This application is divided out of Indian applicaticn no.741/K0LNP/2007
TECHNICAL FIELD
The present invention relates to a single-mode optical fiber (hereinafter, referred
to as an SMF) that has chromatic dispersion characteristics equivalent to those of a
conventional SMF and that has a very small bending loss when bent around a small radius.
Priority is claimed on Japanese Patent Application No. 2004-250039 filed on
August 30,2004 and Japanese Patent Application No. 2004-296369 filed on October 8,
2004, the contents of which are incorporated herein by reference.
BACKGROUND ART
Conventionally, developments of a transmission system and an optical fiber that
use Wavelength Division Multiplexing (WDM) have been actively advanced with the
objective of increasing the data transmission rate in backbone and/or long-distance
systems. The characteristics such as suppression of the nonlinear and control of the
chromatic dispersion effect have been demanded for optical fibers for WDM transmission.
In recent years, fibers in which the dispersion slope is decreased for a system called metro
(metropolitan area network) with a span of about several kilometers or fibers that suffer
virtually no loss increase due to OH are proposed.
On the other hand, when introduction of fibers to offices and homes (Fiber To
The Home; FTTH) is taken into consideration, characteristics different from those for the
above-described optical fibers for transmission are required. In the case of wiring the
fibers in the building or the house, there is the possibility that a very small bending around
a radius such as 15 mm or 10 mm occurs. Furthermore, when the extra length of the
fiber is stored, it is very important that loss increase does not occur even if the fiber is
wound around a small radius. That is, it is a very important characteristic for the optical
fiber for FTTH to be insensitive to a small-radius bend. The connectivity with optical

fibers (many of which are SMFs for transmission at a normal wavelength, i.e., 1300 nm)
used from the base station to the building or to the house is also an important point.
From such viewpoints, many reports on and patent application of optical fibers with
reduced bending loss have been made (for example, refer to Patent Documents 1 to 4 and
Non-Patent Documents 1 to 5).
Patent Document 1: U.S. Patent Application Publication No. 2004/0213531
Patent Document 2: PCT International Publication No. WO 01/27667 pamphlet
Patent Document 3: Japanese Unexamined Patent Application, First Publication No.
2004-133373
Patent Document 4: Japanese Patent No. 2618400
Non-Patent Document 1: Ikeda et al., "Low Bending Loss Optical Fiber with Reduced
Splice Loss," Technical Report of The Institute of Electronics, Information and
Communication Engineers (IEICE), 103, 255, OCS 2003-43 (2003)
Non-Patent Document 2: Sato et al., "Optical Fiber Conforming to Small Bending Radius
for Optical Access," Collected Papers of Lectures of IEICE Society Conference, B-10-30
(2003)
Non-Patent Document 3: S. Matsuo, et al., "Bend-insensitive and Low-splice-loss optical
fiber for indoor wiring in FTTH", Technical Digest of OFC 2004, ThI3 (2004)
Non-Patent Document 4: Ikeda et al., "Low Bending Loss Optical Fiber with Reduced
Splice Loss," Collected Papers of IEICE General Meeting 2004, B-10-1 (2004)
Non-Patent Document 5:1. Sakabe, et al., "Enhanced Bending Loss Insensitive Fiber and
New Cables for CWDM Access Network," Proceedings of the 53rd IWCS, pp. 112-118
(2004)
In the current FTTH system, a Passive Optical Network (PON) that employs
SMFs for transmission in the 1300-nm wavelength band is widely used.
However, these conventional optical fibers have generally allowed bending radius
of about 30 mm. In the wiring of the fibers, close attention has been required to

eliminate an extra bending.
Recently, SMFs have been commercialized that permit allowable bending
radiuses down to 15 mm by reducing the mode field diameter (hereinafter, referred to as
MFD) while maintaining the chromatic dispersion characteristics that comply with ITU-T
G.652 (hereinafter, referred to as G.652), the international standard for the SMF for the
1300-nm wavelength band. However, such optical fibers had a problem in that bending
loss suddenly increases with a bending radius of 15 mm or less. FIG. 1 is a graph
exemplifying a bending radius dependence of the bending loss of an optical fiber with an
allowable bending radius of 15 mm. As shown in FIG. 1, the optical fiber with the
conventional allowable bending radius of 15 mm suffers a sudden increase in bending loss
when the bending radius falls below 10 mm.
For a wiring use in buildings and homes, there are cases where a bending radius
of 15 mm or less is needed. The optical fibers proposed in the above-mentioned Patent
Documents 1 to 3 and Non-Patent Documents 1 to 5 are assumed to be used in an
environment in which a bending around a 15-mm radius or less may be applied. An
optical fiber with enhanced bending characteristics generally has a longer zero-dispersion
wavelength, and thus has a larger absolute value of the chromatic dispersion in the
1300-nm wavelength band compared with the normal SMF. For example, Patent
Document 1 shows an example in which the optical fiber with low bending loss disclosed
therein is -4.6 to -10.7 ps/nm/km in the 1300-nm wavelength band. The chromatic
dispersion of this conventional optical fiber with low bending loss is large as the absolute
value compared with the fact that the chromatic dispersion in the 1300-nm band under
G.652 is in the range of 0 to -2.2 ps/nm/km when calculated by the definition of the
zero-dispersion wavelength and the slope under G. 652. However, the chromatic
dispersion on this level has rarely posed a problem over a distance on the order of several
tens of meters such as in indoor wiring.
On the other hand, there is a need also for optical fibers for transmission lines to
be resistant to bending loss when their handling in a cable or a closure box is taken into
consideration. However, there are cases where the chromatic dispersion value of the low

bending loss optical fiber shown in Patent Document 1 poses a problem when the fiber is
used in the PON system. ITU-T G.983 or the like rules that, in the PON system used for
the FTTH service, the 1500-nm wavelength band is used for the transmission from the
base station to the user and that the 1300-nm wavelength band is used for the transmission
from the user to the base station. As for the light source for the 1300-nm wavelength
band, an inexpensive Fabry-perot laser (hereinafter, referred to as FP laser) is widely used.
Because the FP laser is based on multimode oscillation, the characteristics thereof are
greatly affected by the chromatic dispersion value of the optical fiber that works as the
transmission line. Current transmission apparatuses are designed with the chromatic
dispersion characteristics of G.652 in mind. Therefore, there are cases where a
large-absolute-valued chromatic dispersion value that the conventional, low bending loss
fibers have is not preferable because it may cause a communication failure.
DISCLOSURE OF INVENTION
The present invention has been achieved in view of the above-mentioned
circumstances, and has an object to provide an SMF with a very small bending loss while
conforming to the characteristics defined under G. 652.
To achieve the above-mentioned object, the present invention provides an SMF,
in which a cut-off wavelength is 1260 nm or less, a zero-dispersion wavelength is in the
range of 1300 nm to 1324 nm, a zero-dispersion slope is 0.093 ps/nm2/km or less, an MFD
at a wavelength of 1310 nm is in the range of 5.5 μm to 7.9μm, and a bending loss that is
produced when the fiber is wound around a 10-mm radius for 10 turns is 0.5 dB or less at
a wavelength of 1550 nm.
In the SMF of the present invention, it is preferable that the cut-off wavelength be
any of a cable cut-off wavelength, a fiber cut-off wavelength, and a jumper cut-off
wavelength.
The SMF of the present invention, it is preferable that the chromatic dispersion
value at a wavelength of 1550 nm be +18 ps/nm/km or less, and more preferably +17
ps/nm/km or less.

In the SMF of the present invention, it is preferable that an RDS defined by
dispersion slope/chromatic dispersion value be in the range of 0.003 nm-1 to 0.004 nm-1 at
a wavelength of 1550 nm. ,
In the SMF of the present invention, it is preferable that a bending loss that is
produced when the fiber is wound around a 10-mm radius for 10 turns be 0.1 dB or less at
a wavelength of 1550 nm.
In the SMF of the present invention, it is preferable that a bending loss that is
produced when the fiber is wound around a 7.5-mm radius for 10 turns be 0.5 dB or less at
a wavelength of 1550 nm. It is more preferable that a bending loss that is produced when
the fiber is wound around a 5.0-mm radius for 10 turns be 0.5 dB or less at a wavelength
of l550 nm.
The SMF of the present invention preferably includes: a central core that has a
radius r1 and a refractive index n1; an inner cladding that is provided around the outer
circumference of the central core and has a radius r2 and a refractive index n2; a trench
portion that is provided around the outer circumference of the inner cladding and has a
radius r3 and a refractive index n3; and an outer cladding that is provided around the outer
circumference of the trench portion and has a radius r4 and a refractive index n4, with a
refractive index profile in which the refractive indexes of the individual portions satisfy n1
> n4 ≥ n2 > n3.
In the SMF with the trench portion, it is preferable that with reference to the
refractive index 114 of the outer cladding, a relative refractive index difference Δ1 of the
central core, a relative refractive index difference Δ2 of the inner cladding, and a relative
refractive index differenceΔ3of the trench portion satisfy the relations as follows:
0.40% ≤ Δ1 ≤ 0.85%
-0.20%≤Δ2≤ 0.00%
-1.0%<Δ3<Δ2.
In the SMF with the trench portion, it is preferable that a radius r1 of the central
core, a radius r2 of the inner cladding, and a radius r3 of the trench portion satisfy the
relations as follows:

1.5 < r2/r1 < 3.0
0.5<(r3-r2)/r1<3.0.
In the SMF with the trench portion, it is preferable that the radius r3 of the trench
portion be in the range of 6 μm to 20 μm.
In the SMF with the trench portion, it is preferable that a radius r4 of the outer
cladding be in the range of 28 μm to 64 μm
The SMF of the present invention preferably includes: a central core that has a
radius r1 and a refractive index n1; an inner cladding that is provided around the outer
circumference of the central core and has a radius r2 and a refractive index n2, and an outer
cladding that is provided around the outer circumference of the inner cladding and has a
radius r4 and a refractive index n4, the SMF having a W-shaped refractive index profile in
which the refractive indexes of the individual portions satisfy n1 > n4 > n2.
In the SMF with the W-shaped refractive index profile, letting, a relative
refractive index difference of the central core be A1 and a relative refractive index
difference of the inner cladding be A2 with reference to the outer cladding, it is preferable
that the following relations be satisfied:
0.42% ≤A1≤ 0.85%
1.5≤r2/r1≤5.0
-1.0%≤Δ2≤-0.05%.
In the SMF with the W-shaped refractive index profile, letting y=(r2/r1)-|Δ2|, it is
preferable that the following relations be satisfied:
1.4-Δ1 - 0.8 ≤y≤ 1.4Δ-0.05
y≤ 0.075%.
In the SMF with the W-shaped refractive index profile, it is preferable that the
radius r2 of the inner cladding be in the range of 4.5 μm to 16 μm
In the SMF with the W-shaped refractive index profile, it is preferable that the
radius r4 of the outer cladding be in the range of 28 μm to 64 μm
The SMF of the present invention has the characteristics as follows: a cut-off
wavelength is 1260 nm or less; a zero-dispersion wavelength is in the range of 1300 nm to

1324 nm; a zero-dispersion slope is 0.093 ps/nm2/km or less; an MFD at a wavelength of
1310 nm is in the range of 5.5 um to 7.9 nm; and a bending loss that is produced when the
fiber is wound around a 10-mm radius for 10 turns is 0.5 dB or less at a wavelength of
1550 nm. Therefore, it can actualize an SMF with a very small bending loss while
conforming to the chromatic dispersion characteristics defined under G 652.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph exemplifying a bending radius dependence of a bending loss in
the conventional SMF.
FIG. 2 is a graph exemplifying a Δ3 dependence of the bending loss.
FIG. 3 is a graph showing the refractive index profile of the low bending loss
SMF produced in Example 1.
FIG. 4 is a graph showing the refractive index profile of the low bending loss
SMF produced in Example 2.
FIG. 5 is a graph showing the refractive index profile of the low bending loss
SMF produced in Example 3.
FIG. 6 is a graph showing the refractive index profile of the low bending loss
SMF produced in Example 4.
FIG. 7 is a graph showing the refractive index profile of the low bending loss
SMF produced in Example 5.
FIG. 8 is a graph showing the refractive index profile of the low bending loss
SMF produced in Example 6.
DESCRIPTION OF SYMBOLS
1: central core; 2: inner cladding; 3: trench portion; 4, 5: outer cladding; 10, 20,
30,40, 50, 60: SMF
BEST MODE FOR CARRYING OUT THE INVENTION
The SMF of the present invention has the characteristics as follows: a cut-off

wavelength is 1260 nm or less; a zero-dispersion wavelength is in the range of 1300 nm to
1324 nm; a zero-dispersion slope is 0.093 ps/nm2/km or less; an MFD at a wavelength of
1310 nm is in the range of 5.5 nm to 7.9 μm; and a bending loss that is produced when the
fiber is wound around a 10-mm radius for 10 turns is 0.5 dB or less at a wavelength of
1550 nm.
Furthermore, the cut-off wavelength of the low bending loss SMF of the present
invention is defined by the cable cut-off wavelength, the fiber cut-off wavelength, or the
jumper cut-off wavelength, according to the conditions of use of the fiber. Measurement
methods for each of the cut-off wavelengths are defined under ITU-T G. 650.1,
"Definitions and test methods for linear, deterministic attributes of single-mode fibre and
cable."
In the low bending loss SMF of the present invention, it is preferable that the
chromatic dispersion value at a wavelength of 1550 nm be +18 ps/nm/km or less. G.652
lists 17 ps/nm/km as the typical value for the chromatic dispersion value at a wavelength
of 1550 nm. When the chromatic dispersion value shows an extremely larger value than
this, it is not preferable from a viewpoint of transmission line design.
Furthermore, in the low bending loss SMF of the present invention, it is
preferable that a Relative Dispersion Slope (RDS) at a wavelength of 1550 nm be in the
range of 0.003 nm' to 0.004 nm" . This RDS is a parameter acquired by (dispersion
slope)/(chromatic dispersion value), and is an indicator for determining the matching
between a dispersion compensating fiber and a dispersion compensated fiber. The RDS
of the optical fiber defined under the current G.652 (hereinafter, referred to as G.652 fiber)
is about 0.0032 nm" . When a high-speed, long-distance transmission is performed, a
dispersion compensating fiber is indispensable. If the optical fiber has the same RDS as
the G.652 fiber, which is currently in wide use, it is possible to compensate the chromatic
dispersion of the fiber using the dispersion compensating fiber for the G.652 fiber to be
used as the G.652 fiber. In the SMF of the present invention, a dispersion compensating
fiber for the G.652 fiber can be utilized if the RDS thereof is in the range of 0.003 nm"1 to
0.004 nm"1.

In a preferred embodiment of the present invention, it is preferable that the low
bending loss SMF include: a central core that has a radius r1 and a refractive index n1; an
inner cladding that is provided around the outer circumference of the central core and has
a radius r2 and a refractive index n2; a trench portion that is provided around the outer
circumference of the inner cladding and has a radius r3 and a refractive index n3; and an
outer cladding that is provided around the outer circumference of the trench portion and
has a radius r4 and a refractive index n4, with a refractive index profile in which the
refractive indexes of the individual portions satisfy n1 > 114 > n2 > n3. Note that the
respective radiuses r2, r3, and r4 of the inner cladding, the trench portion, and the outer
cladding are distances from the center of the central core to the outer periphery edge of the
relevant portions. FIG. 3 to FIG. 5 exemplify a refractive index profile of respective low
bending loss SMFs 10,20, and 30 with a trench portion. In the figures, reference
numeral 1 denotes a central core, reference numeral 2 an inner cladding, reference
numeral 3 a trench portion, and reference numerals 4 and 5 outer claddings.
Such refractive index profiles are disclosed in Patent Document 4. The
invention described in Patent Document 4 discloses the effect of this refractive index
profile in the design of a so-called dispersion-shifted optical fiber that takes the
zero-dispersion wavelength near 1550 nm. However, it does not disclose the main object
of the present invention, that is, the effect of this refractive index profile with the
zero-dispersion wavelength near 1300 nm.
In the low bending loss SMF 10,20, and 30 with the trench portion, it is
preferable that with reference to the refractive index 114, a relative refractive index
difference Δ1 of the central core, a relative refractive index difference Δ2 of the inner
cladding, and a relative refractive index differenceΔ3of the trench portion satisfy the
relations as follows:
0.40% ≤ Δ1 ≤ 0.85%
-0.20% ≤Δ2≤ 0.00%
-1.0%<Δ3<Δ2.

Furthermore, it is preferable that a radius r1 of the central core, a radius r2 of the inner
cladding, and a radius r3 of the trench portion satisfy the relations as follows:
1.5 n4 > n2. Note that the respective radiuses r2 and r4 of the
inner cladding and the outer cladding are distances from the center of the central core to
the outer periphery edge of the relevant portions. FIG. 6 to FIG. 8 exemplify refractive
index profiles of the low bending loss SMFs 40, 50, and 60 with the W-shaped refractive
index profile of the present invention. In the figures, reference numeral 1 denotes a
central core, reference numeral 2 an inner cladding, and reference numerals 4 and 5 outer
claddings.

In the low bending loss SMFs 40, 50, and 60 with the W-shaped refractive index
profile, letting a relative refractive index difference of the central core be Δ1 and a relative
refractive index difference of the inner cladding be Δ2 with reference to the outer cladding,
it is preferable that the following relations be satisfied:
0.42% ≤ Δ1 ≤ 0.85%
1.5≤r2/r1≤5.0
-1.0% ≤Δ2≤ -0.05%.
Furthermore, letting y = (r2/r1)-Δ2, it is preferable that the following relations be
satisfied:
1.4-Δ1-0.8≤y≤1.4-Δ1-0.05
y> 0.075%.
When the relative refractive index difference Δ1 of the central core falls below
0.42%, the MFD at a wavelength of 1310 nm exceeds 7.9 . In association with this,
the bending loss produced when the fiber is wound around a 10-mm radius for 10 turns
exceeds 0.5 dB at a wavelength of 1550 nm. Therefore, the actualization of the
characteristic of low bending loss, which is the object of the present invention, cannot be
achieved. On the other hand, when the relative refractive index difference Δ1 exceeds
0.85%, the MFD at a wavelength of 1310 falls below 5.5 μm This may lead to
deteriorated connectivity, and thus is not preferable. As for r2/r1 and Δ2 that define the
inner cladding, it is preferable that r2/r1 be in the range of 1.5 to 5.0 and Δ2 be in the range
of -1.0% to -0.05%. For these parameters, appropriate values are selected according to
the manufacturing methods.
Furthermore, as for r2/r1, Δ1, and Δ2, it is preferable that y defined by y =
(r2/r1)-|Δ2| be set so as to be in the range of (1.4-Δ1 - 0.8) to (1.4-Δ1 - 0.05). Setting y in
the above-mentioned range with respect to Δ1 enables the restriction of the zero-dispersion
wavelength in the range of 13 00 nm to 1324 nm. Note that the above-mentioned two
conditions cannot be satisfied also when y is less than 0.075%.
In the low bending loss SMFs 40, 50, and 60 with the W-shaped refractive index
profile, it is preferable that the radius r2 of the inner cladding be in the range of 4.5 μm to

16 μm
The radius r4 of the outer cladding can be set irrespective of the above-described
parameters. In many cases, the radius r4 of the outer cladding of a general optical fiber
has 62.5 μm (diameter of 125 urn) as its center value. However, the center value for the
radius is appropriately set in the range of 30 μm to 62.5μm according to the purpose of
use. For example, in an optical fiber for component use that is assumed to be stored by
being wound around a small bending radius, the radius is often set to 40 μm Therefore,
it is desirable that r4 be in the range of 28 μm to 64 μm, with the manufacture margin
taken into consideration.
Tables 4 to 6 show design examples, in various conditions, of the low bending
loss SMF according to the present invention with a W-shaped refractive index profile
(refer to Example 23 to Example 44). In these Example 23 to Example 44, the cable
cut-off wavelength is 1220 nm.

wavelength of 1550 nm is +18ps/nm/km or less, and the bending loss produced when the
fiber is wound around a 10-mm radius for 10 turns is 0.5 dB or less at a wavelength of
1550 nm. Therefore, an SMF with very low bending loss can be actualized while
conforming to the chromatic dispersion characteristics defined under G.652.
Examples
(Example 1)
In this example, the low bending loss SMF according to the present invention was
manufactured based on Example 14 shown in Table 2. Fig. 3 shows the refractive index
profile of the low bending loss SMF 10 manufactured in this example. This low bending
loss SMF 10 was manufactured by first layering the central core 1 made of silica-based
glass, the inner cladding 2, the trench portion 3, and a part of the outer cladding 4 by the
MCVD method, then performing an outside vapor deposition to layer the remaining outer
cladding 5, and finally drawing the obtained optical fiber preform in the same manner as
in the normal SMF. The characteristics of the obtained low bending loss SMF 10 were
measured. The results are shown in Table7.


In the low bending loss SMF 10 of this example, the MFD at a wavelength of
1310 was 7.40 urn, which is lower than the range of G.652. However, the
zero-dispersion wavelength was 1316.5 nm. Therefore, this satisfied the definition under
G.652. The chromatic dispersion value at a wavelength of 1550 nm was 16.5 ps/nm/km,
which is a value in conformance with the typical value of G.652. The bending loss
produced when the fiber is wound around a 10-mm radius for 10 turns is very small, that is,

0.03 dB at a wavelength of 1550 nm. Therefore, an SMF with a very small bending loss
while retaining the chromatic dispersion characteristics of G. 652 was obtained.
(Example 2)
In this example, the low bending loss SMF according to the present invention was
manufactured based on Example 6 shown in Table 1. Fig. 4 shows the refractive index
profile of the low bending loss SMF 20 manufactured in this example. This low bending
loss SMF 20 was manufactured by first layering the central core 1 made of silica-based
glass, the inner cladding 2, the trench portion 3, and a part of the outer cladding 4 by the
MCVD method, then performing an outside vapor deposition to layer the remaining outer
cladding 5, and finally drawing the obtained optical fiber preform in the same manner as
in the normal SMF. The characteristics of the obtained low bending loss SMF 20 were
measured. The results are shown in Table 8.


In the low bending loss SMF 20 of this example, the MFD at a wavelength of
1310 was 6.19 um, which is even lower than the MFD of Example 1. However, the
zero-dispersion wavelength was 136.2 nm. Therefore, this satisfied the definition under
G652. The chromatic dispersion value at a wavelength of 1550 nm was 16.6 ps/nm/km,
which is a value in conformance with the typical value of G.652.
As for the bending loss, very small values less than 0.1 dB at a wavelength of
1550 nm were obtained when the fiber was wound around not only a 10-mm radius but

also around a 7.5-mm radius and a 5.0-mm radius for 10 turns. Furthermore, also at a
wavelength of 1650 nm, virtually no bending loss was observed in a 10-mm radius and a
7.5-mm radius. Thus, in this example, an SMF with a very small bending loss while
retaining the chromatic dispersion characteristics of G. 652 was obtained.
(Example 3)
In the example, the low bending loss SMF according to the present invention was
manufactured based on Example 6 shown in Table 1. Fig. 5 shows the refractive index
profile of the low bending loss SMF 30 manufactured in this example. This low bending
loss SMF 30 was manufactured by first layering the central core 1 made of silica-based
glass, the inner cladding 2, the trench portion 3, and a part of the outer cladding 4 by the
MCVD method, then performing an outside vapor deposition to layer the remaining outer
cladding 5, and finally drawing the obtained optical fiber preform in the same manner as
in the normal SMF. The characteristics of the obtained low bending loss SMF 30 were
measured. The results are shown in Table 9.


In the low bending loss SMF 30 of this example, the MFD at a wavelength of
1310 was 7.67 μm, which is larger than the SMF of Example 1. The zero-dispersion
wavelength was 1309.3 run. Therefore, this satisfied the definition under G.652. The
chromatic dispersion value at a wavelength of 1550 nm was 17.3 ps/nm/km, which is a
value in conformance with the typical value of G.652. The bending loss produced when
the fiber is wound around a 10-mm radius for 10 turns is very small, that is, 0.03 dB at a
wavelength of 1550 nm. Therefore, an SMF with a very small bending loss while

retaining the chromatic dispersion characteristics of G. 652 was obtained.
(Example 4)
In this example, the low bending loss SMF according to the present invention was
manufactured based on Example 24 shown in Table 4. Fig. 6 shows the refractive index
profile of the low bending loss SMF 40 manufactured in this example. This low bending
loss SMF 40 was manufactured by first layering the central core 1 made of silica-based
glass, the inner cladding 2 by the VAD method, then layering the outer cladding 5 by the
outside vapor deposition method, and finally drawing the obtained optical fiber preform in
the same manner as in the normal SMF. The characteristics of the obtained low bending
loss SMF 40 were measured. The results are shown in Table 10.


In the low bending loss SMF 40 of this example, the MFD at a wavelength of
1310 was 7.90 μm, which is lower than the range of G.652. However, the
zero-dispersion wavelength was 1313.8 nm. Therefore, this satisfied the definition under
G.652. The chromatic dispersion value at a wavelength of 1550 nm was 16.4 ps/nm/km,
which is a value in conformance with the typical value of G.652. The bending loss
produced when the fiber is wound around a 10-mm radius for 10 turns is very small, that is,
0.14 dB at a wavelength of 1550 nm. Therefore, an SMF with a very small bending loss

while retaining the chromatic dispersion characteristics of G.652 was obtained.
(Example 5)
In this example, the low bending loss SMF according to the present invention was
manufactured based on Example 28 shown in Table 4. Fig. 7 shows the refractive index
profile of the low bending loss SMF 50 manufactured in this example. This low bending
loss SMF 50 was manufactured by first layering the central core 1 made of silica-based
glass, the inner cladding 2, and a part of the outer cladding 4 by the MCVD method, then
performing an outside vapor deposition to layer the remaining outer cladding 5, and finally
drawing the obtained optical fiber preform in the same manner as in the normal SMF.
The characteristics of the obtained low bending loss SMF 50 were measured. The results
are shown in Table 11.


In the low bending loss SMF 50 of this example, the MFD at a wavelength of
1310 was 7.28 μm, which is lower than the range of G.652, but the zero-dispersion
wavelength was 1302.3 nm. Therefore, this satisfied the definition under G.652. The
chromatic dispersion value at a wavelength of 1550 nm was 16.6 ps/nm/km, which is a
value in conformance with the typical value of G.652. The bending loss produced when
the fiber is wound around a 7.5-mm radius for 10 turns is very small, that is, 0.15 dB at a
wavelength of 1550 nm. Therefore, an SMF with a very small bending loss while

retaining the chromatic dispersion characteristics of G.652 was obtained.
(Example 6)
In this example, the low bending loss SMF according to the present invention was
manufactured based on Example 35 shown in Table 5. Fig. 8 shows the refractive index
profile of the low bending loss SMF 60 manufactured in this example. This low bending
loss SMF 60 was manufactured by first layering the central core 1 made of silica-based
glass, the inner cladding 2, and a part of the outer cladding 4 by the MCVD method, then
performing an outside vapor deposition to layer the remaining outer cladding 5, and finally
drawing the obtained optical fiber preform in the same manner as in the normal SMF.
The characteristics of the obtained low bending loss SMF 60 were measured. The results
are shown in Table 12.


In the low bending loss SMF 60 of this example, the MFD at a wavelength of
1310 was 6.27 urn, which is lower than the range of G.652. However, the
zero-dispersion wavelength was 1310.8 nm. Therefore, this satisfied the definition under
G.652. The chromatic dispersion value at a wavelength of 1550 nm was 15.6 ps/nm/km,
which is a value in conformance with the typical value of G.652. The bending loss
produced when the fiber is wound around a 5-mm radius for 10 turns is very small, that is,
0.09 dB at a wavelength of 1550 nm. Therefore, an SMF with a very small bending loss

while retaining the chromatic dispersion characteristics of G.652 was obtained.
While preferred embodiments of the present invention have been described above,
these are not considered to be limitative of the invention. Addition, omission, and
replacement of the constituents, and other modifications can be made without departing
from the spirit or scope of the invention. The present invention is not limited by the
descriptions above, but is limited only by the appended claims.

We Claim :
1. A single-mode optical fiber comprising:
a central core which has a radius r1 and a refractive index n1;
an inner cladding which is provided around an outer circumference of the central core and has a
radius r2 and a refractive index n2; and
an outer cladding which is provided around the outer circumference of the inner cladding and
has a radius n and a refractive index n4, wherein :
a cut-off wavelength is not greater than 1260 nm, a zero-dispersion wavelength is between 1300
nm and 1324 nm, a zero-dispersion slope is not greater than 0.093 ps/nm2/km or less, a mode field
diameter at a wavelength of 1310 nm is between 5.5 μm and 7.9 μm, and a bending loss produced if
the the fiber is wound around a 10-mm radius for 10 turns is not greater than 0.5 dB at a wavelength of
1550 nm;
n1 > n4 > n2;and
Δ1 is a relative refractive index difference of the central core, Δ2 is a relative refractive index
difference of the inner cladding with reference to the outer cladding, and the following conditions are
satisfied:
0.42% ≤Δ1,≤ 0.85%
1.5≤r2/r1,≤5.0
-1.0% ≤Δ2≤ -0.05%.
2. The single-mode optical fiber according to claim 1, wherein y=(r2/r1)-|Δ2|, and the following
conditions are satisfied:
1.4-Δ1, - 0.8 ≤y≤1.4-Δ1,-0.05
y > 0.075%.

3. The single-mode optical fiber according to claim 1, wherein the radius r2 of the inner cladding is
between 4.5 μm and 16 μm
4. The single-mode optical fiber according to claim 1, wherein the radius r4 of the outer cladding is
between 28 μm and 64 μm.

A single-mode optical fiber has a cut-off wavelength of 1260 nm or less, a zero-dispersion wavelength
in the range of 1300 nm tol324 nm, a zero-dispersion slope of 0.093 ps/nm2/km or less, a mode field
diameter at a wavelength of 1310 nm in the range of 5.5 μm to 7.9 μm, and a bending loss of 0.5 dB or
less at a wavelength of 1550 nm, the bending loss being produced when the fiber is wound around a 10-
mm radius for 10 turns. As exemplified in FIG. 6, the single-mode optical fiber has a central core (1)
which has a refractive index n1; an inner cladding (2) which has a refractive index n2; and an outer
cladding (5) which has a refractive index n4, in which n1 > n4 ≥ n4 > n2