MANUFACTURING METHOD OF OPTICAL FIBER PREFORM
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a manufacturing method of a high-quality
optical fiber preform.
Description of Related Art
It is generally known that a glass preform for an optical fiber is manufactured by
using a method in which a porous glass preform manufactured by a soot method, such as
a VAD (vapor phase axial deposition) method or an OVD (outside vapor deposition)
method is sintered to convert it to a transparent glass.
In order to manufacture a porous glass preform in the OVD method, a technique
of supplying an oxyhydrogen gas and a glass source gas, such as silicon tetrachloride
(S1Cl4) or germanium tetrachloride (GeCl4), to a burner, mixing them to generate glass
particles by hydrolysis reaction of both gases, and making the glass particles adhere to be
deposited on the outer periphery of the target (which is equivalent to an optical fiber
preform and indicates a porous glass preform or a start member thereof) which is rotating
at the position facing the burner is used. The target and the burner are surrounded by a
chamber in which an exhaust port is provided. Through the exhaust port, a combustion
gas, non-adhering glass particles, and the like generated during manufacture are
exhausted.
The air flow caused by the exhaust pressure is always formed within the
chamber so that non-adhering glass particles, which are particles that did not adhere to
the target, are efficiently discharged. However, some glass particles may not be

completely exhausted from the exhaust port but adhere to an inner wall of the chamber or
float within the chamber. These glass particles may remain as non-adhering glass
particles in the chamber and be reattached to the target, hi this case, the non-adhering
glass particles remain as bubbles or foreign matter in the optical fiber preform, which
lowers the strength or increases the loss in manufacturing the optical fiber. This causes
a problem in that the reliability of the optical fiber decreases.
To cope with this problem, Japanese Unexamined Patent Application, First
Publication No. H08-198635 discloses that a high-quality glass preform for an optical
fiber can be manufactured by providing a mechanism for guided discharge of the
non-adhering glass particles in a chamber in order to suppress reattachment of
non-adhering glass particles to a target.
Furthermore, Japanese Unexamined Patent Application, First Publication No.
H09-124334 discloses that a high-quality glass preform for an optical fiber can be
manufactured by increasing the exhaust pressure within a chamber according to
deposition and growth of a porous glass preform in order to suppress reattachment of
non-adhering glass particles to a target.
In the technique disclosed in Japanese Unexamined Patent Application, First
Publication No. H08-198635, it is thought that the described effects, such as suppressing
the reattachment of non-adhering glass particles to the target, are obtained. However,
since the structure of the apparatus becomes complicated, it is difficult to lower the cost.
In addition, even if the technique of guided discharge is used, it is theoretically
impossible to discharge all non-adhering glass particles. In addition, when the target
becomes thick by deposition and growth, the amount of non-adhering glass particles
generated also increases. In this case, the amount of glass particles reattached without
being exhausted also increases. However, a method for solving this problem is not

described in Japanese Unexamined Patent Application, First Publication No.
H08-198635.
In the technique disclosed in Japanese Unexamined Patent Application, First
Publication No. H09-124334, it is thought that the described effects, such as suppressing
the reattachment of non-adhering glass particles to the target, are obtained. However,
even if high pressure is set as the exhaust pressure, it is theoretically impossible to
discharge all non-adhering glass particles. Moreover, it becomes difficult to control the
air flow in the chamber if the exhaust pressure is set high. In addition, when the target
is deposited and grown to become thick, the amount of non-adhering glass particles
generated also increases. In this case, the amount of glass particles reattached without
being exhausted also increases. However, a method for solving this problem is not
described in Japanese Unexamined Patent Application, First Publication No.
H09-124334.
An example of a conventional manufacturing method of a porous optical fiber
preform is shown in FIGS. 4A and 4B. Reference numeral 101 denotes a gas burner,
reference numeral 102 denotes the flame of the burner including glass particles, reference
numeral 103 denotes a rod-like target, reference numeral 104 denotes a chamber, and
reference numeral 105 denotes an exhaust port of the chamber. While the target 103
rotates around a central axis, glass particles ejected from the gas burner 101 are adhered
and deposited on the outer periphery of the target 103, and the gas burner 101 makes a
relative parallel movement in the longitudinal direction of the target 103 multiple times.
In this way, a porous optical fiber preform is manufactured.
As shown in FIG 4A, in a manufacturing method of a porous optical fiber
preform, the external diameter of the target 103 is small at the start of deposition of glass
particles. Accordingly, the flame 102 including glass particles ejected from the gas

burner 101 strikes the outer periphery of the target 103 such that the outer periphery is
covered by tip portions 102a of the flame 102. For this reason, the amount of glass
particles rebounding without adhering to a front surface of the target 103, that is, a side
of the target 103 facing the gas burner, is small. However, as the deposition proceeds,
the target 103 is deposited to grow in the second half of the deposition shown in FIG 4B.
Accordingly, since the external diameter of the target 103 becomes large, the flame 102
including glass particles ejected from the gas burner 101 strikes the target 103 in a
condition where the target 103 interrupts the flame 102. For this reason, the rate of
glass particles rebounding without adhering to the front surface of the target 103
increases.
In recent years, the porous optical fiber preform tends to be made large in order
to reduce the manufacturing cost of the optical fiber. Accordingly, the external diameter
of the porous optical fiber preform manufactured by the OVD method also increases
compared with that in the related art. For this reason, the amount of non-adhering glass
particles which rebound on the front surface of the target 103 in the second half of
deposition tends to increase. In addition, the target 103 which became thick interrupts
the air flow at the time of ventilation. For this reason, in the known manufacturing
method shown in FIGS. 4A and 4B, there is a problem that adhered glass particles cannot
be sufficiently exhausted and reattachment of glass particles to the porous optical fiber
preform cannot be sufficiently suppressed.
From the above background, when a glass preform is enlarged, that is, when an
optical fiber preform which is a target becomes larger than the flame emitted from a
burner, a technique for suppressing the reattachment of non-adhering glass particles was
required since the amount of non-adhering glass particles significantly increases.

SUMMARY OF THE INVENTION
The present invention was devised in view of the above circumstances, and has
as an object the provision of a manufacturing method of an optical fiber preform capable
of manufacturing a high-quality optical fiber preform by suppressing the reattachment of
non-adhering glass particles even if the optical fiber preform becomes large.
According to an aspect of the invention, a manufacturing method of an optical
fiber preform includes: a glass particle generating step of generating glass particles by
ejecting an oxyhydrogen flame including a glass source gas from a burner, and a
deposition step of depositing the generated glass particles on an outer periphery surface
of a target In at least a part of the deposition step, an axial direction of the burner is
shifted from a center of the target. Non-adhering glass particles, which do not adhere to
the target in the deposition step, of the generated glass particles are discharged from an
exhaust port provided opposite the burner with the target interposed therebetween.
In the manufacturing method of an optical fiber preform, in the deposition step,
an angle formed by a line connecting a center of a tip portion of the burner with the
center of the target and the axial line of the burner may be continuously changed
corresponding to deposition and growth of the target.
In the manufacturing method of an optical fiber preform, in the deposition step,
an angle formed by a line connecting a center of a tip portion of the burner with the
center of the target and the axial line of the burner may be changed in a step wise manner
corresponding to deposition and growth of the target.
In the manufacturing method of an optical fiber preform, the axial direction of
the burner may be shifted from the center of the target by moving at least one of the
burner and the target.
In the manufacturing method of an optical fiber preform, when an angle formed

by a line connecting a center of a tip portion of the burner with the center of the target
and the axial line of the burner is a burner angle 'a', and an angle formed by the line
connecting the center of the tip portion of the burner with the center of the target and a
tangential line of the target extending from the center of the tip portion of the burner is a
target tangential angle 'b1, the target may be deposited to grow while maintaining the
relationship of a < b in the deposition step.
In the manufacturing method of an optical fiber preform, a rotation direction of
the target at a side where the glass particles are deposited and a direction when shifting
the axial direction of the burner from the center of the target may be the same direction.
In the manufacturing method of an optical fiber preform, a ventilation air flow
for discharging the non-adhering glass particles may occur from a side where the glass
particles are deposited to a side of the exhaust port.
In a conventional manufacturing method of an optical fiber preform, the axial
direction of a burner always faces the center of the target in order to raise the deposition
efficiency.
However, the inventor discovered that generation of glass particles rebounding
from the target could be suppressed, the target did not interrupt the ventilation air flow,
and non-adhering glass particles could be effectively exhausted to the exhaust port by
shifting the axial direction of the burner from the center of the target.
That is, in the manufacturing method of an optical fiber preform of the invention,
since reattachment of glass particles to the target can be suppressed, it becomes possible
to manufacture a high-quality optical fiber preform with few bubbles or foreign matter
thereinside.
Moreover, the inventor confirmed that when the deposition proceeded to make
the optical fiber preform thick, the deposition efficiency hardly changed even if the axial

direction of the burner has shifted from the center of the target.
Therefore, even if the manufacturing method of an optical fiber preform of the
invention is executed, it is possible to maintain the cost and the productivity without
changing the manufacturing time or the amount of raw materials used.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG 1 is a configuration view illustrating the positional relationship of a target, a
burner, and a chamber in an embodiment of a manufacturing method according to the
invention.
FIG 2 is an explanatory view illustrating the angular position relationship
between the target and the axial line of the burner in the embodiment of the
manufacturing method according to the invention.
FIGS. 3A to 3D are explanatory views illustrating the angular position
relationship between the target and the axial line of the burner, in the embodiment of the
manufacturing method according to the invention, according to growth of the target,
FIGS. 3 A and 3C show a state at the start of deposition and growth of a target and a state
at the end of target growth in a first example, and FIGS. 3B and 3D show a state at the
start of deposition and growth of a target and a state at the end of target growth in a
second example.
FIG 4A is an explanatory view illustrating the positional relationship between a
target and a burner in an initial manufacturing stage in a known manufacturing method,
and FIG 4B is an explanatory view illustrating the positional relationship between the
target and the burner at the end of manufacturing in the known manufacturing method.
DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an example of a manufacturing method of an optical fiber preform
according to the invention will be described with reference to the accompanying
drawings.
FIG 1 is ah explanatory view illustrating an embodiment of a manufacturing
method of an optical fiber preform according to the invention.
In FIG 1, reference numeral 1 denotes a gas burner, reference numeral 2 denotes
a flame of the burner including glass particles, reference numeral 3 denotes a rod-like
target, reference numeral 4 denotes a chamber, and reference numeral 5 denotes an
exhaust port of the chamber. In the middle of the chamber 4, the rod-like target 3 is
disposed so as to freely rotate around the central axis. At the side of the target 3, the
burner 1 is disposed with a tip portion 1 a facing one side of a peripheral surface of the
target 3. An exhaust nozzle of the oxyhydrogen flame is formed in the tip portion 1 a of
the burner 1. From the exhaust nozzle, the oxyhydrogen flame 2 including a source gas
from a source gas supply source (not shown) connected to the burner 1 is ejected toward
the target 3.
Moreover, in FIG 1, a partial cross section of the rod-like target 3 is shown and
the burner 1 is disposed at the right side of the target 3. The chamber 4 surrounds the
target 3 and the burner 1. In addition, assuming that a side of the peripheral surface of
the target 3 facing the burner 1 is a front surface 3 a side of the target 3, the exhaust port 5
of the chamber 4 is provided at a back surface 3b side of the target 3.
In the chamber 4, the ventilation air flow caused by the exhaust pressure from
the exhaust port 5 occurs, such that generated exhaust gas and non-adhering glass
particles can be quickly discharged from the exhaust port 5 after growing the target 3 by
ejecting the oxyhydrogen flame 2 including the source gas ejected from the burner 1 onto
the peripheral surface of the target 3.

In the chamber 4, glass particles generated by the source gas ejected from the
burner 1 are adhered and deposited on the outer peripheral surface of the target while the
target 3 rotates around the axis. In addition, the burner 1 makes a relative parallel
movement in the longitudinal direction of the target 3 multiple times as needed. As a
result, the target 3 is grown around the periphery to thereby form an optical fiber preform
(porous glass preform). Although the target 3 is shown in a state of being horizontally
supported in FIG. 1, the target 3 may be rotatably placed in the chamber by holding the
target 3 in an arbitrary direction. For example, the target 3 may be held vertically,
horizontally, or in a slanted state. In the invention, the direction or rotation direction of
the target 3 is not particularly limited.
In the manufacturing method of an optical fiber preform according to the
embodiment, the axial direction of the burner 1 (that is, the direction in which the flame 2
is ejected from the burner 1) is shifted from the center of the target 3. In this case,
generation of non-adhering glass particles which are generated when the flame 2 from the
burner 1 rebounds on the outer peripheral surface of the target 3 is suppressed, and
non-adhering glass particles are efficiently exhausted to the exhaust port 5 without the
target 3 interrupting the air flow to the exhaust port 5.
The axial direction of the burner 1 is preferably shifted from the center of the
target 3 toward the rotation direction on the front surface 3 a side of the target 3 on which
the oxyhydrogen flame 2 from the burner 1 strikes. In this case, since the direction of
movement of the outer surface of the target 3 made by the rotation matches the gas flow
direction, non-adhering glass particles can be exhausted more efficiently.
In addition, by shifting the axial direction of the burner 1 from the center of the
target 3, an extending portion 2a side of the oxyhydrogen flame 2 including glass
particles ejected from the burner 1 is not interrupted by the target 3 and extends to cover

one side of the outer peripheral surface of the target 3 and strikes on it. Accordingly,
generation of glass particles rebounding without adhering to the front surface 3 a side of
the target 3 can be suppressed. Furthermore, since the direction of the oxyhydrogen
flame 2 ejected from the burner 1 is close to the direction of the ventilation air flow
within the chamber, non-adhering glass particles can be efficiently discharged from the
exhaust port 5 without non-adhering glass particles that adhere to an inner wall surface of
the chamber 4 or floats within the chamber 4.
That is, as shown in FIG 1, the extending portion 2a of the oxyhydrogen flame 2
extends from the front surface 3 a side of the target 3 toward an upper surface side and
covers a part of the upper surface and the front surface 3a of the target 3. In addition, a
ventilation air flow 4a on the upper surface side of the target 3 follows the extension
portion so as to flow along the outer peripheral surface of the target 3 and moves toward
the exhaust port 5. As a result, non-adhering glass particles are discharged efficiently
without being reattached to the target 3. In this way, reattachment of non-adhering glass
particles to the optical fiber preform can be suppressed.
FIG 2 is an explanatory view illustrating an example of the angular relationship
between the axial line of the burner 1 and the target 3 when the manufacturing method
according to the invention is executed.
In the invention, assuming that an angle formed by a line 1 A, which connects
the center of the tip portion 1 a of the burner 1 with the center O of the target 3, and a line
m (extending line of the central axis line of the burner 1), which passes through the
center of the tip portion 1a of the burner 1 and extends in parallel with the axial direction
of the burner 1, is a burner angle 'a', and an angle formed by the line 1A, which connects
the center of the tip portion 1a of the burner 1 with the center O of the target 3, and a
tangential line n of the target 3 extending from the center of the tip portion 1a of the

burner 1 is a target tangential angle 'b' the relationship of a < b is preferably satisfied.
That is, it is preferable to deposit glass particles on the target 3 while maintaining this
relationship.
If the burner angle 'a' becomes larger than the target tangential angle 'b' the
oxyhydrogen flame 2 from the burner 1 stops striking the target 3. As a result, the
deposition efficiency of glass particles falls.
Since the external diameter of the target 3 becomes thick as the deposition for
growing the target 3 by glass particles adhering to the outer peripheral surface of the
target 3 proceeds, the optimal burner angle 'a' with respect to the center O of the target 3
changes sequentially. Accordingly, it is preferable to make the burner angle 'a' small
when the target 3 is thin and make the burner angle 'a' large when the target 3 is thick by
changing the burner angle 'a' according to deposition and growth of the target 3. The
change of the burner angle 'a' may be made continuously or in a step wise manner
according to deposition and growth of the target 3.
Moreover, for a moving mechanism of the burner 1, an oscillating mechanism
may be provided in a mechanism of supporting the burner 1 when an oscillating angle of
the burner 1 is controlled. Moreover, when performing a parallel movement of the
burner 1 or a positional movement of the burner 1, a stage or the like may be provided in
the mechanism of supporting the burner 1 in order to move the burner 1.
FIGS. 3 A to 3D show examples of a method of changing the burner angle 'a' in
the embodiment of the manufacturing method of an optical fiber preform of the invention.
Note that, in the normal OVD method, the position of the burner 1 and the position of the
target 3 only change according to deposition and growth of the target 3 such that a
distance between the burner 1 and the surface of the target 3 on which the oxyhydrogen
flame 2 strikes is fixed.

In the present embodiment, in addition to the position change, the burner angle
'a' with respect to the center O of the target 3 is relatively changed by moving at least one
of the burner 1 and the target 3.
A method of changing the burner angle 'a' in the case where the target 3 is
deposited and grown to become thick like a target 13 will be described. In a first
example shown in FIGS. 3A and 3C, a positional adjustment is made such that the
distance between the burner 1 and the surface of the target 3, on which the oxyhydrogen
flame 2 from the burner 1 strikes, is fixed by moving the position of the target 3
according to the growth of the target 3 as indicated by arrow E of FIG 3 A, while the
oxyhydrogen flame 2 is made to strike on the outer peripheral surface of the target 13 so
that the burner angle 'a' changes in a stepwise or sequential manner by rotating the burner
1 as indicated by arrow F of FIG 3C.
Moreover, in a second example shown in FIGS. 3B and 3D, the burner 1 is
disposed in advance such that the axial direction of the burner 1 is slightly shifted from a
moving axis 3c of the target 3 toward the rotation direction on the front surface side of
the target 3, and by moving the position of the target 3 according to the growth and
deposition of the target 3 as indicated by arrow E of FIG 3B, a positional adjustment is
made such that the distance between the burner 1 and the surface of the target 3, on
which the oxyhydrogen flame 2 from the burner 1 strikes, is fixed and the oxyhydrogen
flame 2 is made to strike on the outer peripheral surface of the target 13 so that the burner
angle 'a' relatively changes.
The effects of the invention can be obtained even if any of the method of
direction control of the burner 1 shown in FIGS. 3 A and 3C and the method of direction
control of the burner 1 shown in FIGS. 3B and 3D is used. That is, it is possible to
prevent the growing target 3 from interrupting the ventilation air flow. In addition,

since non-adhering glass particles are effectively exhausted to the exhaust port 5,
generation of rebounding glass particles can be suppressed. As a result, since
reattachment of glass particles to the target 3 can be suppressed, it becomes possible to
manufacture a high-quality optical fiber preform with few bubbles or foreign matter
thereinside.

The flow rate of a silicon tetrachloride (SiCl4) gas as a glass source gas was set
to 5.5 to 7.5 SLM (L/min.), the flow rate of a hydrogen gas was set to 40 to 100 SLM,
the flow rate of an oxygen gas was set to 15 to 40 SLM, and the flow rate of an argon gas
as a seal gas was set to 1 SLM. Each of the gases was supplied to the burner. A target
was grown by ejecting the oxyhydrogen flame from the burner toward the target and
depositing glass particles on the target surface.
A silica glass round bar with an external diameter of 40 mm was used for the
target used as a start member. Fifty porous glass preforms, each of which has a
diameter of 250 mm and a length of 1500 mm, were manufactured by disposing total ten
burners in the longitudinal direction of the target and depositing glass particles over a
plurality of layers on the outer periphery of the target Here, the optical fiber preform
was manufactured by changing the angle of the burner by a predetermined amount during
deposition such that the burner angle at the start of deposition becomes 0° and the burner
angle at the end of deposition becomes 0.8 x b = 21 °. Moreover, the ventilation flow
velocity from the exhaust port was set to 3.0 m/sec.
An optical fiber preform was manufactured by sintering the porous glass
preform manufactured by the above manufacturing method and converting it to a
transparent glass. For this optical fiber preform, the number of bubbles or foreign

matter on the surface or thereinside was counted by visual observation. As a result, the
number of bubbles or foreign matter generated per preform was 0.6 on average. In
addition, the deposition efficiency of glass particles in this method was 54% on average.

Fifty porous glass preforms were manufactured in the same conditions as the
previous example except that the burner angle during deposition was always set to 0° in
the manufacturing method of an optical fiber preform (that is, axial direction of the
burner was always made to face the center of a target). An optical fiber preform was
manufactured by sintering the porous glass preform manufactured by the above method
and converting it to a transparent glass. For this optical fiber preform, the number of
bubbles or foreign matter on the surface or thereinside was counted by visual observation.
As a result, the number of bubbles or foreign matter generated per preform was 5.2 on
average. In addition, the deposition efficiency of glass particles in this method was
55% on average.
In the manufacturing method described in the previous example, since
generation of glass particles that rebound without adhering to the front surface of the
target could be suppressed and non-adhering glass particles could be efficiently
discharged by shifting the axial direction of the burner from the center of the target,
reattachment of the non-adhering glass particles to the target could be reduced.
As a result, the number of bubbles or foreign matter of the fiber preform has apparently
decreased compared with the method in the comparative example.
In addition, when the deposition efficiency in the example is compared with the
deposition efficiency in the comparative example, it could be seen that the deposition
efficiency of glass particles has hardly changed.

From the above result, it has become clear that a high-quality optical fiber
preform with few bubbles or foreign matter on the surface or thereinside can be
manufactured without lowering the deposition efficiency of glass particles, that is,
without lowering the manufacturing efficiency by executing the manufacturing method
of the invention.
While preferred embodiments of the invention have been described and
illustrated above, it should be understood that these are exemplary of the invention and
are not to be considered as limiting. Additions, omissions, substitutions, and other
modifications can be made without departing from the spirit or scope of the present
invention. Accordingly, the invention is not to be considered as being limited by the
foregoing description, and is only limited by the scope of the appended claims.

WE CLAIM :
1. A manufacturing method of an optical fiber preform, comprising:
a glass particle generating step of generating glass particles by ejecting an
oxyhydrogen flame including a glass source gas from a burner; and
a deposition step of depositing the generated glass particles on an outer
periphery surface of a target,
wherein in at least a part of the deposition step, an axial direction of the burner
is shifted from a center of the target, and
non-adhering glass particles, which do not adhere to the target in the deposition
step, of the generated glass particles are discharged from an exhaust port provided
opposite the burner with the target interposed therebetween.
2. The manufacturing method of an optical fiber preform according to claim 1,
wherein in the deposition step, an angle formed by a line connecting a center of
a tip portion of the burner with the center of the target and the axial line of the burner is
continuously changed corresponding to the deposition and growth of the target
3. The manufacturing method of an optical fiber preform according to claim 1,
wherein in the deposition step, an angle formed by a line connecting a center of
a tip portion of the burner with the center of the target and the axial line of the burner is
changed in a stepwise manner corresponding to the deposition and growth of the target.
4. The manufacturing method of an optical fiber preform according to claim 1,
wherein the axial direction of the burner is shifted from the center of the target

by moving at least one of the burner and the target.
5. The manufacturing method of an optical fiber preform according to claim 1, wherein
when an angle formed by a line connecting a center of a tip portion of the burner
with the center of the target and the axial line of the burner is a burner angle 'a', and an
angle formed by the line connecting the center of the tip portion of the burner with the
center of the target and a tangential line of the target extending from the center of the tip
portion of the burner is a target tangential angle *b',
the target is deposited to grow while maintaining the relationship of a < b in the
deposition step.
6. The manufacturing method of an optical fiber preform according to claim 1,
wherein a rotation direction of the target at a side where the glass particles are
deposited and a direction to which the axial direction of the burner is shifted from the
center of the target are the same direction.
7. The manufacturing method of an optical fiber preform according to claim 1,
wherein a ventilation air flow that discharges the non-adhering glass particles occurs
from a side where the glass particles are deposited to a side of the exhaust port.

A manufacturing method of an optical fiber preform includes: a glass particle generating step of generating glass particles by ejecting an oxyhydrogen flame including a glass source gas from a burner; and a deposition step of depositing the generated glass particles on an outer periphery surface of a target. In at least a part of the deposition step, an axial direction of the burner is shifted from the center of the target. Non-adhering glass particles, which do not adhere to the target in the deposition step, of the generated glass particles are discharged from an exhaust port provided opposite the burner with the target interposed therebetween.