailed description present
embodiments of the invention, and are intended to provide an
overview or framework for understanding the nature and character of
the invention as it is claimed. The accompanying drawings are
included to provide a further understanding of the invention, and
are incorporated into, and constitute a part of, this
specification. The drawings illustrate various embodiments of the
invention, and together with the description serve to explain the
principles and operations of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1A is a schematic cross-sectional view of one
embodiment of the optical fiber of the present invention;
[0022] FIG. 1B is a schematic illustration of a relative refractive
index profile of the optical fiber of FIG. 1A;
[0023] FIG. 1C is a schematic cross-sectional view of one
embodiment of the preform blank for making the optical fiber of
FIG. 1A;
[0024] FIG. 2 illustrates schematically one embodiment of the
method of making an optical fiber with airlines;
[0025] FIG. 3 is a photograph illustrating a 47 mm diameter
sintered core blank placed on a ribbon comprising 1 mm diameter
solid silica rods;
[0026] FIG. 4 is a photograph of a silica rod ribbon wrapped around
the core blank shown in FIG. 2;
[0027] FIG. 5 is a photograph of a 10 mm diameter cane, with a
single layer of silica rods attached to the core blank.
[0028] FIG. 6 is a photograph of the enlarged cross-sectional
portion of a 10 mm fiber cane with three layers of silica rods
attached to and surrounding the core blank. The dark areas
represent interstitial spaces between the rods.
[0029] FIG. 7 is a photograph of the enlarged cross-sectional
portion the final preform corresponding to the cane of FIG. 6.
DETAILED DESCRIPTION OF THE EMBODIMENTS.
[0030] Reference will now be made in detail to the present
preferred embodiments of the invention, examples of which are
illustrated in the accompanying drawings. Whenever possible, the
same reference numerals will be used throughout the drawings to
refer to the same or like parts. One embodiment of the optical
fiber of the present invention is shown schematically in FIG. 1A,
and is designated generally throughout by the reference numeral
10.
[0031] One embodiment of the optical fiber 10 comprises a core 12
and a cladding 16 surrounding the core 12. The cladding 16
comprises an annular region, referred to as a cladding ring 18
herein. The cladding ring 18: (a) has a width W not greater than 10
microns; (b) includes at least 50 airlines 20 (also referred to
herein as air holes), each airline 20 having a maximum diameter d
or a maximum width w of not more than 2 microns; and (c) has an
inner radius R.sub.in and an outer radius R.sub.out. According to
some embodiments, 6 .mu.m.ltoreq.R.sub.in.ltoreq.14 .mu.m, and 8
.mu.m.ltoreq.R.sub.out.ltoreq.14 .mu.m. According to some
embodiments, 0.5 .mu.m.ltoreq.d.ltoreq.2 .mu.m or 0.5
.mu.m.ltoreq.w.ltoreq.2 .mu.m. For example, in some embodiments,
0.75 .mu.m.ltoreq.d.ltoreq.1.5 .mu.m. Preferably, the cladding ring
18 includes at least 75 airlines, and more preferably at least 100
airlines. In some embodiments the cladding ring 18 includes at
least 200 airlines. The exemplary optical fibers 10 of FIG. 1A also
include an outer cladding 30 surrounding the cladding 16.
Preferably, the airlines 20 are substantially continuous along the
length of the fiber 10. For example, according to at least some the
embodiments of the present invention, the length of most (>50%)
air holes 20 is greater than 20 m, and is typically greater than 50
m. In the exemplary embodiments described herein the length of most
air holes 20 is greater than 100 m. Advantageously, the optical
fiber 10 according to the embodiments of the present invention can
be designed to offer similar optical performance and similar low
losses as those obtainable with a fiber such as ClearCurve.RTM.
optical fiber available from Corning Inc.
[0032] For example, one embodiment of the optical fiber 10 (see
FIG. 1A) comprises: a glass (e.g., silica based, Ge doped) core 12;
a cladding 16 including a first (optional) cladding region 14 and a
cladding ring 18 with airlines 20; and an outer cladding 30.
Although in the exemplary embodiments the core 12 contains between
3 and 9 wt % GeO.sub.2, the core 12 may also utilize other dopants.
The core 12 of this exemplary fiber 10 has an outer diameter of 9
.mu.m, and a relative refractive index between 0.2% and 0.5% with
respect to the outer cladding 30. Although in this exemplary
embodiment the core 12 is single-moded, in other embodiments the
core 12 may be either single-moded or multi-moded, and may be
either a step index core (see FIG. 1B, solid line), or a graded
index core (see FIG. 1B, dashed line). In the exemplary embodiment
of FIG. 1A, the first cladding region 14 has an outer diameter of
22.5 .mu.m, and is made of pure silica. However, the first cladding
region 14 can also include other dopants in order to increase, or
to decrease its index of refraction relative to that of pure
silica. The outer diameter of the first cladding region 14 is
preferably between 16 and 27 .mu.m. In some exemplary embodiments,
the first cladding region 14 has a maximum relative refractive
index less than 0.05% and a minimum relative refractive index
greater than -0.05%. According to some exemplary embodiments, the
silica based cladding ring 18 containing airlines has a width W of
less than 10 microns. The width W may be, for example, between 2
.mu.m and 8 .mu.m and more preferably is between 3 .mu.m and 6
.mu.m, e.g., 2.2 .mu.m, 2.5 .mu.m, 3 .mu.m, 3.5 .mu.m, 4 .mu.m, 4.5
.mu.m, 5 .mu.m, 5.5 .mu.m or 6 .mu.m. In this embodiment the width
W of the cladding ring 18 is about 3 .mu.m. In some embodiments the
cladding ring 18 includes at least 100 airlines 20. The cladding
ring 18 may comprise silica doped with at least one dopant selected
from the group comprising: germanium, aluminum, phosphorous,
titanium, boron, and/or fluorine. Each airline 20 of this exemplary
embodiment has a maximum diameter d (or, for airlines with a non-
circular or non-elliptical cross-section, maximum width w of no
more than 2 .mu.m (e.g., 0.5 .mu.m to 2 .mu.m). In the exemplary
fiber 10 depicted in FIG. 1A the cladding ring 18 includes four
rows of microstructures 18A (silica micro-rods, with airlines 20
situated between the micro-rods) and the outer cladding 30 which is
made of pure (i.e., undoped) silica. The outer cladding 30 of this
exemplary embodiment has an outer diameter of 125 .mu.m.
[0033] Other embodiments may utilize fewer or more rows of rods
and/or tubes, for example, 2-20 rows of rods and/or tubes,
preferably 2-10 rows, more preferably 3-8 rows, and even more
preferably 3-6 rows. It is noted that although the rods 18A in this
exemplary embodiment were made of pure silica, the rods 18A (or
optional tubes 18B (not shown)) can be also be made from other
optical glasses or from silica doped with at least one dopant
selected from the group comprising: germanium, aluminum,
phosphorous, titanium, boron, and fluorine.
[0034] This exemplary optical fiber 10 of FIG. 1A has a cable
cutoff wavelength less than 1260 nm, zero dispersion wavelength of
between 1302 nm and 1324 nm, and mode field diameter at 1310 nm
between 8.0 and 10.0 microns.
[0035] Some of the advantageous features of fiber 10 of FIG. 1A
include: (a) presence of a first (inner) cladding region 14 between
the Ge-doped core and the airline filled cladding ring 18; and (b)
a large number (more than 50) of small airlines 20 with a maximum
width of no more than 2 .mu.m The first cladding region 14 of the
optical fiber 10 of FIG. 1A advantageously centers the
zero-dispersion wavelength in the G.652 window (1302 nm-1324 nm)
and reduces the optical power and scattering losses in the airline
region. The large number of small airlines 20 enables a low cable
cutoff wavelength (below 1260 nm) and improves the rotational
symmetry of the air filled fraction (AFF) in the cladding ring
18.
[0036] Some embodiments of fiber 10 have the air fill fraction
(AFF) of 0.1%.ltoreq.AFF.ltoreq.10%. This range of AFF is: 1) high
enough to advantageously contribute to good bend loss performance,
for example, less than 1 dB/turn when bent around a 10 mm diameter
mandrel, and/or less than 0.2 dB/turn when bent around a 20 mm
diameter mandrel, and 2) low enough to provide a low cable cutoff
wavelength )c of no more than then 1500 nm (e.g., .lamda.c<1500
nm, or .lamda.c<1400 nm, or .lamda.c<1260 nm).
[0037] The exemplary multimode optical fiber 10 of FIG. 1B has a
graded index core with a diameter greater than 40 .mu.m, for
example 47 to 53 .mu.m, 60-65 .mu.m, greater than 70 .mu.m or in
some cases greater than 90 .mu.m. The maximum relative refractive
index of the graded index core is preferably greater than 0.8%,
e.g. greater than 0.9%, between 0.9 and 1.1%, between 0.9 and 1.0%,
between 1.4 and 1.6% or between 1.8 and 2.2%.
[0038] The numerical aperture of the fiber is preferably greater
than 0.18, e.g. 0.185 to 0.215, greater than 0.24, greater than
0.27, or 0.27 to 0.30. The overfilled bandwidth at 850 nm is
preferably greater than 500 MHz-km, e.g. greater than 750 MHz-km,
greater than 1000 MHz-km, or greater than 1500 MHz-km. The
overfilled bandwidth at 1300 nm is preferably greater than 400
MHz-km, e.g. greater than 500 MHz-km, greater than 700 MHz-km, or
greater than 1000 MHz-km.
[0039] The multimode fiber 10 of FIG. 1B also includes a large
number (more than 50) of small airlines 20 with a maximum width of
no more than 2 .mu.m Some embodiments of fiber 10 have the air fill
fraction (AFF) of 0.1%.ltoreq.AFF.ltoreq.10%. This range of AFF is:
1) high enough to advantageously contribute to good bend loss
performance, for example, less than 1 dB/turn when bent around a 10
mm diameter mandrel, and/or less than 0.2 dB/turn when bent around
a 20 mm diameter mandrel
[0040] The addition of cladding region 14 between the graded index
core and the airline filled cladding ring 18 may increase the
bandwidth at 850 nm by tuning the dispersion of the outer mode
groups, as described in U.S. patent application Ser. No. 12/250,987
filed Oct. 14, 2008, which is incorporated by reference in its
entirety herein. The width of the cladding region 14 is preferably
greater than 0.5 .mu.m and less than 3 .mu.m, e.g. between 0.5 and
2.5 .mu.m, between 0.5 and 2.0 .mu.m or between 0.8 and 1.8
.mu.m.
[0041] An exemplary fiber preform blank 50 for making optical fiber
10 is shown schematically in FIG. 1C. The preform blank 50 of FIG.
1C includes sintered core blank 55 (D of 42.6 .mu.m) surrounded by
a plurality of glass rods 65A (typically 2 to 10 layers of glass
rods) and an overclad silica based layer 80. Preferably, the number
of glass rod layers surrounding the core blank 55 is 2 to 5.
However, in other embodiments, the core blank 55 may be of a
different size, and may include, for example a relatively large
gradient index core portion with a small pure silica layer
surrounding this portion, or a relatively small core portion with a
relatively large layer corresponding to the first cladding region
14 of the fiber 10. This embodiment includes 4 layers of glass rods
65A. It is noted that although glass tubes 65B (not shown) may be
used instead of the glass rods, glass rods are significantly less
expensive than glass tubes, and are adequate for most
applications.
[0042] In accordance with one embodiment of the present invention,
as shown in FIG. 2 a method of making an optical fiber 10 comprises
the steps of: [0043] (i) Providing a sintered core blank 55 (which
can be produced, for example, by steps 1A and 1B shown in FIG. 2).
[0044] (ii) Wrapping (step 2D of FIG. 2) the sintered core blank 55
with a rod ribbon 60 containing at least 50 (and preferably at
least 75, more preferably at least 100) glass: [0045] (a) rods 65A
(which can be produced, for example, by steps 2A-2C shown in FIG.
2), or [0046] (b) tubes 65B (not shown), or [0047] (c) a
combination of rods and tubes, [0048] in order to produce a wrapped
preform 66. [0049] (iii) Heating (Step 2E of FIG. 2) the wrapped
preform 66 so as to fuse or otherwise attach at least a portion of
each of the rods and/or tubes to (a) the core blank 55 and/or (b)
to at least one other rod or tube, thereby creating a core blank
assembly 70. For example, the first row of rods and/or tubes will
be at least partially attached to the core blank 55 and to the rods
and/or tubes of the second row, while the second row of rods and or
tubes will be tacked to the first row of rods and/or tubes and to
the next row of rods and/or tubes. Each of the following rows of
rods and /or tubes, except for the last row will be tacked
(attached) to the preceding row of rods and/or tubes and to the
subsequent row of rods and/or tubes. [0050] (iv) Re-drawing the
core blank assembly 70 to create a smaller diameter cane 75 (Step
2F of FIG. 2). That is, cane 75 has a smaller outer diameter than
the core blank assembly 70. The glass cane 75 will comprise a core
blank portion and one or more rows of tubes and/or rods attached to
one another. According to some exemplary embodiments of the present
invention, the core blank assembly 70 is capped during the re-draw
step, or is pressurized (positive pressure is applied) in order to
keep the interstitial holes open.
[0051] According to some exemplary embodiments the redrawn
(smaller) cane 75 is overclad with glass soot 78 (Step 3A of FIG.
2), and is then consolidated (Step 3B of FIG. 2) to create the
final preform blank 50 (see FIG. 7). Applicants discovered that
surprisingly the glass soot can deposited by a standard deposition
process (e.g., OVD) directly on the uneven surface provided by the
outer layer of rods and/or tubes, and that the soot 78 can then be
consolidated into solid glass (overclad layer 80) by a standard
consolidation process, while the interstitial voids between the
rods (or tubes) remain open, to form the airlines 20 in the
resultant fiber 10. Thus, advantageously, this method introduces
airlines 20 into optical fiber 10, and can be used as an
alternative to other methods of making fibers that contain
airlines. (For example, it results in fiber(s) that can be used as
an alternative to a fiber with random voids of short length (10 m
or less).) In this example, the airlines 67 within the cane 75 are
formed by the spaces between the rods 65A or tubes 65B. Additional
airlines may be formed between rods and tubes or by the cavities
within the tubes, however it is preferable to use simple silica
rods, not expensive tubes, to generate the airlines 67 within the
cane 75. The airlines 67 of cane(s) 75 correspond to the airlines
20 of the optical fiber(s) 10.
[0052] The outer surface of the redrawn cane 75 is uneven due to
the presence of the rods, and the diameter of the cane 75 may vary
up to 1%, but preferably by less than 0.75%. This diameter
variation is essentially periodic or semi-periodic, and is based on
the size of the rods and their location relative to the adjacent
rods. For example, in one embodiment, the average diameter of the
cane 75 was 10 mm (10000 .mu.m) and the average diameter variation
(.DELTA. diameter) was 56 .mu.m. Thus, in this example, the
diameter of the cane 75 was uneven and varied by 0.56% (56
.mu.m/10000 .mu.m). The cane 75 was overclad with silica soot 78
(not shown) which was then consolidated into solid glass (overclad
layer 80) to create the preform blank 50. This preform blank 50 can
then be utilized for drawing of optical fiber 10 (Step 3C of FIG.
2.) Other steps, such as fiber measurement and screening (Step 4A
and 4B of FIG. 2) may also be performed during or after the fiber
draw step (Step 3C).
[0053] It is noted that the redrawn cane 75 can also be inserted
into an overclad tube, heated in order to bond it to the tube to
form a preform blank 50, and then drawn into fiber 10. However, one
advantage of using standard OVD laydown to deposit soot over the
rods 65A, is that costly rod-in-tube processing is avoided.
Therefore, the preferred embodiments of the present method do not
require the purchase or manufacture of high quality large diameter
overclad tube(s) required to contain the plurality of smaller
diameter rods or tubes by the "rod-in-tube" processing methods.
These embodiments of the method according to the present invention
also advantageously avoid the expensive rod stacking step(s)
utilized in rod-in-tube manufacturing.
[0054] Preferably the length of the airlines extends throughout the
fiber length. Because the present method results in a substantially
predetermined and continuous array of airlines the optical fiber 10
can be drawn at high speeds However, it is also understood that
instead of rods 65A with the length that equals or is longer the
length of the core blank, shorter rods 65A may be stacked
length-wise, creating at least some airlines that may not extend
throughout the fiber's length (i.e., in this case the airlines 20
may not be continuous in length).
[0055] One advantage of the present method is that this method
enables the fiber to have a high air fill fraction
(AFF.gtoreq.0.1%) in the cladding ring 18. The AFF the cladding
ring 18 is preferably >0.1%, more preferably >0.5%, even more
preferably >1% and still more preferably >2%. This air fill
fraction AFF enables low bend loss (i.e., bend loss below 1 dB/turn
at 10 mm diameter and/or <0.2 dB/turn at 20 mm diameter.
Preferably, according to some embodiments, the bend loss is less
than 0.1 dB/turn at 20 mm diameter.
[0056] In addition, the present method provides one or more of the
following advantages: [0057] 1. A substantially uniform,
predetermined (designed) airline structure that can be scaled up or
down based on the diameters of rods 65A (or tubes 65B), and/or the
change in airline sizes during the redraw step. [0058] 2. A way to
mechanically create the airlines in a relatively low manual labor
operation as compared to standard stack and draw processes,
resulting in a less expensive fiber; [0059] 3. It enables standard
soot overclad (technical and commercial) for low cost processing as
compared to the cost tube used in standard rod and tube processes;
[0060] 4. The preform can be designed and assembled to provide for
optimum core blank diameter, filler rod diameter, and void space
requirements.
[0061] The invention will be further clarified by the following
exemplary method of making optical fiber.
EXAMPLE 1
[0062] For example, in order to produce a fiber similar to that of
FIG. 1A, the core blank 55 was wrapped in a ribbon 60. The ribbon
60 contained a plurality of glass rods 65A. This ribbon was wrapped
around the core blank 50 four times, producing the wrapped preform.
The wrapped preform was heated by a flame torch to soften the glass
sufficiently so as to tack the rods 65A to each other and to the
core blank 55 in order to produce the core blank assembly 70. This
core blank assembly was then redrawn to a smaller diameter cane 75,
while maintaining the relative interstitial spaces between the
glass rods. The cane 75 was than overclad with glass soot (in this
example it was pure silica soot, but other dopants may also be
present, e.g. chlorine, germanium, fluorine, boron and phosphorus)
and consolidated to form the final consolidated glass preform blank
50. It is noted that the soot deposited on the cane 75, after
consolidation, corresponds to the cladding 30 of fiber 10 of FIG.
1A.
[0063] More specifically, in this example we produced a fiber by
wrapping a sintered core blank 55 with a plurality of solid rods
65A to produce the wrapped preform. (See FIGS. 3 and 4) The wrapped
preform was heated to tack the rods to the blank and to each other.
The resultant core blank assembly was redrawn to a cane, which was
then overclad as a normal cane. The size of the rods and their
separation determined the size and amount of airspaces that were
needed to give the fiber 10 the desired void fraction (AFF) in the
annular ring 18.
[0064] As stated above, the airlines resulted from the interstitial
spaces between rods 65A. Alternatively or in addition, tubes 65B
can be used instead of rods for increased air fill. Glass rods are
cheaper than glass tubes, and their use is adequate for most
applications. The preform can be designed and its size selected to
optimize for core blank diameter, filler rod diameter, and void
space requirements.
[0065] As noted previously, fibers with random voids can be
produced by trapping multitudes of gas bubbles in a glass that are
pulled into gas filled voids of random size, length, and axial
distribution within a void containing region. According to the
exemplary embodiments of the present invention, airlines 20 are
produced by utilizing rods, tubes or a combination thereof with
predetermined sizes with respect to the core to provide a narrower
range of airline sizes with more radial and axial uniformity than
those in fibers with random voids. The airlines of fibers 10 are
usually continuous along the length of the fiber. (Note: a broken
rod may result in an airline that is discontinuous along the length
of the fiber). The ease of manufacturing resulted from rolling the
rods around the core preform also results in some variability in
the rod placement, which leads to some range and variability in
airline sizes, but this variability is relatively small when
compared to the variability of voids in the fiber with random
voids.
[0066] PBG fiber stack and draw methods result in far more ordered
and precise airline placement than that resulted from the method(s)
of manufacturing fiber 10 described herein. Each rod or tube used
in PBG fiber stack and draw method(s) is required to be in a
precise lattice structure, which leads to exacting precision of
process and placement, and typically requires subsequent processing
to obtain extremely high air-fill fractions (like etching
processes). Thus, while the voids in the PBF are highly periodic,
and the airlines in the present invention, the airlines 20 are
distributed with less precision than the voids in PDF, or
"semi-randomly".
EXAMPLE(S)
[0067] In this example, the silica rods 65A are positioned next to
each other on a flat surface and the ends of the rods 65A are taped
or bound together to make a continuous ribbon of glass rods. This
is shown in FIG. 3. Then the core blank 55 is laid on top of the
ribbon and is rolled up "sushi-style" to obtain the desired number
of layers, resulting in a wrapped preform. Note, as shown in this
photograph (FIG. 3), we utilized a re-cycled draw blank for this
experiment--hence the tapered draw root end. In preferred
embodiments, a uniform diameter core cane 55 would be wrapped
inside the roll of rods 65A in a similar fashion.
[0068] The wrapped preform shown in FIG. 4. Although we had used
tape (which can be subsequently burned off), epoxies or other ways
of temporarily binding the rods together at the ends may also be
utilized. Also, while several columns of tape were used in this
example, only the ends of the rods need to be connected together.
Alternatively, even the ends of the rods may not be required to be
taped or bound together before the wrapped preform is tacked
together with heat.
[0069] Subsequently, the wrapped preform of FIG. 4 was taken to a
flame working lathe and the ends of the rods were tacked together
and to the core blank. The tape was then removed. The partially
tacked core blank assembly was then taken for experimentation to
determine operating conditions for tacking the core blank assembly
fully together.
[0070] In this exemplary embodiment, we utilized a precision redraw
furnace to tack the rods 65A fully to each other (along the rod
length) and to the core blank 55. Another option would be to make a
simple and cheap induction coil and traverse apparatus to be used
for this operation, rather than using a relatively expensive
production redraw furnace.
[0071] More specifically, in this exemplary embodiment, in order to
tack the rods 65A to each other and to the core blank 55, the
entire core blank assembly was run through the furnace at 21.2
mm/min downfeed rate and at the temperature of 2000.degree. C. The
core blank assembly 70 was then cooled and examined. The rods 65A
and the core blank were found to be fully tacked together (i.e.,
they were attached to one another along the entire length of the
rods). It is noted that when the tacking process was performed at
the same downfeed (DF) rate, but at the temperature of 1800.degree.
C., some rods were tacked together, and some were not. When the
tacking process was performed at the same downfeed rate, but at the
temperature of 1900.degree. C., more rods were tacked together, but
not all.
[0072] The next step was to determine how to redraw the almost 60
mm diameter core blank assembly 70 to a 10 mm diameter cane 75,
while not melting the rods 65A so much as to close off the void
spaces in between the rods. We utilized a precision redraw furnace.
During the re-draw process, in order to keep the interstitials
holes open, a high torque (15-100 Nm) is utilized. The optimum
torque is a function of preform viscosity. High viscosity requires
high torque. Both the redraw speed and redraw temperature control
preform glass viscosity. For example, higher redraw speed leads to
higher torque and higher temperature leads to lower torque. Capping
the core blank assembly 70 during re-draw process to trap
interstitial gases, or pressurizing interstitial gaps helps to keep
the interstitial holes open and to promote the creation of the
airlines in fiber 10. It is preferable, for some embodiments, that
the torque during redraw is in the 15 to 100 Nm range, for example
15 Nm to 50 Nm, or 20 Nm to 50 Nm, or 25 Nm to 50 Nm.
[0073] Table 2 depicts two exemplary re-draw conditions. In this
embodiment the gob temperature was set to 2150.degree. C. Note the
set points, such as for downfeed (DF) rate, were controllable only
to a single degree C., but reportable to the hundredths. The first
set of conditions (#1) was not successful because many of the
interstitial holes closed. The second set of redraw conditions (#2)
was successful, and the (smaller diameter) redrawn cane 75 retained
the interstitial holes. FIG. 5 is a photograph of an enlarged
cross-section of one exemplary 10 mm diameter redrawn cane 75
(which includes a core blank portion with one row of rods tacked
thereto), and corresponds to the redraw step under condition #2
(see Table 2). This figure shows that the interstitial spaces
between the rods and the core blank were maintained after the
re-draw step. More specifically, the interstitial spaces shown in
this photograph (FIG. 5) are about 26 .mu.m to 36 .mu.m in height
(radial distance) and about 25 .mu.m to 65 .mu.m in width (along
the periphery of the portion corresponding to the core blank).
TABLE-US-00001 TABLE 2 Cane redraw Furnace Tractor DF rate Temp
Speed Conditions (mm/min) (.degree. C.) (mm/min) Torque (Nm)
Comments #1 3.06 2001 125 14.1 voids gone #2 6.12 1998 250 28.2
Excellent void control
[0074] After determining how to successfully build, tack and redraw
canes for the fiber manufacturing process, we fabricated a final
preform blank 50 with 3 to 4 rings of holes. A 40 mm diameter core
blank 55 was wrapped with 1 mm diameter silica rods which
overlapped to produce from 3 to 4 rows of holes. The photograph of
FIG. 6 is a partial cross section of the "lacked assembly" after
the redraw step--i.e., the cross section of the resultant cane 75.
Note that some portions of the rods are missing due to breakage
when the cane 75 was cut with a saw to produce the photograph of
FIG. 6. The scale length measurement in this photograph corresponds
to 117 microns.
[0075] Note that the surface of the cane 75 was "rough" due to the
presence of the rods. About 2700 grams of silica soot overclad 78
was applied to the 1 meter long, 10 mm diameter cane 75. Standard
overclad consolidation of soot 78 was then successfully performed
to produce an overclad layer 80. This layer 80 corresponds to the
glass outer cladding 30 of the Fiber 10 of FIG. 1A. A cross section
was taken from the tip and is shown in FIG. 7. The blank 50 was
drawn into a fiber 10. The resultant fiber was a scaled down
version of the fiber of FIG. 1A, with an outer diameter of 125
.mu.m and an air filled fraction in region 18 greater than 1%.
[0076] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention
without departing from the spirit and scope of the invention. Thus
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *