BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a manufacturing method for a single
mode optical fiber for optical communications. In particular, the present
invention relates to a manufacturing method for a single mode optical fiber
which has a low loss in the 1380 nm wavelength range and superior hydrogen
resistance.
Description of Related Art
As the amount of data traffic increases, technology has improved in
the area of wavelength division multiplexing transmission systems. For
increasing the transmission capacity, it is important to broaden the available
wavelength range. Currently, the C-Band or L-Band are used as such a
wavelength range which can be amplified by an erbium-doped optical fiber.
As a form for realizing a broader wavelength range, a thulium-doped optical
fiber in which amplification can be performed in the S-Band and a Raman
amplifier in which amplification can be performed at any wavelength are
under development. As a result, it is possible to perform amplification in all
ranges of low loss regions in optical fibers; thus, it is necessary to obtain an
optical fiber having a low loss region in all wavelength ranges.
An optical fiber has a low loss region in the 1200 to 1600 nm
wavelength and a large loss peak in the 1380 nm wavelength range due to the
existence of hydroxyHon (OH). The loss peak is caused by the material
which forms an optical fiber. An optical fiber is made from a silica glass
which has a network structure in which SiO2 is united randomly in a
three-dimensional manner. When impurities or defects exist in the network
structure, new bonding and breakage occur; thus, these factors cause optical
absorptions. Among such optical absorptions, it is estimated that the loss at
1380 nm wavelength may be caused by hydroxyHon (OH) existing in the silica
glass. Therefore, the greater the amount of hydroxyl-ion (OH) included
therein, the larger the loss that will occur at 1380 nm wavelength.
Because the loss peak is broad, wavelength ranges on both sides of the
loss peak cannot be used for optical communications. From a practical point
of view, it is possible to perform optical communications in a broad wavelength
range if the loss in 1380 nm wavelength range can be under 0.31 dB/km.
In Japanese Unexamined Patent Application, First Publication No.
Hei 11-171575, it is disclosed that the loss in 1380 nm wavelength range
caused by the existence of the OH can be reduced by controlling the value of
the diameter of the core/clad ratio (D/d ratio) within a certain range.
It is possible to manufacture an optical fiber having a lower loss at
1380 nm than 0.33 dB/km by using a method which is disclosed in Japanese
Unexamined Patent Application, First Publication No. Hei 11-171575. This
method relates to a manufacturing method for a cladding using a jacket made
of a silica glass tube, and an advantage of the method is reducing the
manufacturing cost by using a jacket made of a silica glass tube. However,
there was a problem in that bubbles tend to remain between the core rod and the silica glass tube.
Also the quality of the optical fibers depends on factors such as OH concentration or bending
of the silica glass tube;therefore, there was a problem in that extreme quality control was always
necessary. As a result, product yield decreased; thus the manufacturing cost increased. Also, even
when an initial loss in 1380 nm wavelength range was low, there was a problem in that the loss
increased due to hydrogen which diffused from the outside. However, there has not been an available
countermeasure for such phenomenon.
SUMMARY OF THE INVENTION
The present invention was made in consideration of the above mentioned problems. An
object of the present invention is to provide a manufacturing method for a single mode optical fiber
which has a lower initial loss at 1380 nm wavelength range and can maintain the loss at 1380 nm
wavelength range at a lower level than in a conventional optical fiber even when hydrogen diffuses
from the outside.
Accordingly, the present invention provides a manufacturing method for a single mode
optical fiber, comprising steps of:
forming a glass rod having a core section and a first cladding section having a refractive
index lower than that of the core section;
vapor phase depositing for a second cladding on the first cladding;
sintering the glass rod having the first and second claddings to produce a glass preform; and
performing the drawing operation on the glass preform to produce an optical fiber;
wherein the ratio of a diameter D of the first cladding section to the diameter d of the core
section is in a range of 4.0 to 4.8; and OH concentrations of the core section, the first cladding section,
and the second cladding section are 0.1 ppm or less.
By doing this, it is possible to reduce more bubbles in an interface between the core and the
cladding, or between the first cladding section and the second cladding section than a case in which a
silica glass tube is used for a jacket. It is easy to dehydrate the porous soot to which vapor phase
deposition is performed; therefore, it is possible to control OH concentration desirably. Also, a silica
glass tube is not used, there is no problem such as bending of a core rod and a cladding made of a
silica glass tube; therefore, product yield increases. Accordingly, it is possible to produce a single
mode optical fiber at a low manufacturing cost.
The present invention also provides a manufacturing method for a single mode optical fiber,
comprising steps of:
forming a glass rod having a core section and a first cladding section having a refractive
index lower than that of the core section;
vapor phase depositing for a second cladding on the first cladding;
sintering the glass rod having the first and second claddings to produce a glass preform; and
performing the drawing operation on the glass preform to produce an optical fiber;
wherein the ratio of the diameter of the first cladding section to the diameter of the core
section is > 4.8 ; the OH concentration of the core section and the first cladding section are not more
than 0.1 ppm ; and the OH concentration of the second cladding section is not more than 100 ppm.
In a third aspect of a manufacturing method for a single mode optical fiber, the fiber has an
initial loss in the 1380 nm wavelength range is 0.31 dB/km or less ; and loss in the 1380 nm
wavelength range after hydrogen diffusion is 0.35 dB/km.
By doing this, the peak in the 1380 nm wavelength range becomes small, and both sides of
the wavelength range can be used for optical communications. Also, because it is possible to maintain
a loss under 0.35 dB/km in the 1380 nm wavelength range after hydrogen diffuses, it is possible to
supply a single mode optical fiber in which the loss in the 1380 nm wavelength range is low when
hydrogen diffusion occurs at low manufacturing cost.
In a fourth aspect of the manufacturing method for a single mode optical fiber, in a drawing
process, the drawing operation is performed on the glass preform by using a drawing device having an
annealing unit so as to manufacture an optical fiber.
By doing this, it is possible to maintain an occurrence of SiO at low level. Therefore, it is
possible to manufacture a single mode optical fiber in which the loss does not increase in the 1380 nm
wavelength range even if hydrogen diffuses from the outside of the optical fiber so as to be durable
over
long periods.
In a fifth aspect of the manufacturing method for a single mode optical
fiber, the annealing unit comprises a furnace with inclined heat zone and an
annealing tube.
In a sixth aspect of the manufacturing method for a single mode
optical fiber, in the annealing unit, the annealing atmosphere is any one of an
air, Ar, N2, or mixture thereof.
In a seventh aspect of the present invention, a single mode optical
fiber is manufactured by a manufacturing method according to any one of first
to sixth aspects of the present invention.
As explained above, according to the present invention, by forming a
glass preform by performing vapor phase deposition of SiO2 which forms a
second cladding section around the outside of an outer circumference of a glass
rod comprising a core section and a first cladding section, an optical fiber can
be produced by performing drawing of the glass preform. Therefore, it is
possible to reduce the occurrence of bubbles to a greater extent in an interface
between a core and a clad or between a first cladding section and a second
cladding section as comparing the case in which a silica glass tube is used for
a jacket. Also, because it is easy to dehydrate a porous soot on which vapor
phase deposition is to be performed, it is possible to produce an optical fiber by
controlling its OH concentration desirably. Also, because a silica glass tube is
not used, there is no problem such as bending of a core rod and a silica glass
tube which forms a cladding. Therefore, it is possible to increase product
yield; thus, it is possible to manufacture a single mode optical fiber at low
manufacturing cost.
Also, an optical fiber is manufactured so that a value of D/d such as a
ratio of the diameter D of the first cladding section and the diameter d of the
core section is in a range of 4.0 to 4.8, and the OH concentration of the core
section, the first cladding section, and the second cladding section is 0.1 ppm
or less, a value of D/d such as a ratio of the diameter of the first cladding
section and the diameter of the core section is D/d > 4.8, the OH concentration
of the core section and the first cladding section are 0.1 ppm or less, and the
OH concentration of the second cladding section is 100 ppm or less.
Therefore, it is possible to maintain an initial loss in the 1380 nm wavelength
range under 0.31 dB/km. Also, because the peak in 1380 nm wavelength
range becomes small, it is possible to use both sides of the peak for optical
communications.
Also, because it is possible to restrict a loss in the 1380 nm wavelength
range after hydrogen diffusion to under 0.35 dB/km, it is possible to supply a
single mode optical fiber having a low loss in the 1380 nm wavelength range
even if hydrogen diffusion occurs at a low manufacturing cost.
Also, in a step of drawing, by performing a drawing operation using a
drawing apparatus having an annealing device, it is possible to restrict
generation of SiO • to low level. Therefore, there is a little loss increase due to
hydrogen in the 1380 nm wavelength range even if hydrogen diffuses from the
outside of the optical fiber; thus, it is possible to produce a single mode optical
fiber which is durable over a long period.
Also, an initial loss of a single mode optical fiber which is produced by
an above-mentioned manufacturing method is under 0.31 dB/km in the 1380
nm wavelength range, and the peak in the 1380 nm wavelength range can be
small. Therefore, it is possible to use both sides of the wavelength range for
optical communications. Also, because it is possible to restrict a loss in the
1380 nm wavelength range after hydrogen diffusion to under 0.35 dB/km, it is
possible to perform optical communications in 1380 nm wavelength range
with a low loss even if hydrogen diffusion occurs.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
FIG. 1 is a cross section of a glass preform for producing a single mode
optical fiber according to the present invention.
FIG. 2 is a view showing an example of a drawing apparatus which is
used in a manufacturing method of a single mode optical fiber according to the
present invention.
FIG. 3 is a view showing another example of a drawing apparatus
which is used in a manufacturing method of a single mode optical fiber
according to the present invention.
FIG. 4 is a view showing an example of a conventional drawing
apparatus.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is explained with reference to the drawings as
follows.
FIG. 1 is a cross section of a glass preform for producing a single mode
optical fiber according to the present invention.
In FIG. 1, reference numeral 1 indicates a core section having a high
refractive index. Reference numeral 2 indicates a first cladding section
which is disposed around an outer circumference of the core section 1 and has
a lower refractive index than that of the core section 1. Reference numeral 3
indicates a second cladding section having the same refractive index as that of
the first cladding section 2.
A manufacturing method for a glass preform and an optical fiber which
is formed by performing drawing of the glass preform is explained as follows.
First, a porous soot having a core section 1 having a high refractive
index and a first cladding section having a refractive index lower than that of
the core section 1 is produced by using a common Vapor phase axial deposition
apparatus (hereinafter called a VAD apparatus). The core section 1 is
produced by depositioning particles of GeO2 and that of SiO2. The first
cladding section 2 is produced by depositioning particles of SiO2. Refractive
index difference ? of the core section 1 corresponding to the first cladding
section 2 should preferably be 0.3 to 0.4 %. A value of D/d which indicates a
ratio of the diameter of the core section 1 (having diameter d) and the
diameter of the first cladding section 2 (having diameter D) should preferably
be more than 4.0. The reason why the value of D/d should preferably be such
a value is as follows.
When a value of D/d is in a range of 4.0 to 4.8, it is possible to restrict
an initial loss in the 1380 nm wavelength range to under 0.31 dB/km by
restricting the OH concentration of the second cladding section 3 to under 0.1
ppm. When a value of D/d satisfies a condition such as D/d > 4.8, it is
possible to restrict a loss in the 1380 nm wavelength range to under 0.31
dB/km without performing dehydration using chlorine gas because there is
little influence due to OH concentration in the second cladding section 3.
As explained above, if a loss in the 1380 nm wavelength range can be
restricted to under 0.31 dB/km, it is possible to perform optical
communications using a broader wavelength range.
However, if a value of D/d is under a condition of D/d < 4.0, an initial
loss in the 1380 nm wavelength range is larger than 0.31 dB/km even if the
OH concentration of the second cladding section 3 is restricted to under 0.1
ppm; thus, it is impossible to achieve the objects of the present invention.
As explained above, it is preferable that a value of D/d indicating a
ratio of a diameter D of the first cladding section 2 and a diameter d of the
core section 1 should be in a range of 4.0 to 4.8, and that OH concentration of
the core section 1, the first cladding section 2, and the second cladding section
3 should be under 0.1 ppm.
Otherwise, it is preferable that a value of D/d indicating a ratio of the
diameter D of the first cladding section 2 and the diameter d of the core
section 1 satisfy a relationship such as D/d > 4.8, OH concentration of the core
section 1, and the first cladding section 2 should be less than 0.1 ppm, and the
OH concentration of the second cladding section 3 should be under 100 ppm.
After that, dehydration and sintering are performed on the porous soot
so as to produce a glass rod. Here, if the value of D/d is 4.0 to 4.8,
dehydration operation is performed in chlorine gas or in a mixed atmosphere
of chlorine gas and oxygen gas. Also, a sintering operation is performed in an
atmosphere of 1450°C of helium gas.
A second cladding section 3 is formed by performing vapor phase
deposition of SiO2 particles on the outside of the above-mentioned glass rod.
The thickness of the second cladding section 3 is determined according to that
diameter in which the glass rod is formed. For example, if the diameter of an
optical fiber is 125 µm, it is possible for outer vapor phase deposition of SiO2
particles to be performed so that the thickness of the second cladding section 3
is 43 µ m. or less. When the thickness of the second cladding section 3 is
thicker than 43 µ m, this is not preferable because an initial loss in the 1380
nm wavelength range tends to become large.
If dehydration is necessary according to the value of D/d, the
dehydration is performed in an atmosphere of chlorine gas or in a mixed
atmosphere of chlorine gas and oxygen gas on a glass rod to which the vapor
phase deposition of the second cladding section 3 is performed on the outside.
Also, a sintering operation is performed in an atmosphere of helium gas at
1450°C so as to form a glass preform.
Consequently, an optical fiber is formed by performing a drawing
operation of the glass preform. If the drawing is fast, for example, if the
drawing speed is 600 m/min or faster, the optical fiber cools immediately after
the drawing operation. Therefore, it is preferable to use a drawing apparatus
having an annealing device at an exit of the drawing furnace.
An example of a drawing apparatus which is used in this drawing
process is shown in FIGS. 2 and 3.
In FIG. 2, reference numeral 10 indicates a drawing furnace.
Drawing operation is performed on a glass preform 11 by a heater 12 in the
drawing furnace 10 so as to form a bare optical fiber 13. After the bare
optical fiber 13 is cooled in an annealing tube 14, a resin is applied to the bare
optical fiber 13 by a resin applying apparatus so as to form an optical fiber
strand. On a surface of the annealing tube 14, a gas introducing hole 15 is
formed. For a cooling gas, it is possible to use an air, Ar, N2, or mixture of
any of these gases.
Also, a drawing apparatus shown in FIG. 3 is provided with a furnace
with inclined heat zone 16 in place of the annealing tube 14 which is shown in
FIG. 2 so as to cool the optical fiber core 13. Each reference numeral in FIG.
3 indicates the same structure which is indicated by the same reference
numeral as shown in FIG. 2. It is preferable that the furnace with inclined
heat zone 16 maintain a temperature at lower temperatures than a heater 12
in a unit of the drawing furnace 10, for example 400 to 1800°C. It is more
preferable that the inclined furnace can vary temperatures according to zones
thereinside.
In contrast, in FIG. 4, a conventional drawing furnace which does not
have an annealing apparatus is shown. Each reference numeral in FIG. 4
indicates a structure having the same reference numeral shown in FIG. 2. If
such a drawing furnace which does not have an annealing apparatus is used, ,
the annealing effect is not sufficient, and SiO • tends to remain in the optical
fiber. Therefore, the loss in the 1380 nm wavelength range tends to be higher
after hydrogen diffusion.
After an optical fiber is produced by the above-mentioned method, the
optical fiber is exposed to hydrogen gas under a partial pressure of 0.01 atm
for ten days. After that, the loss after hydrogen diffusion is measured. If a
loss in the 1380 nm wavelength range after hydrogen diffusion is 0.35 dB/km
or less, there is no problem in performing optical communications using a
broad wavelength range. However, if a loss in the 1380 nm wavelength range
after hydrogen diffusion is higher than 0.35 dB/km, it is not possible to
achieve the initial object of the present invention.
Examples of a single mode optical fiber produced by the
above-mentioned manufacturing method are shown as follows.
Example 1
A glass preform was produced so that a D/d indicating a ratio of
diameter d of a core section 1 and diameter D of a first cladding section 2 was
4.3, and the OH concentration of the second cladding section 3 was 0.1 ppm or
less. After that, a single mode optical fiber was produced by drawing using a
drawing apparatus having an annealing apparatus. A loss in the 1380 nm
wavelength range was 0.285 dB/km. This value was lower than 0.31 dB/km;
therefore, the loss in the 1380 nm wavelength range was satisfactory
temporarily. Also, a loss in the 1380 nm wavelength range after the
hydrogen test was measured. As a result, the loss was 0.320dB/km. This
value was less than 0.35 dB/km; therefore, the loss in the 1380 nm wavelength
range was satisfactory as a final result in Example 1.
Example 2
A glass preform was produced so that a D/d indicating a ratio of
diameter d of a core section 1 and diameter D of a first cladding section 2 was
4.9, and the OH concentration of the second cladding section 3 was 40 ppm or
less. After that, a single mode optical fiber was produced by drawing using a
drawing apparatus having an annealing apparatus. A loss in the 1380 nm
wavelength range was 0.308 dB/km. This value was lower than 0.31 dB/km;
therefore, the loss in the 1380 nm wavelength range was satisfactory
temporarily. Also, a loss in the 1380 nm wavelength range after the
hydrogen test was measured. As a result, the loss was 0.341 dB/km. This
value was lower than 0.35 dB/km; therefore, the loss in the 1380 nm
wavelength range was satisfactory as a final result in Example 2.
Comparison Example 1
A glass preform was produced so that a D/d indicating a ratio of
diameter d of a core section 1 and diameter D of a first cladding section 2 was
4.1, and the OH concentration of the second cladding section 3 was 0.1 ppm or
less. After that, a single mode optical fiber was produced by drawing using a
drawing apparatus which did not have an annealing apparatus. A loss in the
1380 nm wavelength range was 0.292 dB/km. This value was lower than
0.31 dB/km,' therefore, the loss in the 1380 nm wavelength range was
satisfactory temporarily. Also, a loss in the 1380 nm wavelength range after
the hydrogen test was measured. However, as a result, the loss was 0.359
dB/km. This value was higher than 0.35 dB/km; therefore, the loss in the
1380 nm wavelength range was not satisfactory as a final result in
Comparison Example 1.
Comparison Example 2
A glass preform was produced so that a D/d indicating a ratio of the
diameter d of a core section 1 and the diameter D of a first cladding section 2
was 3.8, and the OH concentration of the second cladding section 3 was 0.1
ppm or less. After that, a single mode optical fiber was produced by drawing
using a drawing apparatus which did not have an annealing apparatus. A
loss in the 1380 nm wavelength range was 0.320 dB/km. This value was
higher than 0.31 dB/km; therefore, the loss in the 1380 nm wavelength range
was not satisfactory temporarily. Also, a loss in the 1380 nm wavelength
range after the hydrogen test was measured. However, as a result, the loss
was 0.371 dB/km. This value was higher than 0.35 dB/km; therefore, the loss
in the 1380 nm wavelength range was not satisfactory as a final result in
Comparison Example 2.
Comparison Example 3
A glass preform was produced so that a D/d indicating a ratio of
diameter d of a core section 1 and diameter D of a first cladding section 2 was
4.3, and the OH concentration of the second cladding section 3 was 35 ppm.
After that, a single mode optical fiber was produced by drawing using a
drawing apparatus which did not have an annealing apparatus. A loss in the
1380 nm wavelength range was 0.317 dB/km. This value was higher than
0.31 dB/km; therefore, the loss in the 1380 nm wavelength range was not
satisfactory temporarily. Also, a loss in the 1380 nm wavelength range after
the hydrogen test was measured. However, as a result, the loss was 0.365
dB/km. This value was higher than 0.35 dB/km; therefore, the loss in the
1380 nm wavelength range was not satisfactory as a final result in
Comparison Example 3.
By the manufacturing method for a single mode optical fiber which is
shown in the above-explained examples, a single mode optical fiber was
manufactured by forming a glass preform 11 by performing vapor phase
deposition of a second cladding section made from SiO2 particles on an outer
circumference of a glass rod comprising a core section 1 and a first cladding
section 2, and performing drawing of the glass preform 11. By such a
manufacturing method, it is possible to greatly reduce bubbles occurring in an
interface between the core and the clad, or between the first cladding section 2
and the second cladding section 3. Also, it is easy to dehydrate the porous
soot on which vapor phase deposition is performed; therefore, it is possible to
produce an optical fiber while controlling OH concentration desirably.
Also, because a silica glass tube is not used, there is no influence such
as bending of a silica glass tube which forms a core rod or a clad. Therefore,
product yield increases, and it is possible to produce a single mode optical
fiber at a low manufacturing cost.
Also, an optical fiber is manufactured so that a value of D/d such as a
ratio of diameter D of the first cladding section 2 and diameter d of the core
section 1 is in a range of 4.0 to 4.8, and the OH concentration of the core
section 1, the first cladding section 2, and the second cladding section 3 is 0.1
ppm or less, a value of D/d such as a ratio of diameter of the first cladding
section and a diameter of the core section is D/d > 4.8, the OH concentration of
the core section 1 and the first cladding section 2 are 0.1 ppm or less, and the
OH concentration of the second cladding section 3 is 100 ppm or less.
Therefore, it is possible to restrict an initial loss in the 1380 nm wavelength
range to under 0.31 dB/km. Also, because the peak in the 1380 nm
wavelength becomes small, it is possible to use both sides of the wavelength
range for optical communications.
Also, because it is possible to restrict a loss in the 1380 nm wavelength
range after hydrogen diffusion to under 0.35 dB/km, it is possible to supply a
single mode optical fiber with a low loss in the 1380 nm wavelength range at a
low manufacturing cost.
Also, it is possible to restrict generation of SiO • to low levels by-
performing drawing operation by a drawing apparatus having an annealing
apparatus in a drawing process. Therefore, it is possible to supply a single
mode optical fiber having a low loss in the 1380 nm wavelength range so as to
be durable for use over long periods even if a hydrogen diffuses from the
outside.
Also, an initial loss of the single mode optical fiber which is produced
by the above-mentioned manufacturing method is 0.31 dB/km or less.
Therefore, the peak in the 1380 nm wavelength range can be small, thus, it is
possible to use both sides of the peak for optical communications. Also, it is
possible to restrict the loss in the 1380 nm wavelength range after the
hydrogen diffusion to 0.35 dB/km or less. Therefore, it is possible to perform
optical communications in the 1380 nm wavelength range even if hydrogen
diffusion occurs.
WE CLAIM :
1. A manufacturing method for a single mode optical fiber, comprising steps
of
forming a glass rod having a core section and a first cladding section
having a refractive index lower than that of the core section;
vapor phase depositing for a second cladding on the first cladding;
sintering the glass rod having the first and second claddings to
produce a glass preform; and
performing the drawing operation on the glass preform to produce an
optical fiber;
wherein the ratio of a diameter D of the first cladding section to the
diameter d of the core section is in a range of 4.0 to 4.8; and OH
concentrations of the core section, the first cladding section, and the second
cladding section are 0.1 ppm or less.
2. A manufacturing method for a single mode optical fiber, comprising steps
of:
forming a glass rod having a core section and a first cladding section
having a refractive index lower than that of the core section;
vapor phase depositing for a second cladding on the first cladding;
sintering the glass rod having the first and second claddings to
produce a glass preform; and
performing the drawing operation on the glass preform to produce an
optical fiber;
wherein the ratio of the diameter of the first cladding section to the
diameter of the core section is > 4.8; the OH concentration of the core section
and the first cladding section are not more than 0.1 ppm; and the OH
concentration of the second cladding section is not more than 100 ppm.
3. A manufacturing method for a single mode optical fiber as claimed in any one of
claims 1 and 2 wherein the initial loss in the 1380 nm wavelength range is not
more than 0.31 dB/km and the loss in the 1380 nm wavelength range after
hydrogen diffusion is not more than 0.35 dB/km.
4. A manufacturing method for a single mode optical fiber as claimed in claim
3, wherein in the drawing process, the drawing operation is performed on the
glass preform by using a drawing device having an annealing unit.
5. A manufacturing method for a single mode optical fiber as claimed in claim
4 wherein the annealing unit comprises a furnace with inclined heat zone and
an annealing tube.
6. A manufacturing method for a single mode optical fiber as claimed in claim
5, wherein in the annealing unit, the annealing atmosphere is any one of an
air, Ar, N2, or mixture thereof.
7. A single mode optical fiber which is manufactured by a manufacturing
method as claimed in any one of claims 1 to 6.
8. A manufacturing method for a single mode optical fiber, substantially as herein described,
particularly with reference to the accompanying drawings.
9. A single mode optical fiber substantially as herein described, particularly with reference to
the accompanying drawings.

An optical fiber is formed by performing vapor phase deposition of
SiO2 on the outside of a glass rod comprising a core section (1) and a first
cladding section (2) and drawing a glass preform which formed by a second
cladding section (3). Also, a single mode optical fiber is manufactured so that
the ratio of the diameter D of the first cladding section and the diameter d of
the core section is in a range of 4.0 to 4.8, and OH concentration is 0.1 ppm or
less. Also, an optical fiber is manufactured so that a value of D/d > 4.8, and
the OH concentration is 0.1 ppm or less. It is thereby possible to maintain an
initial loss in the 1380 nm wavelength range even if hydrogen diffusion
occurs.