U.S. patent application number 10/231865 was filed with the patent office on 2004-03-25 for low loss optical fiber and method for making same.
Invention is credited to Ball, Laura J., Baney, Bruno P. M., Bookbinder, Dana C., House, Keith L., Khrapko, Rostislav R., Schiefelbein, Susan L..
Application Number | 20040057692 10/231865 |
Document ID | / |
Family ID | 31976846 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040057692 |
Kind Code |
A1 |
Ball, Laura J. ; et
al. |
March 25, 2004 |
Low loss optical fiber and method for making same
Abstract
A method of forming an alkali metal oxide-doped optical fiber by
diffusing an alkali metal into a surface of a glass article is
disclosed. The silica glass article may be in the form of a tube or
a rod, or a collection of tubes or rods. The silica glass article
containing the alkali metal, and impurities that may have been
unintentionally diffused into the glass article, is etched to a
depth sufficient to remove the impurities. The silica glass article
may be further processed to form a complete optical fiber preform.
The preform, when drawn into an optical fiber, exhibits a low
attenuation.
Inventors: |
Ball, Laura J.; (Fountaine
le Port, FR) ; Baney, Bruno P. M.; (Avon, FR)
; Bookbinder, Dana C.; (Corning, NY) ; House,
Keith L.; (Corning, NY) ; Khrapko, Rostislav R.;
(Moscow, RU) ; Schiefelbein, Susan L.; (Ithaca,
NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
31976846 |
Appl. No.: |
10/231865 |
Filed: |
August 28, 2002 |
Current U.S.
Class: |
385/142 |
Current CPC
Class: |
C03B 37/01861 20130101;
C03C 2201/23 20130101; C03B 2207/90 20130101; C03B 2201/31
20130101; C03C 3/06 20130101; C03B 37/01892 20130101; C03C 25/602
20130101; C03B 37/01426 20130101; C03B 2201/54 20130101; C03B
2201/075 20130101; C03C 25/607 20130101; C03C 2201/50 20130101;
C03B 2201/04 20130101; C03B 37/01228 20130101; C03B 2201/03
20130101; C03C 25/68 20130101; C03B 2201/50 20130101; C03B 2201/32
20130101; G02B 6/02 20130101; C03B 2201/20 20130101; C03B 2203/26
20130101; C03B 2201/12 20130101; C03B 37/01807 20130101; C03C
2203/50 20130101; C03B 2201/28 20130101; C03B 37/01211 20130101;
C03C 2201/11 20130101; C03B 2207/30 20130101; C03B 2201/07
20130101 |
Class at
Publication: |
385/142 |
International
Class: |
G02B 006/00 |
Claims
We claim:
1. A method of forming an optical fiber precursor comprising:
diffusing an alkali metal through a surface of a glass article,
said glass article being comprised of at least 80 mole percent
SiO2; and etching the diffusion surface of said glass article, to a
depth sufficient to remove diffused impurities, to form an optical
fiber precursor.
2. The method of claim 1 wherein said glass article in said
diffusing step comprises less than about 100 ppm chlorine.
3. The method of claim 1 wherein said glass article in said
diffusing step comprises less than about 50 ppm chlorine.
4. The method of claim 1 wherein said glass article in said
diffusing step comprises less than about 100 ppb .sup.-OH.
5. The method of claim 1 wherein said glass article in said
diffusing step comprises less than about 20 ppb .sup.-OH.
6. The method of claim 1 wherein said alkali metal in said
diffusing step is K, Na, Li, Cs or Rb.
7. The method of claim 6 wherein prior to said diffusing step said
alkali metal is derived by heating a bromide of K, Na, Li, Cs or
Rb.
8. The method of claim 6 wherein prior to said diffusing step said
alkali metal is derived by heating an iodide of K, Na, Li, Cs or
Rb
9. The method of claim 6 wherein prior to said diffusing step said
alkali metal is derived by heating a fluoride of K, Na, Li, Cs or
Rb
10. The method of claim 1 wherein said glass article in said
diffusing step is a glass tube, and wherein said alkali metal is
diffused through the inside surface of said glass tube.
11. The method of claim 10 wherein said diffusing step comprises
diffusing said alkali metal to a depth of at least about 100
microns from the inside surface of said glass tube.
12. The method of claim 10 wherein said diffusing step comprises
diffusing said alkali metal to a depth of at least about 300
microns from the inside surface of said glass tube.
13. The method of claim 10 wherein said diffusing step comprises
diffusing said alkali metal to a depth of at least about 500
microns from the inside surface of said glass tube
14. The method of claim 1 wherein said glass article in said
diffusing step is a glass rod, and wherein said alkali metal is
diffused through the outside surface of said glass rod.
15. The method of claim 14 wherein said diffusing step comprises
diffusing said alkali metal to a depth of at least about 100
microns from the outside surface of said glass rod.
16. The method of claim 14 wherein said diffusing step comprises
diffusing said alkali metal to a depth of at least about 300
microns from the outside surface of said glass rod
17. The method of claim 14 wherein said diffusing step comprises
diffusing said alkali metal to a depth of at least about 600
microns from the outside surface of said glass rod
18. The method of claim 10 further comprising collapsing said glass
tube to form an intermediate glass article; adding additional glass
to said intermediate glass article to form a complete optical fiber
preform; and drawing said optical fiber preform into an optical
fiber.
19. The method of claim 14 further comprising adding additional
glass to said glass rod to form an optical fiber preform; and
drawing said optical fiber preform into an optical fiber.
20. The method of claim 18 further comprising inserting a glass rod
into said glass tube prior to said collapsing step.
21. The method of claim 1 wherein said etching step comprises
etching away glass from the diffusion surface of said glass article
to a depth of at least about 5% of the diffusion depth of the
alkali metal.
22. An optical fiber wherein the core is comprised of SiO.sub.2 and
an alkali metal oxide of the form X.sub.2O where X is selected from
the group consisting of K, Na, Li, Cs and Rb and
0.035.ltoreq.X.sub.2O.ltoreq- .6 mole percent and wherein said core
is further comprised of less than about 100 ppm chlorine.
23. The optical fiber of claim 22 wherein said core is comprised of
less than about 50 ppm Cl.
24. The optical fiber of claim 22 wherein said core of said optical
fiber is further comprised of less than about 100 ppb .sup.-OH.
25. The optical fiber of claim 22 wherein said core of said optical
fiber is comprised of less than about 20 ppb .sup.-OH.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to a method for
manufacturing a low loss optical fiber, and more particularly,
methods for producing an optical fiber doped with an alkali metal
oxide.
[0003] 2. Technical Background
[0004] Attenuation is a principal limiting attribute of optical
fibers. Optical fiber loss, for example, plays an important role in
setting the limiting distance between optical fiber amplifiers.
This is particularly important in long distance and ultra-long
distance networks such as, for example, undersea applications,
where such amplifiers represent a significant system cost, as well
as a major factor in system reliability. Consequently there is a
tremendous amount of commercial interest in reducing attenuation to
the lowest possible level.
[0005] For silica-based optical fibers used in long distance
telecommunication transmission networks, attenuation losses have
been reduced to the point where most of the remaining attenuation
is due to intrinsic scattering within the glass material. It is
generally accepted that intrinsic scattering is a combination of
losses associated with density and dopant concentration
fluctuations. Density fluctuations are closely proportional to the
glass transition temperature, T.sub.g, defined as the temperature
at which the melt viscosity is 10.sup.13 poise, and may result in
both large and small-angle scattering losses.
[0006] One means of lowering the T.sub.g, and therefore the
attenuation, is to add a modifier to the core glass. Such
modifiers, if chosen appropriately, are capable of significantly
reducing scattering losses in the fiber core, and therefore the
attenuation of the fiber. Alkali metal oxides serve as efficient
modifiers--a concentration of approximately 0.5 mole percent of an
alkali metal oxide can reduce the attenuation of silica glass by as
much as 25%.
[0007] The most common, commercially available optical fibers are
SiO.sub.2-based, and the theoretical lower limit for the
attenuation in such fibers is generally accepted to be about 0.15
dB/km. It is known in the art that some non-SiO.sub.2 glasses, such
as high-alkali alumino-silicate and fluoride glasses, are capable
of achieving losses lower than SiO.sub.2-based fiber, however these
approaches have not yet been commercially realized in long-length
transmission fiber.
[0008] Silica-based glasses containing an alkali metal oxide
dopant, alone or in combination with other compounds, such as, for
example, CaO, Al.sub.2O.sub.3 or F, have been proposed as core
materials for optical fibers having intrinsic scattering losses
lower than that of pure vitreous SiO.sub.2. However, attempts to
manufacture such fibers have resulted in attenuation levels much
higher than the theoretical lower limit. In the case of the
multi-dopant glasses, increased dopant concentration fluctuations
and crystallization have proven difficult to overcome. For both
single- and multi-dopant glasses, high levels of contaminants, such
as, for example, transition metals and .sup.-OH, have made it
difficult to achieve the desired low attenuation. Often these
contaminants are unintentionally introduced during the doping
process.
[0009] Conventional soot-to-glass fiber making processes, such as
outside vapor deposition (OVD) and vapor axial deposition (VAD),
are not well suited to alkali metal oxide doping. One reason for
this unsuitability is the unavailability of simple, easy-to-deploy
high vapor pressure alkali metal source compounds. In addition, the
soot preforms that result from these processes generally contain
H.sub.2O, a combustion by-product generated during the soot laydown
process. This H.sub.2O can disassociate during further processing
of the soot preform to form .sup.-OH. .sup.-OH can have a
deleterious effect on fiber attenuation, particularly when present
in the core of the fiber. Typically, this .sup.-OH is removed by
flowing chlorine through the preform at an elevated temperature.
Unfortunately, this drying step would likewise remove any alkali
metal oxide that would be deposited in an OVD or VAD process by
forming an alkali chloride. Moreover, any alkali chloride that
remained in the preform after the chlorine drying step would form
alkali chloride crystals upon cooling. Such crystals cause the
glass to become opaque, making it unsuitable for the transmission
of light. Further, the alkali metals also increase the
crystallization rate of the silica itself such that the alkali
metal oxide-doped silica soot that would be deposited in an OVD or
VAD process would tend also to crystallize before it can be
sintered into dense, defect-free glass.
[0010] One technique for incorporating an alkali metal oxide into
silica glass is by diffusing an alkali metal directly into
consolidated glass. However, efforts to diffuse alkali metals into
silica glass have suffered from the simultaneous diffusion of
impurities, including transition metals and water, resulting in
losses well above the theoretical minimum. It would be desirable to
develop a method of doping a silica glass optical fiber precursor
with an alkali metal such that a fiber drawn therefrom would have a
low optical loss.
SUMMARY OF THE INVENTION
[0011] The present invention entails the manufacture of an optical
fiber doped with an appropriate material of sufficient purity that
low optical loss can be achieved. By low loss we mean an optical
attenuation preferably less than about 0.18 dB/km at a wavelength
of 1550 nm, more preferably less than about 0.17 dB/km at a
wavelength of 1550 nm, and most preferably less than about 0.16
dB/km at a wavelength of 1550 nm. More specifically, the present
invention relates to a method for diffusing an alkali metal through
a surface of a silica glass article, hereinafter referred to as the
diffusion surface, and etching the diffusion surface to a depth
sufficient to remove impurities that may have been unintentionally
introduced into the glass during the diffusion process. By
diffusion surface we mean the surface of the glass article through
which diffusion of the alkali metal has occurred. The silica glass
article may be in the form of a tube, a rod, or a collection of
tubes or rods, or other glass articles suitable for use as an
optical fiber precursor. Alkali metals exhibit a high diffusion
rate in silica glass when compared with typical contaminants, such
as, for example, transition metals that may be present in minute
quantities in the alkali metal source compound or in the
environment surrounding the silica glass. Whereas the alkali metal
exhibits high diffusivity and is therefore able to move deeply into
the silica glass, transition metal diffusivities are significantly
lower, and the transition metal impurities are consequently able to
diffuse only to shallow depths from the diffusion surface of the
glass. We have discovered that this difference in relative
diffusion depths between the desired alkali metal dopant and the
undesirable impurities advantageously allows these impurities to be
removed by conventional etching techniques subsequent to the
diffusion process, thereby effectively purifying the alkali metal
dopant, with minimal impact on the concentration of alkali metal
oxide in the glass.
[0012] Preferably the silica glass article is essentially free of
chlorine and water. Alkali metals bond strongly with chlorine,
either within the silica glass, or outside of the silica glass, to
form an alkali metal chloride. Such alkali metal chloride, if
formed outside the silica glass, will inhibit the diffusion of the
alkali metal into the glass article. If formed within the silica
glass article, alkali metal chloride crystals would render the
glass opaque and therefore undesirable for the transmission of
light.
[0013] The alkali metal is selected from the group consisting of K,
Na, Li, Cs and Rb. When diffused into a silica glass network, the
alkali metal bonds with oxygen within the glass network to create
an alkali metal oxide of the form X.sub.2O, where X is a member of
the preceding group. Using current manufacturing processes, we have
found that doping with K or Na produces superior optical loss
results over Li, Cs, or Rb and consequently the preferred alkali
metals are K and Na. However, this may change in the future as
manufacturing processes continue to evolve. For the reasons stated
earlier, it is preferred that the alkali metal source compound does
not contain chlorine, but otherwise any alkali metal containing
source compound is suitable.
[0014] 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.
[0015] 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. Where appropriate, identical features
have been identically numbered.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 depicts the diffusion depths for K and Fe, as
determined by measuring the concentrations of K.sub.2O and FeO, and
shows significantly greater diffusion of K than for Fe.
[0017] FIG. 2 illustrates an arrangement to diffuse an alkali into
a silica glass tube showing the relationship between burner and
alkali metal source compound locations.
[0018] FIG. 3 is a comparison of the diffusion depths for .sup.-OH
and K illustrating similar diffusion depths for .sup.-OH and K. The
diffusion depth for K was determined by measuring the concentration
of K.sub.2O.
[0019] FIG. 4 illustrates an arrangement to diffuse an alkali into
a silica glass rod using a plasma burner.
[0020] FIG. 5 illustrates a preferred arrangement for diffusing an
alkali into a silica glass tube.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention relates to a process of manufacturing
a low loss optical fiber. More specifically, the invention relates
to preparing an optical fiber precursor by diffusing an alkali
metal into a silica glass article and, subsequent to the diffusion
process, etching the silica glass article to remove unwanted
contaminants that may have been unintentionally diffused into the
glass. By optical fiber precursor we mean a complete optical fiber
preform, or a precursor to a complete optical fiber preform such
as, for example, a core cane or a deposition tube. By core cane we
mean a consolidated glass precursor to an optical fiber preform
that is not a complete optical fiber preform but which includes at
least a portion of the core. By complete optical fiber preform we
mean a consolidated glass article ready for drawing into an optical
fiber.
[0022] Silica glass doped with an alkali metal oxide has been shown
capable of producing losses below the theoretical lower limit for
pure silica glass. By "doped" or "doping", or equivalent, we mean
the intentional addition of a material or materials to a glass to
achieve desired characteristics (as indicated herein) in such a
glass. One means of producing a low loss optical fiber is by
diffusing an alkali metal into a suitable silica glass article that
is a precursor to an optical fiber. Conventional diffusion
techniques may result in the diffusion of unwanted contaminants,
such as, for example, transition metal ions. However, for a given
set of processing conditions, such as time and temperature, for
example, the diffusion depth of an alkali metal dopant differs
greatly from such contaminants, with the alkali metal exhibiting a
diffusion depth in silica glass much greater than, for example, a
transition metal contaminant. This is illustrated graphically in
FIG. 1. In this example, both potassium and the transition metal
iron were diffused through the interior surface of a silica glass
tube in a method according to the present invention. The tube was
then collapsed without etching. Inside the glass the alkali metal
and the transition metal bond with available oxygen to form
K.sub.2O and FeO within the silica glass network. The depth of
diffusion for both the alkali metal and the transition metal was
determined by measuring the concentration of K.sub.2O and FeO as a
function of radial position using an electron microprobe. Note that
after collapse of the silica glass tube the layer of glass into
which the alkali metal and the transition metal have been diffused,
and which layer extends from the interior surface of the silica
glass tube outward, becomes the central region of the silica glass
rod resulting from the collapse. FIG. 1 shows the relatively
shallow diffusion depth of FeO, depicted by a region about 500
microns in radius concentric with the centerline of the collapsed
silica glass tube, compared to a region approximately 3000 microns
in radius concentric with the centerline of the collapsed silica
glass tube for K.sub.2O. The shallow layer of FeO contaminant
within the glass tube, as indicated by the small diameter region of
FeO in FIG. 1, is preferably removed prior to collapse of the
silica glass tube to achieve a low loss optical fiber. The process
for manufacturing a low loss optical fiber, according to the
present invention, involves diffusion doping a consolidated glass
optical fiber precursor with an alkali metal, and then etching the
diffusion surface with a suitable etchant to remove unwanted
diffused contaminants.
[0023] In one embodiment of the invention, an illustration of which
is provided in FIG. 2, a silica glass tube 10 suitable for the
manufacture of optical fiber is mounted in a glass-working lathe.
One example of an appropriate apparatus is a conventional modified
chemical vapor deposition (MCVD) glass-forming lathe. A reservoir
16 for receiving KBr is formed near one end of tube 10 by forging
two neck-like deformations, 6 and 8, in the wall of tube 10 about 2
cm from each other. The exact composition of tube 10 is dependent
upon the design of the desired optical fiber, however generally
such tubes contain as much as, or more than about 80 mole percent
SiO.sub.2. For the manufacture of many optical fibers tube 10
preferably contains at least about 90 mole percent SiO.sub.2. Such
tubes may also contain dopants, either singly or in combination.
Such dopants may include, for example, F, Al.sub.2O.sub.3, CaO,
GeO.sub.2 or P. Prior to diffusing an alkali metal into tube 10,
additional silica glass may be added to the interior surface of
glass tube 10 through chemical vapor deposition means. Such
additional glass may contain dopants, including, for example, F,
Al.sub.2O.sub.3, CaO, GeO.sub.2 or P. However, for the reasons
indicated previously, it is desirable that tube 10, and any
additional glass deposited on the inside of tube 10, be essentially
chlorine free. By essentially chlorine free we mean exhibiting a
chlorine content sufficiently low that optical losses due to alkali
chloride crystallization is avoided. We have found that a chlorine
content preferably less than about 500 ppm is desired for this
purpose. More preferably, the chlorine content is less than about
100 ppm, and most preferably less than about 50 ppm. In addition,
silica glass tube 10, and any additional glass deposited therein,
should be essentially free of "water". By "water" we mean the
hydroxyl group .sup.-OH. Water is responsible for an absorption
peak at or about 1383 nm, and which absorption peak can extend into
the operating wavelength regions of an optical fiber. This peak has
a detrimental effect on the fiber attenuation. Therefore it is
desirable to reduce the absorption peak, also referred to as the
water peak, by reducing the .sup.-OH content of the glass as much
as possible. This requires that the starting materials be
essentially free of water. By essentially free of water we mean
having an .sup.-OH content preferably less than about 100 ppb, and
more preferably less than about 20 ppb. By way of example, FIG. 3
illustrates a comparison between the diffusivity of potassium and
the diffusivity of .sup.-OH as indicated by the relative depths
between the concentrations of K.sub.2O and .sup.-OH. As shown, the
depth of diffusion for potassium, as determined by measuring the
concentration of K.sub.2O, is similar to the diffusion depth of
.sup.-OH concentration, suggesting that .sup.-OH cannot be removed
from the glass by etching from the diffusion surface without
substantially affecting the concentration of alkali metal oxide.
This further suggests that the most viable means of achieving a low
loss optical fiber according to the present invention is to ensure
that starting glass articles are essentially free from water prior
to diffusing an alkali metal dopant. This can be accomplished, for
example, by conventional chlorine drying techniques during
manufacture of the silica glass tube, and employing suitable
precautions subsequent to its manufacture to prevent rewetting of
the tube. The use of chlorine, however, should be minimized to
reduce chlorine concentrations in the glass. In the case of porous
soot glass articles, drying is preferably accomplished by exposing
the article to a fluorine-containing atmosphere, such as, for
example, CF.sub.4 or SiF.sub.4, or combinations thereof, either
after chlorine drying or in place of it. The exposure to a
fluorine-containing atmosphere is done at temperatures preferably
less than about 1100.degree. C. to avoid doping the glass with high
levels of fluorine. Preferably, the water content of the glass is
less than about 100 ppb, and more preferably less than about 20
ppb.
[0024] Referring again to FIG. 2, once the silica glass tube 10 has
been prepared, including any deposition of additional glass, an
alkali source compound 12 is introduced into tube 10 at reservoir
16 and heated by heat source 18 to form a vapor as tube 10 is
rotated. Alkali metal source compound 12 may be introduced into
reservoir 16 as a liquid or as a solid. Oxygen is flowed into inlet
2 and into tube 10 through rotating seal 4, and the portion of tube
10 downstream of alkali metal source compound 12 is heated to
facilitate diffusion of the alkali metal into the interior surface
of tube 10. The portion of tube 10 downstream of alkali metal
source compound 12 should be heated to a temperature sufficient to
promote rapid diffusion of the alkali and to prevent
devitrification. Preferably, the portion of tube 10 downstream of
alkali metal source compound 12 is heated by heat source 20 to at
least about 1500.degree. C., more preferably at least about
1700.degree. C., and most preferably at least about 2000.degree. C.
Alkali metal source compound 12 is a non-chlorine-containing
compound having as a constituent an element selected from the group
consisting of K, Na, Li, Cs, and Rb. Preferably alkali metal source
compound 12 is a bromide, an iodide, or a fluoride. More preferably
alkali metal source compound 12 is a bromide, an iodide or a
fluoride of K or Na. The peak mole percent of alkali diffused into
the tube, or any additional glass deposited therein, should be
greater than about 0.035%. However, alkali concentrations in excess
of about 6 mole percent are typically not advantageous and may
present significant processing difficulties. The alkali metal is
preferably diffused throughout a depth of at least about 100
microns from the diffusion surface of the tube prior to collapse of
the tube, more preferably at least about 300 microns, and most
preferably at least about 500 microns.
[0025] The diffusion process may be followed by the step of further
heating tube 10 to promote a partial collapse of tube 10 to both
reduce the inside surface area through which the alkali metal might
be lost and to thicken the layer of glass into which the alkali
metal has been diffused. Once the diffusion doping step, or any
partial collapse of the tube, has been completed, the diffusion
surface of the tube is etched with an etchant, suitable for
removing silica glass, to a depth sufficient to remove unwanted
impurities that may have diffused through the diffusion surface of
tube 10. An aqueous HF solution may be used as an etchant. However,
this may result in re-wetting of tube 10 and require subsequent
drying of the tube. Preferably, a fluoride gas such as, for
example, CF.sub.4, SF.sub.6, NF.sub.3, C.sub.2F.sub.6 or a mixture
thereof, is employed. The amount of material removed is dependent
upon processing conditions during diffusion and any partial tube
collapse, but the etching conditions are preferably sufficient to
result in the removal of glass to a depth of at least about 5
percent of the diffusion depth of the alkali metal. Once etching is
finalized, silica glass tube 10 is further heated with heat source
20 to collapse the tube 10 downstream of alkali metal source
compound 12 and form a solid glass rod. The solid glass rod is then
cut to remove that portion of glass containing reservoir 16. The
remaining solid glass rod may constitute an entire optical fiber
preform, or it may constitute core cane, i.e. a portion of an
optical fiber preform that includes at least a portion of the core,
that may be further processed by adding glass material, either
through sleeving with a glass tube, through chemical vapor
deposition, or through other means, to form an entire optical fiber
preform. This additional glass material may constitute core
material, cladding material, or both. The completed preform may be
drawn into an optical fiber that is capable of exhibiting low
optical loss. The optical fiber core made in accordance with the
present invention has a peak alkali metal oxide concentration of at
least about 0.035 mole percent, a water content less than about 100
ppb and preferably less than about 20 ppb, and has a chlorine
content less than about 500 ppm, preferably less than about 100 ppm
and most preferably less than about 50 ppm.
[0026] In another embodiment of the invention, shown in FIG. 4, an
alkali metal source compound is introduced into heat source 40
through inlet 44 while heat source 40 is traversed over rotating
silica glass rod 46. The alkali metal source compound is vaporized
in flame 42, and transported to glass rod 46 by flame 42 such that
alkali metal is diffused into the outside surface of glass rod 46.
The alkali metal is preferably diffused throughout a depth of at
least about 100 microns from the diffusion surface of the rod prior
to further processing of the rod, more preferably at least about
300 microns, and most preferably at least about 600 microns. The
alkali metal source compound may be introduced into heat source 40
as a liquid, as a vapor or as a solid. Preferably the alkali metal
source compound is introduced as a liquid. Silica glass rod 46 is
essentially free of chlorine and water. Silica glass rod 46
preferably contains at least about 80 mole percent SiO.sub.2, with
some fiber designs requiring an SiO.sub.2 content as high as, or
greater than, about 90 mole percent. Because the diffusion surface
of glass rod 46 is exposed directly to heat source 40, it is
preferable that heat source 40 is dry, wherein by dry we mean the
heat source does not contain H.sub.2, .sup.-OH, or H.sub.2O, or
produce H.sub.2, .sup.-OH, or H.sub.2O as a by-product. When carbon
monoxide, for example, is used as a fuel and combined in a burner
flame with oxygen, the only by-product is carbon dioxide. This
carbon dioxide by-product is easily disposed of, and
advantageously, no water is formed from the combustion process.
Thus, the production of essentially water-free silica soot,
preforms and glass is obtainable. As a result, heat source 40 may
be, for example, a CO/O.sub.2 burner or plasma torch. However, this
should not be considered limiting with respect to suitable heat
sources. For the purposes of illustration, a plasma torch is
depicted as the source for flame 42 in FIG. 4.
[0027] When diffusion doping of rod 46 has been completed, the
diffusion surface of rod 46 is etched with an etchant, suitable for
removing silica glass, to a depth sufficient to remove unwanted
impurities that may have diffused through the diffusion surface of
rod 46. Preferably the etching step results in the removal of glass
to a depth of at least about 5 percent of the diffusion depth of
the alkali metal. An aqueous HF solution may, for example, be used
as an etchant. However, this may result in re-wetting of glass rod
46 and require subsequent drying of the rod. Preferably, a fluoride
gas such as, for example, CF.sub.4, SF.sub.6, NF.sub.3,
C.sub.2F.sub.6 or a mixture thereof, is employed. The rod may then
be further processed by adding glass material, either through
sleeving with a glass tube, through chemical vapor deposition, or
through other means, to form an entire optical fiber preform. This
additional glass material may constitute core material, cladding
material, or both. The completed preform may be drawn into an
optical fiber capable of exhibiting low loss. The optical fiber
core has a peak alkali metal oxide concentration of at least about
0.035 mole percent, a water content preferably less than about 100
ppb and more preferably less than about 20 ppb, and has a chlorine
content preferably less than about 500 ppm, more preferably less
than about 100 ppm and most preferably less than about 50 ppm.
[0028] In a further embodiment of the invention, a silica glass
tube 10, as depicted in FIG. 2, is mounted in a conventional MCVD
glass-forming lathe. As in either of the previous embodiments
silica glass tube 10 preferably contains at least 80 mole percent
SiO.sub.2, with as much as, or greater than, about 90 mole percent
SiO.sub.2. Silica glass tube 10 may be doped. Such dopants may
include, for example, F, Al.sub.2O.sub.3, CaO, GeO.sub.2 or P.
Preferably, silica glass tube 10 contains less than about 500 ppm
chlorine, more preferably less than about 100 ppm and most
preferably less than about 50 ppm chlorine. The water content of
silica glass tube 10 is preferably less than about 100 ppb, and
more preferably less than about 20 ppb. An alkali metal source
compound 12 is introduced into tube 10 at reservoir 16 and heated
by heat source 18 to form a vapor. Oxygen is flowed into inlet 2
and into tube 10 through rotating seal 4, and the portion of tube
10 downstream of alkali metal source compound 12 is heated by heat
source 20 to facilitate diffusion of the alkali metal into the
interior surface of tube 10. The alkali metal is preferably
diffused throughout a depth of at least about 100 microns from the
diffusion surface of the tube prior to collapse of the tube, more
preferably at least about 300 microns, and most preferably at least
about 500 microns. Once the diffusion doping step has been
completed, the diffusion surface of the tube is etched with an
etchant, suitable for removing silica glass, to a depth sufficient
to remove unwanted impurities that may have diffused through the
diffusion surface of tube 10. Preferably the etching step results
in the removal of glass to a depth of at least about 5 percent of
the diffusion depth of the alkali metal. An aqueous HF solution may
be used as an etchant. However, this may result in re-wetting of
tube 10 and require subsequent drying of the tube. Preferably, a
fluoride gas such as, for example, CF.sub.4, SF.sub.6, NF.sub.3,
C.sub.2F.sub.6 or a mixture thereof, is employed. After etching is
complete, a silica glass rod is inserted into tube 10. Preferably,
the silica glass rod contains less than about 100 ppb water, and
more preferably less than about 20 ppb. In addition, the chlorine
content of the rod is preferably less than about 500 ppm, more
preferably less than 100 ppm and most preferably less than about 50
ppm. The silica glass rod preferably contains at least about 80
mole percent SiO.sub.2 and for some optical fiber designs the rod
should be comprised of as much as, or more than, 90 mole percent
SiO.sub.2. The silica glass rod may be doped. Such dopants may
include, for example, F, Al.sub.2O.sub.3, CaO, GeO.sub.2 or P. When
the silica glass rod has been inserted into tube 10, the assembly
of tube 10 and the silica glass rod is then heated by a heat source
suitable for collapsing tube 10 onto the silica glass rod to form
an optical fiber precursor. The resulting optical fiber precursor
may constitute an entire optical fiber preform, or it may be
further processed by adding glass material, either through sleeving
with a glass tube, through chemical vapor deposition, or through
other means, to form a complete optical fiber preform. This
additional glass material may constitute core material, cladding
material, or both. The completed preform may be drawn into an
optical fiber capable of exhibiting low loss. In an alternative to
the current embodiment, a silica glass rod 46 may be doped by
diffusing an alkali metal into the outer surface of silica glass
rod 46 as previously described, and as shown in FIG. 4. The alkali
metal is preferably diffused throughout a depth of at least about
100 microns from the diffusion surface of the rod prior to further
processing of the rod, more preferably at least about 300 microns,
and most preferably at least about 600 microns. As in the preceding
description, silica glass rod 46 is essentially free of water and
chlorine, with a water content preferably less than about 100 ppb
and more preferably less than about 20 ppb. Preferably, the
chlorine content silica glass rod 46 is less than about 500 ppm,
more preferably less than about 100 ppm and most preferably less
than about 50 ppm. As before, rod 46 contains at least about 80
mole percent SiO.sub.2, and in some cases, depending upon the
design of the optical fiber, as much as, or more than about 90 mole
percent SiO.sub.2. Silica glass rod 46 may be doped. Such dopants
may include, for example, F, Al.sub.2O.sub.3, CaO or GeO.sub.2 or P
After diffusing the alkali metal into rod 46 in a manner as
previously described, the diffusion surface of rod 46 is etched to
a depth sufficient to remove diffused impurities, and then rod 46
is inserted into a silica glass tube. Preferably the etching step
results in the removal of glass to a depth of at least about 5
percent of the diffusion depth of the alkali metal. The silica
glass tube preferably contains at least 80 mole percent SiO.sub.2,
with as much as, or greater than, about 90 mole percent SiO.sub.2.
The silica glass tube may be doped. Such dopants may include, for
example, F, Al.sub.2O.sub.3, CaO, GeO.sub.2 or P. Preferably, the
silica glass tube contains less than about 500 ppm chlorine, more
preferably less than about 100 ppm and most preferably less than
about 50 ppm chlorine. The water content of the silica glass tube
is preferably less than about 100 ppb, and more preferably less
than about 20 ppb. The assembly of the silica glass tube and rod 46
is heated with a heat source suitable to collapse the tube onto rod
46. The rod and tube assembly of either alternative of this
embodiment may constitute a complete optical fiber preform, or it
may be further processed by adding glass material, either through
sleeving with a glass tube, through chemical vapor deposition, or
through other means, to form an entire optical fiber preform. This
additional glass may constitute core material, cladding material,
or both. The completed preform may be drawn into a fiber that is
capable of exhibiting low optical loss. The optical fiber core has
a peak alkali metal oxide concentration of at least about 0.035
mole percent, a water content less than about 100 ppb and
preferably less than about 20 ppb, and has a chlorine content
preferably less than about 500 ppm, more preferably less than about
100 ppm, and most preferably less than about 50 ppm.
EXAMPLES
[0029] The invention will be further clarified by the following
example.
Example 1
[0030] Referring to FIG. 5, synthetic silica tube 10 having an
inside diameter of 19 mm, an outside diameter of 25 mm and about
0.5 m in length, and containing less than 20 ppm Cl, less than 0.05
weight percent F, less than 10 ppb .sup.-OH and that is
substantially free of transition metal impurities, is mounted in a
conventional MCVD glass forming lathe 38 by chuck 36 and chuck 34
at ends 14 and 24 respectively. Oxygen is flowed into inlet 2 and
into tube 10 through rotating seal 4 at 2 l/min and a backpressure
of 0.75 Torr while tube 10 is simultaneously fire polished at about
2100.degree. C. A reservoir 16 for receiving KBr is formed near end
24 of tube 10 by forging two neck-like deformations, 6 and 8, in
the wall of tube 10 about 2 cm from each other. Deformations 6 and
8 are formed by heating tube 10 to its softening point near end 24
with a hand-held torch while tube 10 rotates and pinching the soft
glass with pincers common to the glass-blowing arts. End 24 is
removed from its respective chuck 34 in glass-making lathe 38 and a
quantity of KBr 12 is positioned inside reservoir 16. End 24 of
tube 10 is then remounted in chuck 34 and reservoir 16 is heated to
about 1000.degree. C. by heat source 18, external to tube 10 and
located at reservoir 16, while dry nitrogen is flowed into inlet 2
and tube 10 is rotated. The KBr 12 melts and is dehydrated. Tube 10
is then cooled and end 24 is removed from chuck 34. A second silica
glass tube 22, having the same characteristics with respect to
size, water content and chlorine content as tube 10, and having
been fire polished in a manner similar to tube 10, is placed in
chuck 34 at end 32, and the free end 26 of tube 22 is brought into
contact with the free end 24 of tube 10 at interface 28. Dry
nitrogen is flowed into inlet 2 and into tube 10 through rotating
seal 4, during which time end 24 of tube 10 is welded to end 26 of
tube 22 at interface 28 to form a composite tube 30 using a
hand-held torch. Composite tube 30 is then rotated continuously and
heat source 18 heats reservoir 16 to produce KBr vapor. Oxygen is
flowed into inlet 2 and into composite tube 30 through rotating
joint 4, over the reservoir of liquid KBr 12, and through composite
tube 30 to transport the KBr vapor through composite tube 30.
Simultaneously, heat source 20 is traversed over composite tube 30
downstream of reservoir 16, heating the portion of composite tube
30 downstream of reservoir 16 to about 2100.degree. C. During this
phase, KBr vapor comes in contact with the inside surface of
composite tube 30 and alkali metal diffuses into the glass through
the interior wall of composite tube 30. Heat source 20 is traversed
a sufficient number of times to achieve an alkali metal oxide depth
of about 300 um and a peak alkali metal oxide concentration of at
least 0.035 mole percent in the interior region of composite tube
30.
[0031] The portion of composite tube 30 downstream of reservoir 16
is next heated and partially collapsed to a bore diameter of about
2 mm, after which the centerline is etched to remove glass to a
depth of about 5% of the diffusion depth of the potassium by
flowing a mixture of C.sub.2F.sub.6 and oxygen into inlet 2 and
through composite tube 30 at a temperature of about 2100.degree. C.
and a back pressure of 0.57 Torr.
[0032] Following the etching step, composite tube 30 is further
collapsed, using conventional techniques, into a solid glass rod.
The solid glass rod is then cut downstream of interface 28 to form
a core cane from that portion of solid glass rod downstream of the
cut. The resulting core cane shows no sign of crystallization and
has a peak K.sub.2O concentration of at least about 0.035 mole
percent. The core cane may be further processed by etching the
outer surface to remove impurities picked up from heat source 20
and the outside environment. The core cane is then placed inside a
synthetic silica tube manufactured by an outside vapor deposition
process and which tube contains less than 20 ppm Cl, less than 10
ppb .sup.-OH, and approximately 1.2 wt. % F. The core cane and tube
assembly is purified and the tube collapsed onto the core cane in a
dry furnace, and then the assembly is further sleeved with a
F-doped silica tube and drawn into a fiber that has an optical loss
less than that of an otherwise identical fiber that is essentially
alkali-free.
[0033] 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.
* * * * *