Increasing the retention of Ge0.sub.2 during production of glass articles

A method for forming a GeO.sub.2 -doped SiO.sub.2 glass article by depositing glass particles to form a porous preform and then drying and sintering the porous preform. A precursor of SnO.sub.2 is also present in the reactant stream used to form the particles, whereby the reaction produces particles of glass that contain GeO.sub.2, SiO.sub.2 and SnO.sub.2. The presence of SnO.sub.2 in the particles reduces the reaction of GeO.sub.2 with chlorine to form GeCl.sub.4 during the drying step. The GeCl.sub.4 that would have formed would have either escaped from the porous preform or caused GeO.sub.2 to be re-deposited in an undesirable portion of the preform. The retention of GeO.sub.2 in the article is therefore enhanced.

BACKGROUND OF THE INVENTION 
This invention relates to an improved method for making GeO.sub.2 -doped 
glass articles. A specific application of the invention is the production 
of optical waveguide fibers and, in particular, preforms from which such 
fibers can be produced. 
Optical waveguide fibers consist of a core surrounded by cladding material 
having a refractive index lower that that of the core. Depending on the 
type of fiber and its desired performance characteristics, the radial 
distribution of the refractive index across the face of the fiber can be 
simple or complex. For example, single-mode fibers typically have a 
refractive index profile which is a simple step, i.e., a substantially 
uniform refractive index within the core and a sharp decrease in 
refractive index at the core-cladding interface. On the other hand, to 
produce a high bandwidth, multimode fiber requires achieving a nearly 
parabolic radial refractive index profile in the fiber core so as to 
minimize intermodal dispersion. 
Optical fibers can be prepared by various known techniques. The present 
invention is concerned with those techniques such as the outside vapor 
deposition (OVD) technique and the axial vapor deposition (AVD) technique 
wherein a porous glass preform is formed and then consolidated. 
Preforms produced by vapor deposition techniques typically are composed of 
silicon dioxide (SiO.sub.2) selectively doped with at least one metal or 
metalloid oxide to provide the desired refractive index profile. The 
preferred dopant in commerical use today is germanium dioxide (GeO.sub.2). 
In processes for manufacturing optical fibers, precursors for the 
deposition of metal oxide dopants are relatively expensive raw 
ingredients. It is therefore important that the dopant be effectively 
incorporated in the preform with a minimum of dopant loss during 
processing. 
In accordance with the OVD technique, which will be discussed during a 
description of the invention, glass particles can be formed by oxidizing 
and/or hydrolyzing the halide materials SiCl.sub.4 and GeCl.sub.4 in a 
burner. The preform is formed from the glass particles by moving the 
burner back and forth along the length of a rotating mandrel. See U.S. 
Pat. No. 4,486,212, for example. The distance between the mandrel and the 
burner is selected so that the glass particles collect on the mandrel in 
thin layers with each pass of the burner. The amount of halide materials 
supplied to the burner is adjusted during the glass laydown process so as 
to produce a dopant concentration in the preform which varies with radius. 
This dopant concentration profile is selected so that the finished fiber 
will have the desired refractive index profile. 
The mandrel is removed from the porous preform, thereby forming an 
aperture. The porous preform is then placed in a consolidation furnace 
where it is dried and sintered. During the drying step or during the 
entire consolidation process, depending upon the particular consolidation 
process employed, a first drying gas mixture, which usually contains 
helium and a drying agent such as chlorine or fluorine, flows into the 
aperture. A drying agent can also be flowed through the furnace (see, for 
example, U.S. Pat. No. 4,165,223). The drying step reduces the residual OH 
content of the preform, thereby reducing in the resultant optical fiber 
the absorption loss caused by OH groups in the vicinity of the 1300 nm 
operating wavelength. The step of sintering a porous preform produces a 
dense, substantially clear glass article which itself can be drawn into 
the optical fiber or which can be provided with additional cladding and 
then drawn into an optical fiber. The entire porous preform can be dried 
before the sinter step begins; alternatively, the preform can be subjected 
to a gradient consolidation process whereby the temperature of each 
individual element of the preform increases and decreases with the 
approach and passing of the hot zone, respectively. As the hot zone 
approaches, the preform element becomes sufficiently hot that the drying 
gas mixture can react with the OH ions in the glass, but the preform 
temperature is not so high that preform porosity is decreased to the point 
that drying gas flow is impeded. As the preform element is subjected to 
the maximum temperature region of the hot zone, the pore size decreases 
and the preform element then completely sinters and clarifies. 
During the consolidation process, dopant from the core portion of a porous 
preform can migrate through the pores to the cladding portion, thereby 
creating a dopant depleted region at the edge of the core and a 
corresponding dopant rich region in the adjacent cladding; this 
combination is known as a "diffusion tail". Moreover, in a multimode fiber 
wherein a central region of the core has a higher dopant concentration 
than an adjacent region of greater radius, dopant can migrate from the 
region of higher concentration to the region of lesser concentration to 
alter the core refractive index profile. 
Two features of the refractive index profile, the central dip and the 
diffusion tail, have been recognized as limiting the optical performance 
of optical fibers. The central dip has been shown to be correlated with 
decreasing the optical bandwidth of the fiber. Modelling has revealed that 
the diffusion tail has the effect of increasing the optical attenuation of 
the fiber. Moreover, the migration of germanium out of the preform is very 
costly, especially in a process for fabricating multimode optical fibers. 
A multimode fiber fabrication process requires a large amount of germanium 
source material in order to produce cores having greater radii and greater 
refractive indices as compared to single-mode fibers. Therefore, processes 
which could retain more germania in the sample could result in the 
production of more product (increased select), a better performance 
distribution, and a capital avoidance of buying germanium-containing 
source materials. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to improve the process 
for producing porous GeO.sub.2 -doped glass preforms. More particularly, 
the objects of the invention include: 1) reducing the amount of germanium 
containing precursors used in the formation of consolidated draw blanks 
from which optical fibers are drawn and 2) improving the refractive index 
profiles of germanium containing optical fiber draw blanks. 
Briefly, the present invention relates to a process for forming a GeO.sub.2 
-doped SiO.sub.2 -based glass article. A reactant stream which includes 
precursors of SiO.sub.2 and GeO.sub.2 is flowed to a reaction zone. The 
precursors are reacted to form a stream of glass particles, and the 
particles are collected to form a porous preform. The porous preform is 
dried and sintered to form a clear glass article. In accordance with the 
invention, the reactant stream includes a precursor of an oxide of a metal 
M which in its oxide state is not a glass former with SiO.sub.2 and which 
decomposes to provide oxygen to reduce the reaction of GeO.sub.2 with 
chlorine to thereby enhance the retention of GeO.sub.2 in the article 
during the step of drying.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To achieve the above-mentioned and other objects, the invention provides a 
stabilized optical fiber preform fabrication process in which 1) the 
efficiency of GeO.sub.2 incorporation into the porous preform produced by 
the laydown step is slightly increased and 2) the tendency of the 
GeO.sub.2 to move within the preform and either redeposit in an 
undesirable location or leave the preform is reduced. Since this technique 
is especially applicable to a method for forming draw blanks from which 
optical fibers are drawn, that method will be specifically described 
herein. The improvement in efficiency of retention of GeO.sub.2 in a 
silica-based glass article is accomplished by initially co-doping the 
deposited GeO.sub.2 --SiO.sub.2 porous glass with an oxide of a metal M 
selected from the group consisting of tin, antimony and bismuth. For 
example, tin in the form of SnCl.sub.4 can be mixed with chlorides of 
germania and silica in the reactant gas mixture that is fed to the burner 
to produce SnO.sub.2 -doped germania silicate porous preforms. The 
addition of SnO.sub.2 to GeO.sub.2 -containing SiO.sub.2 glass particles 
in an OVD soot deposition technique has resulted in the retention of up to 
37% more GeO.sub.2 in the consolidated core preform or core cane as 
compared to a similar process wherein no SnO.sub.2 is deposited. By "core 
cane" is meant a preliminary glass article that includes the core portion 
of the resultant optical fiber and optionally includes some of the 
cladding portion. The core cane is then overclad with cladding glass 
particles and consolidated to form a draw blank from which the optical 
fiber is drawn. It is noted that other source materials such as 
organometallics can also be used to deposit these oxides. The improvement 
in the retention of GeO.sub.2 in a silica-based glass article is 
accomplished without adversely affecting the refractive index profile of 
the resultant preform. Although the formation of germania silicate glass 
articles is specifically discussed herein, the present invention is also 
applicable to the formation of germania silicate glasses that also contain 
one or more dopants such as P.sub.2 O.sub.5, B.sub.2 O.sub.3 and the like. 
There is only a small increase in germania laydown efficiency when tin 
oxide is employed as a co-dopant in the deposition of germania-silicate 
soot (glass particles) on a bait rod to form a porous preform. 
Unconsolidated germania silicate porous preforms were prepared with and 
without tin to compare the germania concentration. The deposition system 
employed a burner of the type shown in FIG. 3 of U.S. Pat. No. 4,165,223, 
which is incorporated herein by reference. The burner included a face 
having a centrally located fume orifice surrounded by concentric rings of 
orifices. The rings of orifices, which are referred to in Tables 1 and 2, 
are the inner shield orifices IS, the premix orifices, and the outer 
shield orifices OS, named in order of increasing radius. The reactant 
compounds emanate from the fume orifice where they are subjected to heat 
from a flame produced by the fuel gas and oxygen emanating from the premix 
orifices. Streams of oxygen flow from orifices IS and OS. These soot 
samples were made using a bubbler delivery system of the type disclosed in 
U.S. Pat. No. 3,826,560. The heated bubblers contained SiCl.sub.4, 
GeCl.sub.4, and SnCl.sub.4 maintained at the temperatures indicated in 
Tables 1 and 2. The tables summarize the temperature and flow conditions 
used; all flows are in liters per minute (lpm). The flows through the 
reactants (first three columns) indicate the oxygen flows through the 
bubblers containing those reactants. In each case a soot preform was built 
up for 30 minutes on an alumina bait rod and removed for analysis. 
TABLE 1 
______________________________________ 
SiCl.sub.4 
GeCl.sub.4 
SnCl.sub.4 
Premix 
Premix 
IS OS Fume 
@43.sub.E C 
@43.sub.E C 
@55.sub.E C 
CH.sub.4 
O.sub.2 
O.sub.2 
O.sub.2 
O.sub.2 
______________________________________ 
2.0 0.5 0.1 6.0 5.82 2.0 3.6 1.0 
2.0 0.5 0.2 6.0 5.82 2.0 3.6 1.0 
2.0 0.5 0.3 6.0 5.82 2.0 3.6 1.0 
2.0 0.5 0.4 6.0 5.82 2.0 3.6 1.0 
2.0 0.5 0.5 6.0 5.82 2.0 3.6 1.0 
2.0 0.5 0.6 6.0 5.82 2.0 3.6 1.0 
2.0 0.5 0.8 6.0 5.82 2.0 3.6 1.0 
2.0 0.5 1.0 6.0 5.82 2.0 3.6 1.0 
______________________________________ 
Curve 10 of FIG. 1 is a graph showing GeO.sub.2 concentration in 
unconsolidated soot preforms as a function of the concentration of 
SnO.sub.2 in the soot particles. The diamond-shaped data points on curve 
10 show the effect of the addition of SnCl.sub.4 at the different 
concentrations indicated in Table 1 to fixed flow rates of germania and 
silica. The flow rates of oxygen through the SnCl.sub.4 -containing 
bubbler are given for most of the data points on curve 10. 
The second set of data (represented on the plot of FIG. 1 by the square 
data points) shows four samples made under the same flow conditions 
described above except that SnCl.sub.4 is not added to the reactant gas 
mixture. As shown in the column labeled "Fume O.sub.2 " of Table 2, the 
oxygen which would have normally flowed through the tin tetrachloride 
bubbler is added to the fume stream to be certain that that oxygen which 
had delivered the tin was not responsible for the increase in GeO.sub.2 
retention. Even though the extra oxygen in the fume stream slightly 
increases the GeO.sub.2 content of the unconsolidated soot preform, it is 
not responsible for the large GeO.sub.2 increase that has been observed in 
the consolidated glass article. 
TABLE 2 
______________________________________ 
SiCl.sub.4 
GeCl.sub.4 
SnCl.sub.4 
Premix 
Premix 
IS OS Fume 
@43.sub.E C 
@43.sub.E C 
@55.sub.E C 
CH.sub.4 
O.sub.2 
O.sub.2 
O.sub.2 
O.sub.2 
______________________________________ 
2.0 0.5 0.0 6.0 5.82 2.0 3.6 1.0 
2.0 0.5 0.0 6.0 5.82 2.0 3.6 1.2 
2.0 0.5 0.0 6.0 5.82 2.0 3.6 1.4 
2.0 0.5 0.0 6.0 5.82 2.0 3.6 1.6 
______________________________________ 
These experiments show that there is only about a 10% increase in germania 
concentration in the unconsolidated porous preform due to the presence of 
SnO.sub.2. Such a small increase in germania concentration at this point 
in the process cannot account for the enhancements in germania 
concentration of up to 37% that have been observed after the porous 
preforms are dried and sintered. 
The main beneficial effect of the tin occurs in the consolidation furnace 
where the system kinetics and thermodynamics support an enhancement in 
germania retention. Stannous dioxide exists in two phases, mostly 
crystalline SnO.sub.2 (cassiterite) with some amorphous SnO.sub.2 in the 
SiO.sub.2 /GeO.sub.2 matrix. The crystalline SnO.sub.2 reacts with 
chlorine faster than that tied up in the matrix to produce O.sub.2 in 
accordance with equation (1). 
EQU SnO.sub.2 +2Cl.sub.2 .div.SnCl.sub.4 +O.sub.2 (1) 
Due to the high reactivity of the SnO.sub.2 crystals with chlorine, very 
little SnO.sub.2 remains in the glass. Some quantity of SnO.sub.2 
decomposes into SnO and provides additional O.sub.2 in accordance with 
equation (2). 
EQU 2SnO.sub.2 .div.2SnO+O.sub.2 (2) 
The additional O.sub.2 from the above reactions causes GeO.sub.2 to be 
retained instead of producing volatile GeCl.sub.4. One likely mechanism is 
that oxygen from reactions (1) and (2) shifts the reaction of GeO.sub.2 
with chlorine to the left as shown in reaction (3) to form GeO.sub.2. 
EQU GeO.sub.2 +2Cl.sub.2 }.about..vertline. GeCl.sub.4 +O.sub.2(3) 
Chlorine will react at a fast rate with species that are not bound to the 
glass matrix. If the additional oxygen was not available to cause 
GeO.sub.2 to be retained, GeCl.sub.4 would either leave the glass as a gas 
species, or react with oxygen to re-deposit GeO.sub.2 in another portion 
of the porous preform such as the cladding. 
Bismuth and antimony are expected to have an effect similar to that of tin. 
The oxides of these elements are not expected to form an amorphous network 
with silica or germania. These oxides easily give up oxygen as they are 
heated to 900 C. As can be observed in Table 3, it would be expected that 
the oxygen provided from the decomposition of oxides of bismuth and 
antimony would also help to retain germania. The bismuth and antimony 
should be easily removed by the chlorine drying step. 
TABLE 3 
______________________________________ 
Oxide Melting Point (.sub.E C) 
Boiling Point (.sub.E C) 
______________________________________ 
Sb.sub.2 O.sub.5 
-0 380 Sb.sub.2 O.sub.3 1.550 sublimes 
-20 930 
Sb.sub.2 O.sub.3 656 
Bi.sub.2 O.sub.5 
-0 150 Bi.sub.2 O.sub.3 1890 
-20 357 
Bi.sub.2 O.sub.3 825-860 
______________________________________ 
Microprobe analyses of optical fiber "core canes" (described below) 
demonstrate that consolidated glass that is formed by a process that 
employs SnCl.sub.4 in the soot laydown step has a much larger 
concentration of germania than consolidated glass that is formed by a 
process that does not employ SnCl.sub.4 in that step. Single-mode fibers 
are often made by forming a soot core preform that includes the layers of 
glass soot that are required to form the fiber core and a few layers of 
cladding soot (see U.S. Pat. No. 4,486,212). The soot core preform is 
consolidated to form a core cane that is thereafter overclad with cladding 
soot to form a preform that is consolidated and drawn into an optical 
fiber. The germania retention enhancement feature of the present invention 
can be shown by analyzing the consolidated core cane. 
Core canes were made by a process similar to the process described above in 
connection with Tables 1 and 2 for forming soot preforms. The rates of 
oxygen flow through the bubblers containing SiCl.sub.4, GeCl.sub.4 and 
SnCl.sub.4 and the inner shield oxygen, outer shield oxygen and fume 
oxygen are given in Table 4. 
TABLE 4 
______________________________________ 
Porous SiCl.sub.4 
GeCl.sub.4 
SnCl.sub.4 
IS OS Fume 
Preform 
@43.sub.E C 
@43.sub.E C 
@55.sub.E C 
O.sub.2 
O.sub.2 
O.sub.2 
______________________________________ 
#8055-18 
1.4 0.8 0.0 2.0 3.6 1.0 
#8055-19 
1.4 0.8 1.0 2.0 3.6 1.0 
______________________________________ 
Premix CH.sub.4 was linearly ramped from 8.5 lpm to 15 lpm and the premix 
O.sub.2 was linearly ramped from 8.24 lpm to 14.55 lpm during the 
formation of both preforms. The concentration of SnCl.sub.4 in the 
reactant gas mixture used to produce porous preform #8055-19 was 13 vol. 
%, whereas no SnCl.sub.4 was used to make porous preform #8055-18. The 
porous preforms were consolidated to form core canes, the identifying 
numbers of which are the same as those of the corresponding preforms. The 
porous preforms were consolidated under identical conditions. The mandrel 
was removed from the preform to form a tubular porous preform having a 
longitudinal aperture. The preform was lowered into a 1510.sub.E C hot 
zone of a consolidation furnace at a rate of 6 mm/minute. Helium flowed 
upwardly through the furnace muffle at a rate of 40 lpm. A drying gas 
mixture of 0.66 lpm helium and 0.042 lpm chlorine flowed into the 
longitudinal aperture. 
FIG. 2 shows microprobe analyses for the resultant consolidated core canes. 
Curve 14 represents the microprobe analysis for core cane 8055-19, and 
curve 16 represents the microprobe analysis for core cane 8055-18. The 
total amount of germania in core cane 8055-19 is about 37% greater than 
that in core cane 8055-18. The total amount of germania in each of core 
canes 8055-19 and 8055-18 was determined by fitting curves 14 and 16 
between points A and B to high order polynomials (12-14th order) and 
integrating over the radial coordinate. It is noted that the shapes of 
curves 14 and 16 are substantially independent of whether tin is present 
during the soot laydown process. These results demonstrate that the 
concentration of germania could be increased without sacrificing the 
control of the desired refractive index profile. A tin precursor reactant 
can therefore be used without radically altering the manufacturing recipe. 
Microprobe analysis has not detected tin in the final glass samples (below 
detection limit of 0.01 wt. %). Apparently, tin performs its function of 
enhancing the retention of germania in the consolidated preform and then 
for all practical purposes vacates the preform. 
To ascertain whether the mechanical and optical properties of optical 
fibers are adversely affected by adding SnCl.sub.4 to the reactant gas 
mixture, optical fibers were made by a process similar to the that 
described above in connection with Tables 1 and 2 for forming soot 
preforms. Bubblers containing SiCl.sub.4 and GeCl.sub.4 were held at 
43.sub.E C, and a bubbler containing SnCl.sub.4 was held at 55.sub.E C. 
Initial flow rates to the burner were: 2.0 lpm oxygen through the bubbler 
containing SiCl.sub.4, 0.5 lpm oxygen through the bubbler containing 
GeCl.sub.4, 0.4 lpm oxygen through the bubbler containing SnCl.sub.4, 6 
lpm Premix CH.sub.4, 5.82 lpm Premix O.sub.2, 2.0 lpm IS oxygen, 3.6 lpm 
OS oxygen and 1.0 lpm fume oxygen. The flow rates of SiCl.sub.4, IS 
oxygen, OS oxygen and fume oxygen remained constant during the entire run, 
the duration of which was 14,040 seconds. The flow rate of SnCl.sub.4 
remained constant at 0.4 lpm during the first 5400 seconds of the run, 
during which the core portion of the preform was deposited. The flow rates 
of CH.sub.4 and Premix O.sub.2 are listed in Table 5 as well as the flow 
rate of oxygen through the bubbler containing GeCl.sub.4. 
TABLE 5 
______________________________________ 
Time GeCl.sub.4 Premix Premix 
(sec) @43.sub.E C CH.sub.4 
O.sub.2 
______________________________________ 
0 0.5 6.0 5.82 
675 0.475 6.0 5.82 
676 0.475 6.0 5.82 
1350 0.475 6.0 5.82 
1351 0.475 6.0 5.82 
3375 0.35 6.0 5.82 
3376 0.35 6.0 5.82 
4725 0.4 6.0 5.82 
4726 0.4 6.0 5.82 
5400 0.4 6.0 5.82 
5401 6.0 5.82 
14040 14.0 13.58 
______________________________________ 
When there is a difference between two adjacent flow rates listed in the 
same column in Table 5, there is a linear change in flow rate between the 
two listed rates. For example, the flow rate of GeCl.sub.4 is listed as 
0.5 lpm at 0 seconds and 0.475 lpm at 675 seconds. The flow rate of 
GeCl.sub.4 changes linearly from 0.5 lpm to 0.475 lpm between the start of 
the run and 675 seconds. Similarly, the flow rate of Premix CH.sub.4 
changes linearly from 6 lpm to 14 lpm between 5401 seconds and 14,040 
seconds. 
The burner deposited a porous core preform by traversing back and forth 
over a 50.8 cm length of an alumina mandrel having a diameter that tapered 
from 5.88 mm to 4.95 mm. The mandrel was pulled from the porous preform to 
form an aperture therein. The tubular porous preform was inserted into a 
consolidation furnace where it was consolidated as described in U.S. Pat. 
Nos. 4,165,223 and 4,486,212. The maximum furnace temperature was set at 
1472.sub.E C. The downfeed rate of the porous preform through the hot zone 
was 3.5 mm/minute. A gas mixture of 20 lpm helium and 0.2 lpm chlorine 
flowed upwardly through the furnace muffle, while a gas mixture of 0.5 lpm 
helium and 0.1 lpm chlorine flowed into the preform aperture. 
The consolidated core cane was mounted in a draw furnace where its tip was 
heated. A vacuum source was affixed to the opposite end. While an 
intermediate fiber having a diameter of 7 mm was drawn, the aperture 
closed. The intermediate fiber was severed into rods, one of which was 
employed as a mandrel upon which cladding soot was deposited to a diameter 
of approximately 80 mm. The resultant composite preform was consolidated 
to form a draw blank that was drawn into an optical fiber having an 
outside diameter of 125 .mu.m and a core diameter of about 9 .mu.m. 
The spectral attenuation curve for the resultant fiber is shown in FIG. 3. 
The attenuation at 1570 nm is 0.280 dB/km. The strength of the resultant 
fiber was as good as that of a standard telecommunication fiber. 
The entire core portion of the above-described draw blank contained 
GeO.sub.2, and the cladding portion consisted of pure silica. The method 
of this invention could also be employed to make optical fibers having 
cores containing other dopants in addition to GeO.sub.2. Moreover, to form 
an optical fiber such as a dispersion shifted fiber, the core could be 
made of more than one annular region, at least one of which contained 
GeO.sub.2 and at least one of which contained no GeO.sub.2. The cladding 
could be formed of silica doped with fluorine or boron or even with a 
dopant that increases the refractive index of silica, provided that the 
refractive index of the cladding does not exceed that of the core.