Process for producing glass preform for optical fiber

A process for producing a porous glass preform for optical fiber by depositing fine glass particles on an outer surface of a glass material while moving the glass material, including the steps of: preheating a portion of the glass material for not less than 5 minutes to clean the portion of the glass material in an apparatus for depositing fine glass particles; and depositing fine glass particles on the portion of the glass material cleaned by the preheating, in the apparatus for depositing fine glass particles.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for forming an aggregate or 
agglomerate of fine glass particles on an outer surface of a starting 
material (e.g., on the circumference of a cylindrical starting material). 
More particularly, the present invention relates to a process for 
producing a glass preform for optical fiber, which comprises a glass 
material and deposit of fine glass particles formed on the circumference 
thereof. Such a glass preform may suitably be used as an intermediate 
product particularly in the fabrication of an optical fiber glass preform 
which is required to have high purity. 
2. Related Background Art 
Heretofore, the so-called "outside vapor-phase deposition process" as 
disclosed in Japanese Laid-Open Patent Application (KOKAI) No. 73522/1973 
(i.e., Sho 48-73522), is known as a process for producing a silica glass 
(or fused silica) tube or preform for optical fiber. In this process, a 
glass tube is produced in the following manner. 
Thus, fine particles of glass such as SiO.sub.2, which may be formed by 
hydrolysis of a raw material such as SiCl.sub.4, are caused to be 
deposited on the circumference of carbon or a refractory or fire-resistant 
starting material such as silica glass or alumina rotating about its 
horizontal axis as the rotational axis. The deposition of the glass is 
stopped after a predetermined amount of the glass particles are deposited, 
and the starting material is pulled out to provide a tubular glass 
aggregate. This tubular glass aggregate is sintered for 
transparency-imparting vitrification under a high temperature atmosphere 
in an electric furnace so as to provide a transparent tubular glass. 
Alternatively, a glass tube or preform may also be formed in the following 
manner. 
In such a process, a solid optical fiber preform (i.e., a preform having 
the inside completely filled up) may be used as a starting material in a 
similar manner as described above to provide a complex of the starting 
material and deposit of fine glass particles formed on the circumference 
of the starting material. Then, the complex is subjected, without pulling 
out the above-mentioned starting material, to heat treatment in a high 
temperature furnace so as to sinter the portion of the deposit of fine 
glass particles, whereby a transparent glass layer is further formed on 
the circumference of the optical fiber glass preform as the starting 
material. 
In addition, Japanese Laid-Open Patent Application No. 186240/1986 (i.e., 
Sho 61-186240) discloses another process. In this process, as shown in 
FIG. 3, a glass raw material is supplied into flame 3 fed from a burner 2 
for synthesizing fine glass particles to produce fine glass particles. The 
thus produced fine glass particles start to be deposited on the 
circumference of a substantially cylindrical or cylindrical tubular 
starting material 1 rotating about its axis (in a vertical direction) as a 
rotational axis 7, from the neighborhood of one end of the starting 
material 1. Then, the burner 2 is relatively moved in parallel with the 
axis of the starting material 1 so that deposit 6 of fine glass particles 
is formed around the circumference of starting material 1 with respect to 
the axis direction. Thereafter, the thus obtained deposit of fine glass 
particles is heated at a high temperature for vitrification to provide a 
transparent product. In FIG. 3, reference numeral 8 denotes a reaction 
vessel (soot deposition furnace) and reference numeral 9 denotes an 
exhaust port. 
In the conventional processes as described above, however, there could 
occur a case such that foreign matter such as dust is attached to the 
starting glass material in the above-mentioned deposition process, and the 
attached foreign matter would cause a defect in the transparent vitrified 
glass product. Examples of such a defect may include a crystal and/or a 
bubble. In either case, a portion where the defect is caused cannot be 
used as a good glass product. 
As a solution to the above problem, there is known a method wherein fine 
glass particles are deposited on a glass rod while the glass rod is purged 
with a clean gas, as disclosed in Japanese Laid-Open Patent Application 
No. 162646/1987 (i.e., Sho 62-162646). This method is effective to a 
certain extent. However, according to the present inventors' 
investigation, it has been found that the foreign matter such as dust 
could not be removed completely, when the glass rod is simply placed in a 
clean gas. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process for producing a 
glass preform for optical fiber which is capable of reducing the 
occurrence of a defect in the resultant glass preform. 
According to the present invention, there is provided a process for 
producing a porous glass preform for optical fiber by depositing fine 
glass particles on an outer surface of a glass material while moving the 
glass material, comprising: 
preheating a portion of the glass material for not less than 5 minutes to 
clean the portion of the glass material in an apparatus for depositing 
fine glass particles; and 
depositing fine glass particles on the portion of the glass material 
cleaned by the preheating, in the apparatus for depositing fine glass 
particles. 
In the present invention, when the glass material is preheated in a gas 
containing water vapor in a reaction vessel, the preheating may preferably 
be effected by using oxyhydrogen flame fed from a preheating burner 
disposed below the fine glass particle-synthesizing burner, or by using an 
electric heating device disposed outside of the reaction vessel and below 
the fine glass particle-synthesizing burner while supplying a gas 
containing water vapor to the surface of the glass material. 
The present invention will become more fully understood from the detailed 
description given hereinbelow and the accompanying drawings which are 
given by way of illustration only, and thus are not to be considered as 
limiting the present invention. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art form this detailed description.

DETAILED DESCRIPTION OF THE INVENTION 
As a result of earnest study by the present inventors on the occurrence of 
a bubble and/or crystal at the boundary between a starting material and a 
glass product formed on the circumference of the starting material in a 
transparent vitrified glass product, it has been found that the bubble 
and/or crystal is based on, as one of the causes therefor, contamination 
on the circumference of a starting material before the step of depositing 
fine glass particles on the circumference thereof, and that such 
contamination cannot be removed sufficiently only by the conventional 
cleaning means as described above. 
According to the present inventors' investigation, examples of the above 
contaminant may include: carbon, organic substance such as fiber, 
inorganic substance such as metal (inclusive of Fe, Na, K, Ca, etc.), and 
glass particles per se inclusive of quartz (SiO.sub.2), etc. If a metal is 
attached to the starting material, it can provide a nucleus or seed for 
crystallization and a crystal is liable to be formed during the 
transparency-imparting vitrification. On the other hand, if an organic 
substance or glass particle is attached to the starting material, it can 
invite irregularity (or mismatching) during the deposition of fine glass 
particles, and the irregularity is liable to provide a bubble during the 
vitrification. 
Among these contaminants, the metals and glass particles may be present in 
the environment for the fabrication of a preform. Accordingly, the 
attachment of such contaminants can be prevented by tracking down the 
source of the contaminants. However, the organic substance can originate 
from clothes of operators, etc., and it is more difficult to prevent the 
attachment of such organic substance. 
The present inventors have tried to eliminate a cause for the defect by 
sufficiently removing the contamination component attached to a starting 
material. As a result of the present inventors' study, it has been found 
that the contamination component attached to a starting material may 
sufficiently be removed by heating a surface of the starting material in 
an apparatus wherein fine glass particles are to be deposited on the 
surface of the starting material, before the fine glass particles are 
deposited on the surface thereof. As a result of further study by the 
present inventors, it has further been found that the contamination 
component attached to the starting material may more effectively be 
removed by heating the surface of the starting material in a reaction 
vessel for depositing fine glass particles, immediately before the fine 
glass particles are deposited on the surface of the starting material by 
use of a burner for synthesizing fine glass particles. Based on such a 
discovery, the present inventors have completed the present invention. 
In the present invention, the heating may desirably be conducted at a 
temperature of not lower than 1000.degree. C. and lower than 1600.degree. 
C. If the temperature is lower than 1000.degree. C., the efficiency of the 
surface cleaning is not so good. On the other hand, if the temperature is 
not lower than 1600.degree. C., there may be posed a problem of 
deformation on the basis of a decrease in the viscosity of glass. 
Experiments were further conducted in detail and the results obtained 
thereby are shown hereinbelow. 
Experiment Example 
In order to examine the effect of removal of foreign matter such as dust by 
heating, the following experiments were conducted. 
First, dust in the manufacturing environment was sampled and dispersed in 
pure water at a concentration of 1 g/l. The thus obtained dispersion was 
attached to a cleaned glass rod. The glass rod used herein was one made of 
synthetic silica glass having an outer diameter of 13 mm. 
The rod was placed in a deposition apparatus shown in FIG. 1 and soot (of 
fine glass particles) was deposited on the circumference of the rod 1 
through a burner 2 for synthesizing soot (i.e., burner for synthesizing 
fine glass particles). In FIG. 1, reference numeral 1 denotes a glass 
material (starting material); numeral 2, the burner for synthesizing fine 
glass particles; numeral 3, flame; numeral 4, a burner for preheating; 
numeral 5, oxyhydrogen flame; numeral 6, deposit of fine glass particles; 
numeral 7, a rotation axis; numeral 8, a reaction vessel (soot deposition 
furnace); and numeral 9, exhaust port. 
In this experiment, the heating temperature was adjusted by changing the 
H.sub.2 /O.sub.2 ratio in the preheating burner (or auxiliary burner) 4. 
The soot (deposit of fine glass particle deposit) thus obtained was 
subjected to heat treatment at a heater temperature of 1670.degree. C. 
under ambient of He (flow rate: 10 l/min) in a reaction tube of quartz 
glass (silica glass) to be converted into a transparent glass product. 
Further, it was considered that the glass rod could be elongated by the 
heating. Accordingly, the lengths of the glass rod before and after the 
soot deposition were measured to obtain a change in the length as 
elongation. 
The elongation was obtained from the following equation: 
EQU Elongation=[{(total length after soot deposition) -(total length before 
soot deposition)}/(total length before soot deposition)].times.100(%). 
In this experiment, the temperature of a portion heated by the preheating 
burner 4 was measured by means of an infrared radiation thermometer (trade 
name: Thermo Tracer TH-1102, mfd. by NEC San-ei Instrument Ltd.). The 
number of defect was obtained from the number of observed light-scattering 
point when white light emitting from a halogen lamp (100 W) was incident 
to an end of the resultant glass preform. 
The results thus obtained are shown in the following table. 
TABLE 
______________________________________ 
&lt;Temp. &lt;Defect 
&lt;No.&gt; (.degree.C.)&gt; 
(number/cm)&gt; 
&lt;Elongation (%)&gt; 
______________________________________ 
1 0.000 50.000 0.000 
2 1000.000 10.000 0.000 
3 1100.000 7.000 0.000 
4 1200.000 2.000 0.000 
5 1300.000 0.500 0.000 
6 1400.000 0.200 0.200 
7 1500.000 0.100 0.400 
8 1600.000 0.050 0.900 
9 1670.000 0.020 2.500 
10 1700.000 -- melt 
______________________________________ 
As shown in the above table, in a case where heat treatment was not 
conducted by using the auxiliary burner 4, 50 defects/cm (bubbles and/or 
crystals) were observed. When the temperature of the heat treatment was 
increased, the number of defects was remarkably reduced. As shown in FIG. 
5, the defect points are represented by circles, while the elongation 
points are represented by darkened triangles. For instance, the circle to 
the far left represents the defect under the condition of no temperature 
treatment. 
However, the elongation due to the weight of the glass rod per se began to 
be observed at a temperature of higher than 1500.degree. C., and the glass 
was melted at 1700.degree. C. If a considerable elongation of the glass 
preform occurs, the design profile (or intended profile) would be changed 
when the glass preform is drawn into an optical fiber. Accordingly, such 
an elongation is not preferred. However, an elongation of 0.2% or less is 
generally within a tolerance of design, while the degree of the tolerance 
also depends upon an intended structure of the glass preform or optical 
fiber. 
As shown in the above Experiment Example, the effect of cleaning can be 
recognized even under heat treatment at 1000.degree. C., and the effect is 
a decrease in the number of defects to 20% as compared with that in the 
untreated case (the number of defects=50). A more desirable temperature 
range is not lower than 1200.degree. C. On the other hand, if the 
temperature of the heat treatment is 1400.degree. C. or higher, a 
considerable elongation of the starting glass material can occur. 
Accordingly, the temperature of the heat treatment may preferably be lower 
than 1400.degree. C. 
In view of deformation of glass, it is considered that a change in the 
composition of a starting glass material can provide a change in the glass 
viscosity thereof even at the same temperature, and the degree of the 
deformation can also change even at the same temperature. 
For example, a glass rod doped with fluorine (or a rod of a 
fluorine-containing glass) shows a viscosity corresponding to about one 
tenth of the viscosity of silica glass at a relative refractive index 
difference of 0.5%, as described in J. Material Science, 28 (1993), 
2738-2744. As a standard for glass deformation, a softening point is 
defined by ASTM (American Society for Testing Materials), which is a 
temperature at which the viscosity of a material to be tested becomes 
about 4.times.10.sup.7 poises. At a temperature higher than the softening 
point, glass begins to be rapidly fluidized or fluidified. From the 
viewpoint of safety, it is desired that the heat treatment is conducted at 
a temperature of not higher than the temperature at which the viscosity of 
glass is at least ten times higher than the viscosity thereof at the 
softening point (i.e., at a temperature of not higher than the temperature 
at which the viscosity of the glass is at least 5.times.10.sup.8 poise). 
The above-mentioned softening point according to ASTM is a temperature at 
which an upper portion (length=10 cm) of a sample having a diameter of 
0.55-0.75 mm and a length of 23.5 cm and being uniform in thickness shows 
an elongation of 1 ram/min., when the sample is heated at a temperature 
increasing rate of 5.degree. C./min. In the case of a glass having a 
density of 2.5 g/cm.sup.3, the softening point is a temperature at which 
the glass shows a viscosity of 10.sup.7.6 poise. 
More specifically, in the present invention, the maximum temperature of the 
preheated portion of the glass material (starting material) measured by 
the above-mentioned infrared radiation thermometer may preferably be not 
lower than 1000.degree. C. and lower than 1600.degree. C., more preferably 
not lower than 1200.degree. C. and lower than 1400.degree. C., in view of 
the efficiency of the cleaning and prevention of the elongation of the 
glass preform. 
In an embodiment wherein a glass material (such as fluoride glass material) 
having a relatively low melting or softening point, the maximum 
temperature of the preheated portion of the glass material may preferably 
be a temperature at which the glass material shows a viscosity of not less 
than 5.times.10.sup.8 poise, more preferably 1.times.10.sup.10 poise . 
Such a viscosity may for example be measured by the penetration method. 
In other words, the maximum temperature of the preheated portion of the 
glass material may preferably be a temperature which is lower than the 
softening point of the glass material by about 80.degree.-120.degree. C. 
(particularly preferably, by about 100.degree. C.). 
The heat treatment should preferably be conducted for a period of time 
until the attached dust component is reacted and removed substantially 
perfectly. The reaction for the removal of the dust may vary depending on 
the kind of the dust attached to the starting material. For example, when 
there is considered a case wherein a protective layer is formed on the 
surface by the reaction, a treatment for approximately 370 seconds at 1100 
.degree. C. is preferred in order to remove 90% of particles of 1 .mu.m. 
When the temperature for the treatment is raised, the treatment efficiency 
is improved. If the treatment is conducted at 1200.degree. C. for the same 
period of time, substantially 100% of dust can be removed. 
In the present invention, the period of time for the preheating may 
preferably be not less than 5 min., more preferably not less than 10 min. 
On the other hand, the period of time for the preheating may preferably be 
not more than 50 min. In the present invention, the preheating may 
preferably be conducted at a temperature of not lower than 1100.degree. C. 
for a period of time of not less than 5 min, more preferably at a 
temperature of not lower than 1200.degree. C. for a period of time of not 
less than 5 min. 
In the present invention, the period of time between the preheating and the 
heating for the synthesis of glass fine particles, or the distance between 
the preheating means (such as preheating burner 4) and the glass fine 
particle-synthesizing burner 2 may very depending on the temperature of 
the preheating and/or the heating for glass fine particle synthesis, the 
moving speed of the starting material, etc. In the present invention, in 
general, the period of time between the preheating and the heating for 
glass fine particle synthesis, or the distance between the preheating 
means and the glass fine particle-synthesizing burner 2 may preferably be 
such that the minimum temperature (measured by the above-mentioned 
infrared radiation thermometer) within the portion of the glass material 
located between the preheating means and the particle-synthesizing burner 
2 (e.g., the temperature at the middle point of such a portion) is about 
500.degree. C. or higher, more preferably about 500.degree.-900.degree. C. 
(particularly preferably, about 600.degree.-800.degree. C.). 
In the present invention, with respect to the number of the defects, when a 
portion of the resultant preform (length: 50 cm) is subjected to the 
light-scattering point measurement using the above-mentioned halogen lamp, 
it is preferred that a defect (bubble or crystal) having a diameter 
exceeding 1 mm is not observed in the length of 50 cm, and the number of 
the defects having a diameter of 1 mm or less (e.g., a diameter of about 
0.5-1 mm) may preferably be 5 cm or less, more preferably 4 or less 
(particularly preferably 3 or less). The presence and/or diameter of the 
defect in the preform may for example be determined by placing the preform 
to be examined on a section paper and determining the presence or diameter 
of the defect on the basis of observation with naked eye. In this case, 
the preform per se may function as a magnifying glass. 
In the present invention, it is preferred that the preheating atmosphere 
contains water vapor or steam. The reason for this is as follows. 
Thus, when an oxygen or oxidative atmosphere is employed for the purpose of 
oxidizing a carbon-type or carbon-containing component, the reaction is 
liable to be strongly exothermic, and the resultant heat can cause a 
nucleus or seed for crystallization. On the other hand, when the 
preheating is conducted in an atmosphere containing water vapor, the heat 
of reaction due to the oxidation of carbon may be reduced or compensated 
on the basis of an endothermic reaction between the water vapor and 
carbon, as shown in the following table. 
TABLE 
______________________________________ 
Oxidation Initiation Temp. and Relative Oxidation Rate 
&lt;Reaction&gt; 
&lt;Relative Rate&gt; 
&lt;Oxidation Initiation Temp.&gt; 
______________________________________ 
C--O.sub.2 
1 .times. 10.sup.5 
400.degree. C. 
C--H.sub.2 O 
3 700.degree. C. 
C--CO.sub.2 
1 900.degree. C. 
C--H.sub.2 
3 .times. 10.sup.-3 
-- 
______________________________________ 
The water vapor-containing gas or preheating atmosphere may preferably 
contain water vapor in an amount of not less than 1% by volume, more 
preferably not less than 2% by volume. If the water vapor content is less 
than 1% by volume, the effect of treatment with water vapor is not 
sufficient. 
Specific examples of the means for preheating in the water vapor-containing 
atmosphere may include a preheating burner 4 as shown in FIG. 1. In FIG. 
1, a surface of a glass material (starting material) 1 is treated with 
water vapor under heating on the basis of the reaction of: 
EQU 2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O 
in oxyhydrogen flame 5 of a steam heating burner (preheating burner) 4 
disposed below a burner 2 for synthesizing fine glass particles. In other 
words, the preheating burner 4 is disposed upstream of the fine glass 
particle-synthesizing burner 2 with respect to the moving direction of a 
glass starting material 1. 
In this embodiment, the quantitative ratio of H.sub.2 to O.sub.2 (i.e., 
ratio of H.sub.2 /O.sub.2) in the oxyhydrogen flame 5 may preferably be 2 
or larger (more preferably 2.5 or larger) in order to avoid excess of 
O.sub.2. On the other hand, the quantitative ratio of H.sub.2 to O.sub.2 
in the fine glass particle-synthesizing burner 2 (i.e., H.sub.2 /O.sub.2) 
is usually less than 2.0. For example, H.sub.2 :O.sub.2 =30:45 in Example 
1 appearing hereinbelow. 
In order to form the oxyhydrogen (H.sub.2 /O.sub.2) flame 5, it is 
preferred to use a preheating burner 4 comprising a plurality of ports 12, 
13 and 14 concentrically arranged as shown in a schematic cross-sectional 
view of FIG. 4. When such a burner having a concentric structure is used, 
it is preferred to supply H.sub.2, O.sub.2 and an inert gas to the 
respective ports arranged concentrically in the burner. According to the 
present inventors' experiment, it has been confirmed that the heating 
temperature can be elevated by supplying O.sub.2 to the central port 12 in 
FIG. 4. FIG. 6 is a graph showing a temperature characteristic of a glass 
rod (i.e., a relationship between H.sub.2 /O.sub.2 ratio and temperature 
of the heated glass rod) in a case where H.sub.2 or O.sub.2 is caused to 
flow through the central port 12 of the same burner. As shown in the graph 
of FIG. 6, a high temperature of 1200.degree. C. or higher can be realized 
in a case where O.sub.2 is caused to flow through the central port 12. In 
FIG. 6, the measured values for the flow of O.sub.2 through the central 
port 12 are represented by the circles, whereas the measured values for 
the flow of H.sub.2 through the central port 12 are represented by the 
triangles. Further, the glass rod had a diameter of 13 mm, and the flow of 
O.sub.2 was 1.5 slm. 
Further, when an inert gas such as Ar is caused to flow through a port 13 
disposed between the port for supplying H.sub.2 and the port for supplying 
O.sub.2, burning or seizing at the tip of the burner can be prevented. The 
reason for this may be that the flow of the inert gas can prevent the 
formation of flame in the burner. 
FIG. 2 shows another embodiment of apparatus for soot deposition. The 
apparatus shown in FIG. 2 has substantially the same structure as that of 
the apparatus shown in FIG. 1, except that a heater 10 such as electric 
heater is disposed as preheating means in place of the preheating burner 4 
in FIG. 1. In the present invention, the preheating may also be conducted 
by electric resistance heating by use of a heater 10 provided outside a 
reaction vessel (soot deposition furnace) 8 as shown in FIG. 2. In this 
case, when water vapor is contained in the preheating atmosphere, it is 
preferred to supply a water vapor-containing gas from a water 
vapor-supplying device 11 to the reaction vessel 8. The device 11 for 
supplying water vapor-containing gas may preferably be disposed upstream 
of the preheating means (electric heater) 10, and upstream of the burner 2 
for synthesizing fine glass particles, with respect to the moving 
direction of the glass material. 
In the present invention, the preheating may also be conducted by using 
another heating means such as high-frequency heating and laser (e.g., 
CO.sub.2 -laser). 
In the present invention, a starting material is cleaned by heating in an 
apparatus for glass fine particle synthesis (e.g., by use of oxyhydrogen 
flame immediately before the starting material is subjected to the 
deposition of fine glass particles). According to the present inventors' 
investigation, it has been confirmed that such a cleaning treatment does 
not substantially affect the transmission loss of an optical fiber to be 
finally produced from the resultant glass preform. More specifically, when 
optical fibers were produced by the conventional process for simple 
deposition of fine glass particles (without preheating of a starting 
material), the resultant optical fibers showed a transmission loss of 
0.350 dB/km (average for fiber of 1000 km) at a wavelength of 1.30 .mu.m 
as wavelength for communication, and a transmission loss of 0.670 dB/km 
(average for fiber of 1000 km) at a wavelength of 1.38 .mu.m as a 
wavelength for absorption by an OH group. On the other hand, optical 
fibers produced according to Examples 1 and 2 of the present invention 
appearing hereinbelow showed a transmission loss of 0.348 dB/km (average 
for fiber of 500 km) at the wavelength of 1.30 .mu.m, and a transmission 
loss of 0.700 dB/km (average for fiber of 500 km) at the wavelength of 
1.38 .mu.m. 
The cylindrical or cylindrical tubular glass material, which may preferably 
be used as the starting material in the present invention, may comprise 
any of glass materials having various refractive index arrangements (or 
refractive index structure) such as glass material for providing a core, 
and glass material comprising a portion for providing a core and a portion 
for providing a cladding. Particularly, a glass material having a 
structure for providing a core and a cladding may preferably be used, 
since the effect of the above-mentioned OH group poses substantially no 
problem in such a glass material. 
The composition of the glass material to be used in the present invention 
may be any of those conventionally used for the starting material for an 
optical fiber glass preform of such a type. 
In the present invention, it is preferred to clean the surface of the glass 
material as the starting material by conventional means before it is set 
or placed in the reaction vessel (i.e., before the preheating of the 
starting material in the present invention). Examples of such conventional 
means may include etching of the surface or surface layer with an aqueous 
HF solution, and heating of the glass material set in a glass lathe, in an 
oxyhydrogen flame. 
Hereinbelow, the present invention will be described in further detail with 
reference to specific Examples. However, it should be understood that the 
present invention is by no means restricted by such specific Examples. 
EXAMPLE 1 
The starting material used in this Example was a glass rod (diameter=20 mm, 
length=600 mm) having a central (or core) portion comprising SiO.sub.2 
doped with 0.3% of GeO.sub.2 in terms of relative refractive index 
difference, which had been produced by a VAD (vapor-phase axial 
deposition) method. The outer surface of the starting material was cleaned 
by heating at a temperature of from 1700.degree. C. to 1800.degree. C. 
using oxyhydrogen flame and a glass lathe. Then, the resultant starting 
material 1 was set in a reaction vessel (soot deposition muffle) 8 having 
a structure as shown in FIG. 1. 
In the reaction vessel 8 shown in FIG. 1, a preheating burner 4 was 
disposed below a fine glass particle-synthesizing burner 2 (distance 
between the preheating burner and fine glass particle-synthesizing 
burner=10 cm). The preheating burner 4 had a structure as shown in FIG. 4, 
in which three-layered gas-supplying ports 12, 13 and 14 were 
concentrically arranged (inside diameter, central port 12=2 mm, second 
port 13=5 mm, third port 14=8 mm). O.sub.2 gas was supplied to the central 
port 12 at 5 l(liter)/min., Ar as an inert gas was supplied to the second 
port 13 at 2 l/min. and hydrogen was supplied to the third port 14 at 12 
l/min. The maximum temperature in a portion of the glass rod 1 preheated 
by flame was 1300.degree. C. and the length of a region showing a 
temperature of 1100.degree. C. or higher was 15 mm. SiCl.sub.4 (2 l/min.), 
O.sub.2 (45 l/min.), H.sub.2 (30 l/min.) and Ar (20 l/min.) were supplied 
to the fine glass particle-synthesizing burner 2 so that fine glass 
particles were deposited on the starting material 1 immediately after the 
preheating thereof. During the soot deposition, the pull-up rate of the 
starting material 1 was 1.4 mm/min. on average and the preheating period 
of time was about 11 minutes. 
As a result, there was produced a fine glass particle deposition product 
having a diameter of 160 mm and a length of 600 mm. The thus obtained 
deposition product was subjected to transparency-imparting vitrification 
by heating at 1650.degree. C. in an He atmosphere thereby to provide an 
optical fiber preform having a diameter of 70 mm and a length of 450 mm. 
One end of the thus obtained preform was irradiated with a halogen lamp 
(i.e., the light emitting from the halogen lamp was incident to the 
above-mentioned one end of the preform), and defective points which 
scattered the light were observed. As a result, no formation of bubbles or 
crystals was observed. 
In addition, ten preforms were produced and evaluated in the same manner as 
described above. The number of defects in the preforms thus obtained was 
as small as 0.005 defect/cm. Thus, preforms with very small number of 
defects were produced. 
The resultant preform was subjected to fiber drawing to obtain an optical 
fiber having a diameter of 125 .mu.m. The transmission loss of the thus 
obtained fiber was 0.348 dB/km (wavelength=1.30 .mu.m) on average, and the 
fiber showed good characteristic. 
EXAMPLE 2 
The starting material used in this Example was a glass rod having a central 
portion comprising SiO.sub.2 doped with 0.35% of GeO.sub.2 in terms of 
relative refractive index difference, which had been produced by a VAD 
method. The outer surface of the starting material was cleaned by heating 
at a temperature of from 1700.degree. C. to 1800.degree. C. using 
oxyhydrogen flame and a glass lathe. Then, the resultant starting material 
1 was set in a reaction vessel (soot deposition muffle) 8 having a 
structure as shown in FIG. 1. 
In the reaction vessel 8 shown in FIG. 1, a preheating burner 4 was 
disposed below a fine glass particle-synthesizing burner 2. The preheating 
burner 4 had a structure as shown in FIG. 4, in which three-layered 
gas-supplying ports 12, 13 and 14 were concentrically arranged. H.sub.2 
gas was supplied to the central port 12 at 10 l(liter)/min., Ar as an 
inert gas was supplied to the second port 13 at 2 l/min. and O.sub.2 was 
supplied to the third port 14 at 4 l/min. The maximum temperature in a 
portion of the glass rod 1 preheated by flame was 1150.degree. C. and the 
length of a region showing a temperature of 1100.degree. C. or higher was 
10 mm. 
SiCl.sub.4 (2 l/min.), O.sub.2 (45 l/min.), H.sub.2 (30 l/min.) and Ar (20 
l/min. ) were supplied to the fine glass particle-synthesizing burner 2 so 
that fine glass particles were deposited on the starting material 1 
immediately after the preheating thereof. During the soot deposition, the 
pull-up rate was 1.5 mm/min. on average and the preheating period of time 
was about 7 minutes. 
As a result, there was produced a fine glass particle deposition product 
having a diameter of 160 mm and a length of 600 mm. The thus obtained 
deposition product was subjected to transparency-imparting vitrification 
by heating at 1650.degree. C. in an He atmosphere thereby to provide an 
optical fiber preform having a diameter of 70 mm and a length of 450 mm. 
One end of the thus obtained preform was irradiated with a halogen lamp, 
and defective points which scattered the light were observed. As a result, 
few bubbles and crystals were observed in a number of 0.1 defect/cm. 
In addition, ten preforms were produced and evaluated in the same manner as 
described above. The number of defects in the preforms thus obtained was 
as relatively small as 0.08 defect/cm. Thus, the level of number of 
defects was somewhat higher than that obtained in Example 1. 
The resultant preform was subjected to fiber drawing to obtain an optical 
fiber having a diameter of 125 .mu.m. The transmission loss of the thus 
obtained fiber was 0.351 dB/km (wavelength=1.30 .mu.m) on average, and the 
fiber showed good characteristic. 
EXAMPLE 3 
The starting material used in this Example was a glass rod having a 
cladding portion comprising SiO.sub.2 doped with 0.35% of F (fluorine) in 
terms of relative refractive index difference (decrease), which had been 
produced by a VAD method. The outer surface of the starting material was 
cleaned by heating at a temperature of from 1700.degree. C. to 
1800.degree. C. using oxyhydrogen flame and a glass lathe. Then, the 
resultant starting material 1 was set in a reaction vessel (soot 
deposition muffle) 8 having a structure as shown in FIG. 1. 
In the reaction vessel 8 shown in FIG. 1, a preheating burner 4 was 
disposed below a fine glass particle-synthesizing burner 2. The preheating 
burner 4 had a structure as shown in FIG. 4, in which three-layered 
gas-supplying ports 12, 13 and 14 were concentrically arranged. O.sub.2 
gas was supplied to the central port 12 at 4 l(liter)/min., Ar as an inert 
gas was supplied to the second port 13 at 2 l/min. and hydrogen was 
supplied to the third port 14 at 12 l/min. The maximum temperature in a 
portion of the glass rod 1 preheated by flame was 1250.degree. C. and the 
length of a region showing a temperature of 1100.degree. C. or higher was 
12 mm. SiCl.sub.4 (2 l/min.), O.sub.2 (45 l/min.), H.sub.2 (30 l/min.) and 
Ar (20 l/min.) were supplied to the fine glass particle-synthesizing 
burner 2 so that fine glass particles were deposited on the starting 
material 1 immediately after the preheating thereof. During the soot 
deposition, the pull-up rate was 1.4 mm/min. on average and the preheating 
period of time was about 9 minutes. 
As a result, there was produced a fine glass particle deposition product 
having a diameter of 160 mm and a length of 600 mm. The thus obtained 
deposition product was subjected to fluorine doping and 
transparency-imparting vitrification by heating in an He atmosphere 
containing a fluorine compound thereby to provide an optical fiber preform 
comprising a core of pure silica glass and having a diameter of 70 mm and 
a length of 450 mm. One end of the thus obtained preform was irradiated 
with a halogen lamp, and defective points which scattered the light were 
observed. As a result, no formation of bubbles or crystals was observed. 
In addition, ten preforms were produced and evaluated in the same manner as 
described above. The number of defects in the preforms thus obtained was 
as small as 0.01 defect/cm. Thus, preforms with very small number of 
defects were produced. 
The resultant preform was subjected to fiber drawing to obtain an optical 
fiber having a diameter of 125 .mu.m. The transmission loss of the thus 
obtained fiber was 0.335 dB/km (wavelength=1.30 .mu.m) on average, and 
0.172 dB/km (wavelength=1.55 .mu.m) on average, and the fiber showed good 
characteristic. 
EXAMPLE 4 
The starting material used in this Example was a glass rod having a central 
(or core) portion comprising SiO.sub.2 doped with 0.35% of GeO.sub.2 in 
terms of relative refractive index difference, which had been produced by 
a VAD method. The outer surface of the starting material was cleaned by 
heating at a temperature of from 1700.degree. C. to 1800.degree. C. using 
oxyhydrogen flame and a glass lathe. Then, the resultant starting material 
1 was set in a reaction vessel (soot deposition muffle) 8 having a 
structure as shown in FIG. 2. 
In the reaction vessel 8 shown in FIG. 2, a preheating heater 10 comprising 
an electric resistance heating SiC heater (heater length=50 mm, inner 
diameter =30 mm) was disposed below a fine glass particle-synthesizing 
burner 2 (distance between the center of the preheating heater and fine 
glass particle-synthesizing burner=15 cm). When the maximum temperature of 
the heater 10 was 1250.degree. C., the temperature of the surface of the 
glass rod 1 was elevated to 1150.degree. C., and the length of a region 
showing a temperature of 1100.degree. C. or higher was 25 mm. 
SiCl.sub.4 (2 l/min. ), O.sub.2 (45 l/min. ), H.sub.2 (30 l/min. ) and Ar 
(20 l/min. ) were supplied to the fine glass particle-synthesizing burner 
2 so that fine glass particles were deposited on the starting material 1 
immediately after the preheating thereof. During the soot deposition, the 
pull-up rate was 1.4 mm/min. on average and the preheating period of time 
was about 18 minutes. 
As a result, there was produced a fine glass particle deposition product 
having a diameter of 160 mm and a length of 600 mm. The thus obtained 
deposition product was subjected to transparency-imparting vitrification 
by heating at 1650.degree. C. in an He atmosphere thereby to provide an 
optical fiber preform having a diameter of 70 mm and a length of 450 mm. 
One end of the thus obtained preform was irradiated with a halogen lamp, 
and defective points which scattered the light were observed. As a result, 
no formation of bubbles or crystals was observed. 
In addition, ten preforms were produced and evaluated in the same manner as 
described above. The number of defects in the preforms thus obtained was 
as small as 0.02 defect/cm. Thus, preforms with small number of defects 
were produced. 
The resultant preform was subjected to fiber drawing to obtain an optical 
fiber having a diameter of 125 .mu.m. The transmission loss of the thus 
obtained fiber was 0.351 dB/km (wavelength=1.30 .mu.m) on average, and the 
fiber showed good characteristic. 
EXAMPLE 5 
The starting material used in this Example was a glass rod having a central 
(or core) portion comprising SiO.sub.2 doped with 0.35% of GeO.sub.2 in 
terms of relative refractive index difference, which had been produced by 
a VAD method. The outer surface of the starting material was cleaned by 
heating at a temperature of from 1700.degree. C. to 1800.degree. C. using 
oxyhydrogen flame and a glass lathe. Then, the resultant starting material 
1 was set in a reaction vessel (soot deposition muffle) 8 having a 
structure as shown in FIG. 2. 
In the reaction vessel 8 shown in FIG. 2, a preheating heater 10 comprising 
an electric resistance heating SiC heater (heater length=50 mm, inner 
diameter=30 mm) was disposed below a fine glass particle-synthesizing 
burner 2. When the maximum temperature of the heater 10 was 1000.degree. 
C., the temperature of the surface of the glass rod 1 was elevated to 
950.degree. C. 
SiCl.sub.4 (2 l/min. ), O.sub.2 (45 l/min. ), H.sub.2 (30 l/min. ) and Ar 
(20 l/min. ) were supplied to the fine glass particle-synthesizing burner 
2 so that fine glass particles were deposited on the starting material 1 
immediately after the preheating thereof. During the soot deposition, the 
pull-up rate was 1.4 mm/min. on average. 
As a result, there was produced a fine glass particle deposition product 
having a diameter of 160 mm and a length of 600 min. The thus obtained 
deposition product was subjected to transparency-imparting vitrification 
by heating at 1650.degree. C. in an He atmosphere thereby to provide an 
optical fiber preform having a diameter of 70 mm and a length of 450 mm. 
One end of the thus obtained preform was irradiated with a halogen lamp, 
and defective points which scattered the light were observed. As a result, 
bubbles and crystals were observed at a density of 0.5 defect/cm. 
Comparative Example 1 
The starting material used in this Comparative Example was a glass rod 
having a central (or core) portion comprising SiO.sub.2 doped with 0.35% 
of GeO.sub.2 in terms of relative refractive index difference, which had 
been produced by a VAD method. The outer surface of the starting material 
was cleaned by heating at a temperature of from 1700.degree. C. to 
1800.degree. C. using oxyhydrogen flame and a glass lathe. Then, the 
resultant starting material 1 was set in a reaction vessel (soot 
deposition muffle) 8 having a structure as shown in FIG. 3. 
SiCl.sub.4 (2 l/min.), O.sub.2 (45 l/min.), H.sub.2 (30 l/min.) and Ar (20 
l/min.) were supplied to the fine glass particle-synthesizing burner 2 in 
FIG. 3 so that fine glass particles were deposited on the starting 
material 1 without preheating thereof. During the soot deposition, the 
pull-up rate was 1.4 mm/min. on average. 
As a result, there was produced a fine glass particle deposition product 
having a diameter of 160 mm and a length of 600 mm. The thus obtained 
deposition product was subjected to transparency-imparting vitrification 
by heating at 1650.degree. C. in an He atmosphere thereby to provide an 
optical fiber preform having a diameter of 70 mm and a length of 450 mm. 
One end of the thus obtained preform was irradiated with a halogen lamp, 
and defective points which scattered the light were observed. As a result, 
bubbles and crystals were observed at a density of 1.5 defect/cm. 
Comparative Example 2 
The starting material used in this Example was a glass rod having a central 
(or core) portion comprising SiO.sub.2 doped with 0.3% of GeO.sub.2 in 
terms of relative refractive index difference, which had been produced by 
a VAD method. The outer surface of the starting material was cleaned by 
heating at a temperature of from 1700.degree. C. to 1800.degree. C. using 
oxyhydrogen flame and a glass lathe. Then, the resultant starting material 
1 was set in a reaction vessel (soot deposition muffle) 8 having a 
structure as shown in FIG. 1. 
In the reaction vessel 8 shown in FIG. 1, a preheating burner 4 was 
disposed below a fine glass particle-synthesizing burner 2. The preheating 
burner 4 had a structure as shown in FIG. 4, in which three-layered 
gas-supplying ports 12, 13 and 14 were concentrically arranged. O.sub.2 
gas was supplied to the central port 12 at 7 l(liter)/min., Ar as an inert 
gas was supplied to the second port 13 at 2 l/min. and hydrogen was 
supplied to the third port 14 at 20 l/min. The maximum temperature in a 
portion of the glass rod 1 preheated by flame was 1620.degree. C. and the 
length of a region showing a temperature of 1300.degree. C. or higher was 
15 mm. 
SiCl.sub.4 (2 l/min.), O.sub.2 (45 l/min.), H.sub.2 (30 l/min.) and Ar (20 
l/min.) were supplied to the fine glass particle-synthesizing burner 2 so 
that fine glass particles were deposited on the starting material 1 
immediately after the preheating thereof. During the soot deposition, the 
pull-up rate was 1.4 mm/min. on average. 
The thus obtained deposition product was formed into a preform and the 
resultant preform was evaluated in the same manner as in Example 1. As a 
result, there was no defect and the preform showed good characteristic, 
but the glass rod portion of the preform showed an elongation of 2.5% as 
compared with the initial length, which was somewhat large in view of 
practical use thereof. 
EXAMPLE 6 
The starting material used in this Example was a glass rod having a central 
(or core) portion comprising SiO.sub.2 doped with 0.35% of GeO.sub.2 in 
terms of relative refractive index difference, which had been produced by 
a VAD method. The outer surface of the starting material was cleaned by 
heating at a temperature of from 1700.degree. C. to 1800.degree. C. using 
oxyhydrogen flame and a glass lathe. Then, the resultant starting material 
1 was set in a reaction vessel (soot deposition muffle) 8 having a 
structure as shown in FIG. 1. 
In the reaction vessel 8 shown in FIG. 1, a preheating burner 4 was 
disposed below a fine glass particle-synthesizing burner 2. The preheating 
burner 4 had a structure as shown in FIG. 4, in which three-layered 
gas-supplying ports 12, 13 and 14 were concentrically arranged. O.sub.2 
gas was supplied to the central port 12 at 6 l(liter)/min., Ar as an inert 
gas was supplied to the second port 13 at 2 l/min. and hydrogen was 
supplied to the third port 14 at 14 l/min. The maximum temperature in a 
portion of the glass rod 1 preheated by flame was 1400.degree. C. and the 
length of a region showing a temperature of 1100.degree. C. or higher was 
20 min. SiCl.sub.4 (2 l/min.), O.sub.2 (45 l/min. ), H.sub.2 (30 l/min. ) 
and Ar (20 l/min.) were supplied to the fine glass particle-synthesizing 
burner 2 so that fine glass particles were deposited on the starting 
material 1 immediately after the preheating thereof. During the soot 
deposition, the pull-up rate was 1.4 mm/min. on average and the preheating 
period of time was about 15 minutes. 
As a result, there was produced a fine glass particle deposition product 
having a diameter of 160 mm and a length of 610 mm. The thus obtained 
deposition product was subjected to transparency-imparting vitrification 
by heating at 1650.degree. C. in an He atmosphere thereby to provide an 
optical fiber preform having a diameter of 70 mm and a length of 450 mm. 
One end of the thus obtained preform was irradiated with a halogen lamp, 
and defective points which scattered the light were observed. As a result, 
no formation of bubbles or crystals was observed. 
In addition, ten preforms were produced and evaluated in the same manner as 
described above. The number of defects in the preforms thus obtained was 
not larger than 0.002 defect/cm. Thus, preforms with very small number of 
defects were produced. In this Example, the preheating temperature was 
relatively high and the resultant preform showed an elongation of 0.2% on 
average in the glass rod portion. 
The resultant preform was subjected to fiber drawing to obtain an optical 
fiber having a diameter of 125 .mu.m. The transmission loss of the thus 
obtained fiber was 0.353 dB/km (wavelength=1.30 .mu.m) on average, and the 
fiber showed relatively good characteristic. 
As described hereinabove, according to the present invention, the surface 
of a starting glass material can be cleaned substantially perfectly in an 
apparatus for depositing glass fine particles (e.g., immediately before 
the deposition of the fine glass particles on the starting material), and 
the occurrence of a defect due to bubble and/or crystal possibly appearing 
at the boundary between the starting material and the deposit glass 
portion can be effectively prevented in the resultant glass product after 
transparency-imparting vitrification thereof. As a result, according to 
the present invention, a glass preform for optical fiber substantially 
free from a defect can be produced with a good yield. 
From the invention thus described, it will be obvious that the invention 
may be varied in many ways. Such variations are not to be regarded as a 
departure from the spirit and scope of the invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
to be included within the scope of the following claims.