Technical Field
[0001]
The present invention relates to an optical fiber
production method.
Background Art
[0002]
In optical fiber communication systems, in order to
increase the reach and the rate of optical transmission,
the optical signal-to-noise ratio has to be increased.
Thus, a decrease in transmission losses in optical fibers
is demanded. Nowadays, since an optical fiber production
method is highly sophisticated transmission losses caused
by impurities contained in optical fibers are closed to the
lower limits. A remaining main cause of transmission
losses is scattering losses in association with
fluctuations in the structure or composition of glass
forming optical fibers. This is inevitable, because
optical fibers are formed of glass.
[0003]
As a method of decreasing fluctuations in the
structure of glass, a method is known to cool molten glass
slowly. As a method of slowly cooling molten glass, an
1
GFK16015IN
attempt is made to slowly cool an optical fiber drawn from
a drawing furnace immediately. Specifically, it is
investigated to decrease the cooling rate of the optical
fiber that an optical fiber drawn from a drawing furnace is
heated in an annealing furnace, or an optical fiber drawn
from a drawing furnace is surrounded by a heat insulator
immediately.
[0004]
Patent Literature 1 below discloses a method of
setting the temperature of a heating furnace (an annealing
furnace) is ±100°C or less of the target temperature found
by a recurrence formula in 70% or more of a region from a
position at which the outer diameter of a silica based
optical fiber having a core and a cladding becomes smaller
than 500% of the final outer diameter to a position at
which the temperature of the optical fiber is 1, 400°C.
Since the temperature history of the optical fiber is
controlled in this manner, the fictive temperature of glass
forming the optical fiber is decreased to reduce
transmission losses.
[0005)
[Patent Literature 1] JP2014-62021 A
Summary of Invention
[0006]
However, the technique disclosed in Patent Literature
1 above is required to repeat complex calculations in order
2
GFK16015IN
to adjust the temperature of the optical fiber to an ideal
temperature change found by the recurrence formula. The
technique disclosed in Patent Literature 1 permits the
temperature of the optical fiber to have a temperature
shift of as large as ±50°C to 100°C with respect to the
target temperature found by the recurrence formula. When
the temperature shift of the optical fiber is permitted in
such a large deviation, it is difficult to say that the
temperature history is sufficiently optimized. For example,
supposing that the temperature of the optical fiber slowly
cooled is changed in a range of ±100°C and the fictive
temperature of glass forming the optical fiber is also
changed in a similar range, transmission losses of the
obtained optical fiber caused by light scattering fluctuate
as large as about ±0. 007 dB/km. In such the disclosed
production methods in which the temperature history of the
optical fiber is not sufficiently optimized, the annealing
furnace is elongated more than necessary, resulting in
excessive capital investment, or the drawing rate is
decreased more than necessary,
productivity.
[0007]
resulting in degraded
The present inventors found that the temperature
difference between the fictive temperature of glass forming
the optical fiber and the temperature of the optical fiber
is controlled in a predetermined range in the slow cooling
process, resulting in the promotion of the relaxation of
3
GFK16015IN
the structure of glass forming the optical fiber and easily
reducing transmission losses in the optical fiber.
[0008]
Therefore,
optical fiber
the present invention is
production method that
transmission losses in the optical fiber.
[0009]
to provide an
easily reduces
To solve the problem, an optical fiber production
method according to the present invention includes: a
drawing process of drawing an optical fiber from an optical
fiber preform in a drawing furnace; and a slow cooling
process of slowly cooling the optical fiber drawn in the
drawing process in an annealing furnace. In the method,
when the optical fiber is delivered into the annealing
furnace, a temperature difference between a temperature of
the optical fiber and a fictive temperature of glass
forming a core included in the optical fiber is higher than
20°C and lower than 100°C.
[0010]
As described above, in the slow cooling process, the
temperature difference between the temperature of the
optical fiber and the fictive temperature of glass forming
the core included in the optical fiber is controlled in a
predetermined range, and hence the fictive temperature of
glass forming the core is decreased for a shorter time.
That is, the relaxation of the structure of glass forming
the core is promoted in the slow cooling process for a
4
GFK16015IN
shorter time. Consequently, scattering losses caused by
fluctuations in the structure of glass forming the core in
the transmission of light are reduced, and transmission
losses in the optical fiber are reduced.
[0011]
When the optical fiber is delivered into the
annealing furnace, a temperature difference between a
temperature of the optical fiber and a fictive temperature
of glass forming the core is preferably higher than 4 0°C
and lower than 60°C. As described above, the temperature
of the optical fiber delivered into the annealing furnace
is controlled in a more suitable range, and hence the
effect of promoting the relaxation of the structure of
glass forming the core included in the optical fiber is
easily increased, and transmission losses in the optical
fiber are easily reduced.
[0012]
When a time constant of relaxation of a structure of
glass forming the core is defined as 1(T), a temperature of
the optical fiber at a certain point in time in the slow
cooling process is defined as T,
glass forming the core at the
a fictive temperature of
certain point in time is
defined as Teo, and a fictive temperature of glass forming
the core after a lapse of time ~t from the certain point in
time is defined as Tc, Equation (1) below is preferably
held.
5
GFK16015IN
20°C < T1 - T = (T1D - T) exp (- 1'1 t I r (T)) < 1 00°C · · · ( 1)
The temperature difference (Tc T) between the
temperature T of the optical fiber and the fictive
temperature Tc of glass forming the core included in the
optical fiber is controlled in the predetermined range when
the optical fiber is delivered into the annealing furnace
as well as in a given period in which the optical fiber is
delivered into and out of the annealing furnace, and hence
the relaxation of the structure of glass forming the core
included in the optical fiber is easily promoted, and
transmission losses in the optical fiber are easily reduced.
[0013]
Equation (2) below is preferably held.
40°C < T1 - T = (T1D - T) exp (- 1'1 t I r (T)) < 60°C · · · (2)
As described above, the temperature difference (Tc -
T) between the temperature T of the optical fiber and the
fictive temperature Tc of glass forming the core included
in the optical fiber is controlled in a more suitable range
in a given period in the slow cooling process, and hence
the relaxation of the structure of glaE)s forming the .core
included in the optical fiber is more easily promoted, and
transmission losses in the optical fiber are more easily
reduced.
[0014]
The optical fiber preferably stays in the annealing
furnace in at least a time in a period in which a
6
GFK16015IN
temperature of the optical fiber is in a range of 1, 300°C
to 1, 500°C, both inclusive. The optical fiber is slowly
cooled when the temperature of the optical fiber is in this
range, and hence the fictive temperature of glass forming
the core included in the optical fiber is easily decreased
for a shorter time, and transmission losses in the optical
fiber are easily reduced.
[0015]
As described
invention, an optical
above, according to the present
fiber production method that easily
reduces transmission losses ln the optical fiber is
provided.
Brief Description of Drawings
[0016]
FIG. 1 is a flowchart of the processes of an optical
fiber production method according to the present invention.
FIG. 2 is a schematic diagram of the configuration of
devices for use in an optical fiber production method
according to the present invention.
FIG. 3 is a graph of the relationship of the
temperature of glass and the fictive temperature of the
glass with slow cooling time.
FIG. 4 is a graph of the relationship of the
temperature difference (T£ 0 T) between the fictive
temperature of glass and the temperature of glass with the
decrease rate ((Tf- T£ 0 )/~t) of the fictive temperature of
7
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glass per unit time.
FIG. 5 is a graph of a temporal change in the
temperature difference between the fictive temperature of
glass and the temperature of glass.
FIG. 6 is a graph of a temporal change in the
temperature difference between the fictive temperature of
glass and the temperature of glass under the conditions
different from the conditions in FIG. 5.
FIG. 7 is a graph of the optimized temperature
difference (Tt - T) expressed by a solid line in FIG. 6 and
the upper limit and the lower limit of a variation over
time of the temperature difference (Tt T) where the
transmission loss caused by scattering is not increased by
0.001 dB/km or more.
Description of Embodiments
[0017]
In the following, a preferred embodiment of an
optical fiber production method according to the present
invention will be described in detail with reference to the
drawings ..
[0018]
FIG. 1 is a flowchart of the processes of an optical
fiber production method according to the present invention.
As illustrated in FIG. 1, the optical fiber production
method according to the embodiment includes a drawing
process Pl, a precooling process P2, a slow cooling process
8
GFK16015IN
P3, and a rapid cooling process P4. In the following,
these processes will be described. Note that, FIG. 2 is a
schematic diagram of the configuration of devices for use
in the optical fiber production method according to the
embodiment.
[0019]

The drawing process P1 is a process in which one end
of an optical fiber preform 1P is drawn in a drawing
furnace 110. First, the optical fiber preform 1P is
prepared. The optical fiber preform lP is formed of glass
having refractive index profiles the same as the refractive
index profiles of a core and a cladding forming a desired
optical fiber 1. The optical fiber 1 includes one or a
plurality of cores and a cladding surrounding the outer
circumferential surface of the core with no gap. The core
and the cladding are formed of silica glass. The
refractive index of the core is higher than the refractive
index of the cladding. For example, in the case in which
the core is formed of silica glass doped with a dopant,
such as germanium, which increases the refractive index,
the cladding is formed of pure silica glass. For example,
in the case in which the core is formed of pure silica
glass, the cladding is formed of silica glass doped with a
dopant, such as fluorine, which decreases the refractive
index.
[0020]
9
Subsequently, the optical fiber
suspended so that the longitudinal
GFKl6015IN
preform IF
direction
is
is
perpendicular. The optical fiber preform 1P is disposed in
the drawing furnace 110, a heating unit 111 is caused to
generate heat, and then the lower end portion of the
opt.ical fiber· preform 1P is heated. At this time, the
lower end portion of the optical fiber preform 1P is heated
at a temperature of 2, 000°C, for example, to be molten.
From the heated lower end portion of the optical fiber
preform 1P, molten glass is drawn out of the drawing
furnace 110 at a predetermined drawing rate.
[0021]

The precooling process P2 is a process in which the
optical fiber drawn out of the drawing furnace 110 in the
drawing process P1 is cooled to a predetermined temperature
suitable for delivering the optical fiber into an annealing
furnace 121, described later. A predetermined temperature
of the optical fiber sui table for delivering the optical
fiber into the annealing furnace 121 will be described
later in detail.
[0022]
In the optical fiber production method according to
the embodiment, the precooling process P2 is performed by
passing the optical fiber drawn in the drawing process Pl
through the hollow portion of a tubular body 120 provided
directly below the drawing furnace 110. The tubular body
10
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120 is provided directly below the drawing furnace 110,
causing the atmosphere in the inside of the hollow portion
of the tubular body 120 to be almost the same as the.
atmosphere in the inside of the drawing furnace 110. Thus,
a sudden change in the atmosphere and the temperature
around the optical fiber immediately after drawn is reduced.
[0023]
The temperature of the optical fiber delivered into
the annealing furnace 121 is mainly determined by the
drawing rate and the atmosphere inside the drawing furnace
110. The precooling process P2 is provided, which further
finely adjusts the cooling rate of the optical fiber for
easy adjustment of the incoming temperature of the optical
fiber to be delivered into the annealing furnace 121 to a
sui table range. Based on the temperature of the optical
fiber to be drawn out of the drawing furnace 110 and the
temperature
the optical
of the optical fiber sui table for delivering
fiber into the annealing furnace 121, the
distance from the annealing furnace 121 to the drawing
furnace 110 and the length of the tubular body 120 can be
appropriately selected. The tubular body 120 is formed of
a metal tube, for example. The cooling rate of the optical
fiber may be adjusted by air-cooling the metal tube or by
providing a heat insulator around the metal tube.
[0024]

The slow cooling process P3 is a process in which the
11
GFK16015IN
optical fiber, which is drawn out of the drawing furnace
110 in the drawing process Pl and whose temperature is
adjusted to a predetermined temperature in the precooling
process P2, is slowly cooled in the annealing furnace 121.
The temperature in the inside of the annealing furnace 121
is adjusted to a predetermined temperature different from
the temperature of the optical fiber to be delivered into
the annealing furnace 121, and the cooling rate of the
optical fiber is decreased by the temperature around the
optical fiber delivered into the annealing furnace 121.
Because the cooling rate of the optical fiber is decreased
in the annealing furnace 121, the structure of glass
forming the core included in the optical fiber is relaxed,
and the optical fiber 1 with decreased scattering losses is
obtained, as described below.
[0025]
In the disclosed optical fiber production methods
having slow cooling process, the temperature of the optical
fiber is not sufficiently optimized when the optical fiber
is delivered into the annealing furnace. Specifically, the
. optical fiber is sometimes delivered into the annealing
furnace with the temperature of the optical fiber being too
high or too low. When the temperature of the optical fiber
to be delivered into the annealing furnace is too high, the
rate to relax the structure of glass forming the optical
fiber is too fast, hardly expecting the effect of slowly
cooling the optical fiber. On the other hand, when the
12
GFK16015IN
temperature of the optical fiber to be delivered into the
annealing furnace is too low, the rate to relax the
structure of glass forming the optical fiber is decreased,
sometimes causing a necessity to heat up again the optical
fiber in the annealing furnace, for example. As described
above, in the disclosed slow cooling processes, it is
difficult to say that the relaxation of the structure of
glass forming the optical fiber is
Thus, the annealing furnace is
necessary, which might demand
efficiently performed.
elongated more than
an excessive capital
investment, or the drawing rate is decreased more than
necessary, which might degrade productivity.
[0026]
According to the optical fiber production method of
the embodiment, the temperature of the optical fiber is
controlled in a suitable range in the slow cooling process
P3 as describe below. Thus, the relaxation of the
structure of glass forming the core included in the optical
fiber is promoted. As a result, the optical fiber 1 having
decreased transmission losses can be obtained with no
requirement of excessive capital
excellent productivity. According
production method of the embodiment,
not necessary unlike the technique
Literature 1 described above.
[0027]
investment and with
to the optical fiber
complex calculation is
disclosed in Patent
In silica glass classified as so-called strong glass,
13
GFK16015IN
the time constant c(T) of the structural relaxation, which
is thought to correspond to the viscosity flow of glass,
follows the Arrhenius equation. Thus, the time constant
'(T) is expressed as Equation ( 3) as a function of the
temperature T of glass using a constant A and an activation
energy Eact determined by the composition of glass. Note
that, kB is Boltzmann constant .
. . • (3)
(Here, Tis absolute temperature of glass.)
[0028]
Equation (3) above shows that the structure of glass
is relaxed faster as the temperature of glass is higher and
reached faster in the equilibrium state at the given
temperature. That is, the fictive temperature of glass
comes close to the temperature of glass faster as the
temperature of glass is higher.
[0029]
FIG. 3 shows the relationship of the temperature of
glass and the fictive temperature of the glass with time in
slowly cooling glass. In the graph of FIG. 3, the
horizontal axis expresses time, and the vertical axis
expresses temperature. In FIG. 3, a solid line expresses
the transition of the temperature of glass under certain
slow cooling conditions, and a broken line expresses the
transition of the fictive temperature of glass at that time.
A dotted line expresses the transition of the temperature
14
GFK16015IN
of glass in the case in which the cooling rate is decreased
more slowly than the slow cooling conditions expressed by
the solid line, and an alternate long and short dash line
expresses the transition of t~he fictive temperature of
glass at that time.
[0030]
As expressed by the solid line and the broken line in
FIG. 3, when the temperature of glass is decreased over a
lapse of time in the high temperature area, the fictive
temperature of glass is also similarly decreased. As
described above, in the state in which the temperature of
glass is sufficiently high, the rate of the relaxation of
the structure of glass forming the optical fiber is very
fast. However, as the temperature of glass is decreased,
the rate of the relaxation of the structure of glass is
decreased, and the fictive temperature of glass fails to
follow a decrease in the temperature of glass after a while.
The temperature difference between the temperature of glass
and the fictive temperature of glass is increased. Here,
when the cooling rate of glass is slowed, the optical fiber
is held in a relatively higher temperature state for a
longer time, compared with the case in which the cooling
rate is fast. Thus, as expressed by the dotted line and
the alternate long and short dash line in FIG. 3, even
though the temperature of glass is decreased, the
temperature difference between the temperature of glass and
the fictive temperature of glass becomes smaller, and the
15
GFK16015IN
fictive temperature of glass is lower than the example
described above. That is, when the cooling rate of glass
is slowed, the relaxation of the structure of glass is
easily promoted.
[0031]
As described above, when the temperature of glass is
high, the structure of glass is relaxed fast. However, the
fictive temperature of glass does not reach to the below of
the temperature of glass. Thus, when the temperature of
glass is high, the fictive temperature of the glass also
remains high. That is, when the temperature of glass is
too high, the effects obtained by slow cooling are poor.
From this viewpoint, the temperature of the optical fiber
staying in the annealing furnace 121 is preferably 1,600°C
or less, and more preferably 1,500°C or less. On the other
hand, in the case in which the temperature of glass is low,
the fictive temperature can be decreased to a lower
temperature, but the decrease rate of the fictive
temperature is slowed. That is, when the temperature of
glass is too low, it will take longer time for slow cooling
in order to sufficiently decrease the fictive temperature.
From this viewpoint, the temperature of the optical fiber
staying in the annealing furnace 121 is preferably 1,300°C
or more, and more preferably 1, 400°C or more. Therefore,
the optical fiber preferably stays in the annealing furnace
121 at least a time in a period in which the temperature of
the optical fiber is in a range of 1,300°C to 1,500°C, both
16
GFK16015IN
inclusive. As described above, in the slow cooling process
P3, the optical fiber is slowly cooled when the temperature
of the optical fiber is in a predetermined range. Thus,
the fictive temperature of glass forming the core included
in the optical fiber is easily decreased for a shorter time,
and transmission losses in the optical fiber are easily
reduced.
[0032]
Next, the following is the description in which the
relaxation of the structure of glass forming the core is
efficiently promoted to reduce transmission losses in the
optical fiber by what manner of slowly cooling the optical
fiber by means of the relationship between the temperature
of glass and the fictive temperature of glass.
[0033]
Under the conditions in which the time constant of
the relaxation of the structure of glass forming the core
included in the optical fiber is defined as ~(T}, the
temperature of the optical fiber at a certain point in time
in the slow cooling process P3 is defined as T, and the
fictive temperature of
certain point in time
glass forming the core at that
is defined as Tt0 , the fictive
temperature Tt of glass forming the core after a lapse of
time f..t from the certain point in time is expressed as
Equation (4) below based on Equation (3} above. Note that,
f..t is a short period of time, and the temperature of the
optical fiber T for this period is supposed to be constant.
17
T1 - T = (T1° - T) exp (- LH I 7: (T))
[0034]
... ( 4)
GFK16015IN
Equation ( 4) above shows that the temperature
difference (Tf - T) between the fictive temperature Tf of
glass forming the core and the temperature T of the optical
fiber depends on the temperature difference (Tf0 T)
between the fictive temperature Tf0 of glass forming the
core and the temperature T of the optical fiber at a
certain point in time as well as the fictive temperature Tf
of glass forming the core depends on the time constant t(T)
of the relaxation of the structure. The time constant t(T)
of the relaxation of the structure is defined as time until
the temperature difference (Tf T) between the fictive
temperature Tf of glass and the temperature T of glass
reaches 1/e when the temperature of glass whose fictive
temperature is Tf0 is T. A change in the fictive
temperature Tf per unit time is greater as the temperature
difference (Tf0 - T) is great to some extent.
[0035]
FIG. 4 schematically shows the relationship between
the temperature difference (Tf0 - T) where the temperature
of the optical fiber including the core formed of glass
whose fictive temperature is Tfo is T and a change ( (Tf -
Tf0 ) /Ll.t) in the fictive temperature Tf per unit time. As
shown in FIG. 4, under the conditions in which the fictive
temperature Tf0 of glass forming the core coincides with the
18
GFK16015IN
temperature T of the optical fiber T) ' the
relaxation of the structure of glass forming the core does
not occur, and a change in the fictive temperature per unit
time is zero ( (Tt 0) . The conditions are
thought in which the temperature T of the optical fiber is
decreased from this point and the temperature difference
(Tt0 T) between the fictive temperature Tt0 of glass
forming the core and the temperature T of the optical fiber
is increased. Under the conditions, although the time
constant -r (T) of the relaxation of the structure of glass
forming the core is increased, the change rate of the
fictive temperature Tt per unit time ( (Tt
negatively increased. However, the conditions are thought
in which the temperature T of the optical fiber is further
decreased and the temperature difference (Tto - T) between
the fictive temperature Tto of glass forming the core and
the temperature T of the optical fiber is further increased.
Under the conditions, the time constant -r(T) of the
relaxation of the structure of glass forming the core is
now gradually increased, and the absolute value of a change
in the fictive temperature Tt per unit time ( (Tt - Tt0 ) /.'1t)
is decreased. That lS, FIG. 4 shows that as a peak
expressed in the graph, a change in the fictive temperature
per unit time ((Tt- Tt0 )/L1t) takes a minimum value when the
temperature difference (Tto T) between the fictive
temperature Tto of glass forming the core and the
temperature T of the optical fiber is at a certain value.
19
GFK16015IN
[0036]
Here, solving Equation ( 4) above shows that the
relationship of Equation (5) below is held between the
t:emperature 'r of glass and the fictive temperature Tf when
the decrease rate of the fictive temperature T£ of glass is
the maximum.
[0037]
When Equation ( 5) above is further solved on T as
Equation (6) below, the temperature T of glass can be found,
at which the fictive temperature Tf of glass can be most
efficiently decreased. In the following, the temperature
of glass, at which the fictive temperature Tf of glass can
be most efficiently decreased, is sometimes referred to as
"the optimized temperature of glass", and the fictive
temperature that has been most efficiently decreased is
sometimes referred
temperature".
T=
[0038]
Eact ---+
ks
2
to as "the optimized fictive
... ( 6)
As described so far, when the temperature difference
(T£0 - T) between the fictive temperature T£0 of glass and
the temperature T of glass at a certain point in time is a
20
GFK16015IN
predetermined value, a change in the fictive temperature Tf
of glass per unit time is maximized. That is, when the
fictive temperature Tf after a lapse of a certain time ,',t
of glass having the fictive temperature Tf0 is thought, the
temperature T of glass is present at which fictive
temperature Tf can be minimum value.
[0039]
FIG. 5 shows, on a standard single-mode optical fiber
having a core doped with Ge0 2 , a variation over time of the
temperature difference (Tf - T) between the value where the
fictive temperature Tf of glass forming the core, which is
found from Equation (4) above, takes the lowest value and
the temperature T of the optical fiber at that value. Here,
for the constant A and the activation energy Eact, values
described in Non-Patent Literature 1 are used (K. Saito, et
al., Journal of the American Ceramic Society, Vol. 89, pp.
65-69 (2006)). In the graph shown in FIG. 5, the vertical
axis expresses the temperature difference (Tf - T) between
the value where the fictive temperature Tf of glass forming
the core takes the lowest value and the temperature T of
the optical fiber at that value, and the horizontal axis
expresses the slow cooling time of the optical fiber. Here,
supposing that a temperature T0 of the optical fiber is
1, 900°C at the beginning of slow cooling, at which slow
cooling time is zero second, time required for relaxing the
structure of glass forming the core at this temperature is
as very short as less than 0.0001 second. Thus, it can be
21
GFK16015IN
thought that the fictive temperature TeD of glass forming
the core at the beginning of slow cooling is also a
temperature of 1, 900°C. That is, the initial value is
assumed as TeD - TD ~ 0°C.
[0040]
In a
difference
variation
(Tc T)
over time of the temperature
derived from the assumption, the
temperature difference (Te - T) is suddenly increased in a
time domain up to about 0.01 second. This shows that since
the temperature of the optical fiber is high and the
temperature difference (Te - T) is small, it is necessary
to decrease the fictive temperature Te of glass forming the
core by quickly cooling the optical fiber to increase the
temperature difference (Te - T) . On the other hand, it is
shown that in a time domain after about 0.01 second, the
temperature difference (Tt - T) is gradually decreased and
the temperature T of the optical fiber is maintained in a
range suitable for a decrease in the fictive temperature Tt
of glass forming the core. Under these conditions, the
fictive temperature of glass forming the core where slow
c~:JOling time is 0. 5 second is found as 1, 390°C.
[0041]
Since the maximum value of the temperature difference
(Tc T) between the fictive temperature Tc and the
temperature of the optical fiber shown in FIG. 5 is about
60°C, initial values are assumed where Te0 - TD 60°C for
further investigation. That is, initial values are assumed
22
GFK16015IN
where the temperature TO of the optical fiber at the
beginning of slow cooling, at which slow cooling time is
zero second, is 1,540°C, and the fictive temperature T£0 of
glass forming the core at this time is 1,600°C. Similarly
to the result shown in FIG. 5, the solid line in FIG. 6
shows a variation over time of the temperature difference
(T£ - T) between the value where the fictive temperature Tf
of glass forming the core takes the lowest value and the
temperature T of the optical fiber at that value. In the
graph in FIG. 6, the vertical axis expresses the
temperature difference (Tf - T) between the value where the
fictive temperature Tf of glass forming the core takes the
lowest value and the temperature T of the optical fiber at
that value, and the horizontal axis expresses the slow
cooling time of the optical fiber. The solid line
6 shows that the temperature difference (Tf
in FIG.
T) is
monotonously decreased in all the time domains and the
temperature T of the optical fiber is maintained in a range
sui table for a decrease in the fictive temperature Tf of
glass forming the core. Under these conditions, the
fictive temperature of glass forming the core where slow
cooling time is 0.5 second is derived as 1,388°C. Thus, it
is revealed that the fictive temperature of glass forming
the core is decreased more than the fictive temperature
under the conditions shown in FIG. 5.
[0042]
The investigation that derives the result shown in
23
GFK16015IN
FIG. 5 was similarly performed using the constant A and the
activation energy E~t described in Non-Patent Literature 2
(K. Saito, et al., Applied Physics Letters, Vol. 83, pp.
5175-5177 (2003)). This derives the maximum value of the
temperature difference (Tt T) between the fictive
temperature Tt and the temperature of the optical fiber,
which is about 55°C. The initial values where Tt0 - TD ~
55°C are assumed for further investigation. FIG. 6 shows,
by a broken line, the case in which the temperature T0 of
the optical fiber is 1,485°C at the beginning of slow
cooling, at which slow cooling time is zero second, and the
fictive temperature Tto of glass forming the core at this
time is 1, 54 0°C, and Tt T 55°C. Also under the
conditions shown by the broken line, it is revealed that
the temperature difference (Tt - T) is (kept) monotonously
decreased in all the time domains, and the temperature T of
the optical fiber is maintained in a range sui table for a
decrease in the fictive temperature Tt of glass forming the
core. Under these conditions, the fictive temperature of
glass forming the core where slow cooling time is 0.5
second is derived as 1,321°C.
[0043]
The results shown in FIG. 6 reveal the following.
That is, even though slight differences are present in the
values of the constant A and the activation energy Eact
determined based on the composition, it is revealed that
when the temperature difference (Tt T) between the
24
GFK16015IN
fictive temperature Tc of glass and the temperature T of
glass is in a predetermined range, the fictive temperature
Tc of glass is efficiently decreased. Thus, in so-called
typical optical fibers in which the concentration of dopant
is low and its principal component is silica glass, the
optical fiber is delivered into the annealing furnace 121
when the temperature differe:1ce ('rc T) between the
fict l ve temperature Tc of glass forming the core and the
temperature T of the optical fiber is in a
range, and hence the fictive temperature
predetermined
Tc of glass
forming the core is efficiently decreased. For example,
also in cores made of silica glass doped with a dopant,
such as Ge02, and claddings substantially made of pure
silica glass, the fictive temperature is efficiently
decreased.
[0044]
Note that, from the viewpoint of energy savings in
production of the optical fiber, the temperature of the
optical fiber is preferably monotonously decreased with no
temperature rise after the optical fiber heated and molten
in the drawing furnace 110. In this case, the temperature
of the optical fiber when delivered into the annealing
furnace 121 is the highest temperature in a period in which
the optical fiber stays in the annealing furnace 121. That
is, the rate of the relaxation of the structure of glass
forming
fastest
the core in the annealing furnace 121 is
when the optical fiber is delivered into
25
the
the
GFK16015IN
annealing furnace 121. Thus, the temperature of the
optical fiber when delivered into the annealing furnace 121
greatly affects the relaxation of the structure of glass
forming the core in
it is specifically
temperature of the
the slow cooling process P3. Therefore,
important to appropriately adjust the
optical fiber when delivered into the
annealing furnace 121.
[0045]
From the description above, it is revealed that when
the optical fiber is delivered into the annealing furnace
121, the temperature difference (Tc T) between the
temperature of the optical fiber and the fictive
temperature of glass forming the core included in the
optical fiber is preferably about 55°C to 60 °C. However,
some errors occur in the optimum value of this temperature
difference (Tc - T) depending on the composition of glass.
Therefore, when the optical fiber is delivered into the
annealing furnace 121, the temperature difference between
the temperature of the optical fiber and the fictive
temperature of glass forming the core included in the
optical fiber is preferably higher than 40°C and lower than
60°C. The temperature of the optical fiber and the fictive
temperature of glass forming the core included in the
optical fiber are controlled in predetermined ranges in the
slow cooling process P3 in this manner, and hence the
fictive temperature of glass forming the core is decreased
for a shorter time. That is, the relaxation of the
26
GFK16015IN
structure of glass form] ng t_he r:ore is promoted in the
annealing furnace for a shorter time. Consequently,
scattering losses during the transmission of the light in
the core caused by fluctuations ln the structure of glass
forming the core are reduced, and transmission losses in
the optical fiber are reduced. The lower limit of the
temperature difference (Tt - T) between the temperature of
the optical fiber and the fictive temperature of glass
forming the core included in the optical fiber when
delivered into the annealing furnace 121 is more preferably
45°C or more, still more preferably 50°C or more, and
specifically preferably 55°C or m0re. As described above,
the temperature of the optical fiber delivered into the
annealing furnace is controlled in a more suitable range,
and hence the relaxation of the structure of glass forming
the core included in the optical fiber is easily promoted,
and transmission losses in the optical fiber are easily
reduced.
[0046]
The temperature difference
temperature T of the optical
(Tt T) between the
fiber and the fictive
temperature Tt of glass forming the core included in the
optical fiber is controlled in the predetermined range in a
period in which the optical fiber is delivered into and out
of the annealing furnace 121, i.e.
which the slow cooling process P3 is
well as when the optical fiber is
27
in a given period
started and ended,
delivered into
in
as
the
GFK16015IN
annealing furnace 121, and hence the_ relaxation of the
structure of glass forming the core included in the optical
fiber is easily promoted, and transmission losses in the
optical fiber are easily reduced. ·That is, when the time
constant of the relaxation of the structure of glass
forming the core is defined as c(T), the temperature of the
optical fiber at a certain point in time in the slow
cooling process P3 is defined as T, and the fictive
temperature of glass forming the core at that certain point
in time is defined as Tt0 , and the fictive temperature of
glass forming the core after a lapse of time ilt from the
certain point in time is defined as Tt, Equation (2) below
is preferably held.
40°C < T1 - T = (T1° - T) exp (- LH I r (T)) < 60°C · · · (2)
As described above, in the slow cooling process P3,
the temperature difference (Tt - T) between the temperature
T of the optical fiber and the fictive temperature Tt of
glass forming the core included in the optical fiber is
maintained in a predetermined range, and hence the
relaxation of the structure of glass forming the core
included in the optical fiber is more easily promoted.
Therefore, transmission losses in the optical fiber are
more easily reduced.
[0047]
Note that,
difference (Tt
the conditions for the temperature
T) between the temperature T of the
28
GFK16015IN
optical fiber and the fictive temperature Tt of glass
forming the core included in the optical fiber in order to
most efficiently decrease the fictive temperature Tt of
glass forming the core are as described above. However,
transmission losses in the optical fiber can also be
sufficiently reduced under the conditions described below.
[0048]
The fictive temperature Tt of glass forming the core
included in the optical fiber can be correlated to
transmission losses in the optical fiber by a relational
expression below. A Rayleigh scattering coefficient Rr is
proportional to the fictive temperature Tt·of glass forming
the core, and a transmission loss aT caused by Rayleigh
scattering is expressed by Equation ( 7) below where the
wavelength of light to be transmitted is A (~).
aT=R,/A. 4 =BT1 /A 4 ···(7)
[0049]
Here, based on Non-Patent Literature 2 (K. Saito, et
al., Applied
(2003)), B ~
Physics Letters, Vol.
4.0 x 10-4 dB/km/f.!m4 /K.
transmission loss at the wavelength A
8 3' pp. 517 5-517 7
Let us consider a
1.55 f.!m. When the
fictive temperature Tt of glass forming the core is
increased by 14°C, the Rayleigh transmission loss aT caused
by Rayleigh scattering is increased by about 0.001 dB/km.
That is, when errors caused by the fictive temperature Tt
of glass forming the core, at which the fictive temperature
29
GFK16015IN
Tt is most efficiently decreased, can be controlled to
below l4°C, an increase in the Rayleigh transmission loss
ar caused by Rayleigh scattering can be controlled to below
0.001 dB/km.
[0050]
As described above, in the case of
account of permissive errors based on
taking into
the fictive
temperature Tt of glass forming the core, at which the
fictive temperature Tt is most efficiently decreased, the
optical fiber only has to be delivered into the annealing
furnace 121 under the temperature conditions in which the
temperature
temperature
temperature
difference
glass
of the optical
T)
forming
fiber is
lower than 100°C as described below.
[0051]
between the
the core
higher than
fictive
and
20°C
the
and
To the transmission loss predicted from the fictive
temperature Tt of glass forming the core through the slow
cooling process at the optimized temperature difference (T£
- T) expressed by the solid line in FIG. 6 for 0.5 second,
the temperature difference, at which an increase in a
transmission loss caused by scattering is controlled to
below 0.001 dB/km, can be predicted from Recurrence Formula
(4) above. When Recurrence Formula (4) is solved under the
assumption in which the fictive temperature Tt0 of glass
forming the core of the optical fiber at the beginning of
slow cooling, at which slow cooling time is zero second, is
30
GFK16015IN
1, 540°C and the temperature difference (Tt - T) is almost
constant in the slow cooling pro~ess P3, a graph shown in
FIG. 7 is obtained. In FIG. 7, the optimized temperature
difference (Tt - T) expressed by the solid line in FIG. 6
is again expressed by a solid line. FIG.
limit expressed by a broken line and
7 shows the upper
the lower limit
expressed by an alternate long and short dash line of a
variation over time of the temperature difference (Tt - T)
at which the transmission loss caused by scattering is not
increased by 0.001 dB/km or more. Here, for the constant A
and the activation energy Eact, the values described in NonPatent
Literature 1 (K. Saito, et al., Journal of the
American Ceramic Society, Vol.89, pp.65-69
The result shown in FIG. 7 reveals
(2006))
that
are used.
when the
temperature of the annealing furnace 121 is set to control
the temperature history of the optical fiber in which the
temperature difference (Tt - T) is in a range of above 20°C
and below 100°C during the slow cooling process P3, the
fictive temperature of glass forming the core is controlled
in a range in which the fictive temperature is not
increased by about l4°C or more to the fictive temperature
Tt of glass forming the core at which the fictive
temperature Tt is most efficiently decreased. Thus, when
the optical fiber is delivered into the annealing furnace
121 under the temperature conditions in which the
temperature difference (Tt T) between the fictive
temperature Tt of glass forming the core and the
31
GFK16015IN
temperature of the optical fiber is higher than 20°C and
lower than 100°C, an increase in the transmission loss can
be controlled to an increase of 0.001 dB/km or less to the
value under the optimized conditions in which the fictive
temperature is most decreased.
[0052]
Thus, the temperature difference (Tc - T) between the
temperature T of the optical fiber and the fictive
temperature Tc of glass forming the core included in the
optical fiber is maintained in a range of above 2 0°C and
below 100°C in a period in which the optical fiber ls
delivered into and out of the annealing furnace 121, i.e:
in a given period in which the slow cooling process P3 is
started and ended as well as when the optical fiber is
delivered into the annealing furnace 121, and hence the
relaxation of the structure of glass forming the core
included in the optical fiber is easily promoted, and
transmission losses in the optical fiber are easily reduced.
That is, Equation (1) below is preferably held.
20°C < T1 - T = (T1° - T) exp (- 1:1 t I 7: (T)) < 1 00°C · · · ( 1)
[0053]
Note that, the relationship between the fictive
temperature Tc of glass forming the core, at which the
fictive temperature Tc is most efficiently decreased, and
the temperature T of the optical fiber depends only on the
slow cooling time t. The slow cooling time t, the length L
32
GFK16015IN
of the annealing furnace, and the drawing rate v can be
correlated to each other by the relationship of Equation
(8) below.
t = L I v . .. ( 8)
[0054]
Therefore, when the targeted fictive temperature Tt of
glass forming the core included in the optical fiber to be
produced is set and the drawing rate v taking into account
of productivity is determined, a necessary length L of the
annealing furnace is derived. For example, the slow
cooling time t needs about 0.1 second to set the fictive
temperature Tt to 1, 500°C. Thus, it is revealed that when
the drawing rate v is set to 20 m/second, the length L of
the annealing furnace needs two meters. The slow cooling
time t needs about 0.4 second in order to set the fictive
temperature Tt to 1, 400°C, for example, and hence it is
revealed that when the drawing rate v is set to 10 m/second,
the length L of the annealing furnace needs four meters.
On the other hand, the length L of the annealing furnace
has only two meters, it is revealed that it is necessary to
set the drawing rate v to 5 m/second. However, from the
viewpoint of productivity, for example,
is preferably selected from a range
the drawing rate v
of about 10 to 50
m/second, the length L of the annealing furnace is
preferably selected from a range of about one to ten meters,
and the slow cooling time t is preferably one second or
33
GFK16015IN
less.
[0055]

After the slow cooling process P3, the optical fiber
is covered with a coating layer to enhance the resistance
against external flaws, for example. Typically, this
coating layer is formed of an ultraviolet curable resin.
In order to form such a coating layer, it is necessary to
sufficiently cool the optical fiber at a low temperature
for preventing the coating layer from being burn, for
example. The temperature of the optical fiber affects the
viscosity of a resin to be applied, and as a result, this
affects the thickness of the coating layer. A sui table
temperature of the optical fiber in forming the coating
layer is appropriately determined sui table for the
properties of a resin forming the coating layer.
[0056]
the
In the optical
embodiment, the
fiber production method according to
annealing furnace 121 is provided
between the drawing furnace 110 and a coater 131 to
decrease the section for sufficiently cooling. the optical
fiber. More specifically, the optical fiber production
method according to the embodiment also includes the
precooling process P2, further decreasing the section
sufficiently cooling the optical fiber. Thus, the optical
fiber production method according to the embodiment
includes the rapid cooling process P4 in which the optical
34
GFK16015IN
fiber delivered out of the annealing furnace 121 is rapidly
cooled using a cooling device 122. In the rapid cooling
process P4, the optical fiber is rapidly cooled faster than
in the slow cooling process P3. Since the rapid cooling
process P4 performed in this manner is provided the
temperature of the optical fiber can be sufficiently
decreased in a shorter section,
layer. The temperature of the
delivered out of the cooling
easily forming the coating
optical fiber when it is
device 122 ranges from
temperatures of 40°C to 50°C, for example.
[0057]
As described above, the optical fiber, which has been
passed through the cooling device 122 and cooled to a
predetermined temperature, is passed through the coater 131
containing an ultraviolet curable resin to be the coating
layer that covers the optical fiber, and the optical fiber
is covered with this ultraviolet curable resin. The
optical fiber is further passed through an ultraviolet
irradiator 132, ultraviolet rays are applied to the optical
fiber, the coating layer is formed, and then the optical
fiber 1 is formed. Note that, the coating layer is
typically formed of two layers. In the case of forming a
two-layer coating layer, after the optical fiber is covered
with ultraviolet curable resins forming the respective
layers, the ultraviolet curable resins are cured at one
time, and then the two-layer coating layer can be formed.
Alternatively, after forming a first coating layer, a
35
GFK16015IN
second coating layer may be formed. The direction of the
optical fiber 1 is changed by ·a turn pulley 141, and then
the optical fiber 1 is wound on a reel 142.
[0058]
As described above, the present invention is
described as the preferred embodiment is taken as an
example. The present invention is not limited to this
embodiment. That is, the optical fiber production method
according to the present invention only has to include the
drawing process and the slow cooling process described
above. The precooling process and the rapid cooling
process are not essential processes. The optical fiber
production method according to the present invention is
applicable to the production of any types of optical fibers.
For example, the optical fiber production method according
to the present invention is applicable also to production
methods for optical fibers having different materials, such
as chalcogenide glass and fluorine glass, as a principal
component, as well as production methods for optical fibers
having silica glass as a principal component, if the
constant A and the activation energy Eact in Equation (3)
above are derived.
[0059]
According to the present invention, there is provided
a production method for an optical fiber with which an
optical fiber with decreased transmission losses can be
produced, and the method can be used in the field of
36
GFK160151N
optical fiber communications. The method can also be used
for fiber laser devices and other devices using optical
fibers.
Reference Signs List
[00601
1 ... optical fiber
1P ... optical fiber preform
110 drawing furnace
111 heating unit
120 tubular body
121 annealing furnace
122 cooling device
131 ... coater
132 ultraviolet irradiator
141 turn pulley
142 reel
P1 drawing process
P2 precooling process
P3 slow cooling process
P4 rapid cooling process
37
GFK16015IN

Claims
1. An optical fiber production method comprising:
a drawing process of drawing an optical fiber from an
optical fiber preform in a drawing furnace; and
a slow cooling process of slowly cooling the optical
fiber drawn in the drawing process in an annealing furnace,
wherein
when the optical fiber is delivered into the
annealing furnace, a temperature difference between a
temperature of the optical fiber and a fictive temperature
of glass forming a core included in the optical fiber is
higher than 20°C and lower than 100°C.
2. The optical fiber production method according to
claim 1, wherein
when the optical fiber is delivered into the
annealing furnace, a temperature difference between a
temperature of the optical fiber and a fictive temperature
of glass forming the core is higher than 4 ooc and lower
than 60°C.
3. The optical fiber production method according to
claim 1 or 2, wherein
when a time constant of relaxation of a structure of
glass forming the core is defined as ~(T), a temperature of
the optical fiber at a certain point in time in the slow
cooling process is defined as T, a fictive temperature of
38
GFK16015IN
glass forming the core at the certain point in time is
defined as Tt0 , and a fictive temperature of glass forming
the core after a lapse of time ~t from the certain point in
time is defined as Tt, Equation (1) below is held.
4. The optical fiber production method according to
claim 3, wherein
Equation (2) below is held.
5. The optical fiber production method according to any
one of claims 1 to 4, wherein
the optical fiber stays in the annealing furnace in
at least a time in a period in which a temperature of the
optical fiber is in a range of 1,300°C to 1,500°C, both
inclusive.