DESCRIPTION
OPTICAL FIBER AND OPTICAL FIBER PREFORM
Technical Field
[0001]
The present invention relates to an optical fiber and an optical fiber preform.
There was a problem in that when an optical analog signal or an optical baseband signal
is transmitted for a long distance by using an optical fiber, due to an effect of stimulated
Brillouin scattering (hereinafter, referred to as SBS), only a predetermined intensity of
light (SBS threshold power) can be transmitted through the optical fiber although light
with higher power was intended to be transmitted, and the remaining light becomes
backscattered light and returns to the incoming side, so that transmissible power of signal
light is limited. The invention relates to an optical fiber capable of suppressing
occurrences of SBS and transmitting higher power of signal light.
Priority is claimed on Japanese Patent Application No. 2006-249360, filed on
September 14, 2006, the content of which is incorporated herein by reference.
Background Art
[0002]
Recently, fiber to the home (FTTH) services in which homes are connected via
optical fibers to exchange information using the optical fibers have been expanded. In
FTTH services for transmitting various types of information, there is a system for
simultaneously transmitting a broadcasting signal and other communications signals in
their respective modes using a single optical fiber. In general, in this system, in many
cases, the broadcasting signal is an analog signal, a baseband signal, or an optical SCM

signal. The characteristics of this system from the point of optical fiber as a
transmission medium are as follows:
(i) FTTH is based on a typical double-star PON (passive optical network), and a
distribution loss increases.
(ii) In order to transmit an analog signal, a baseband signal, or an optical SCM
signal, a receiver needs a high CNR (carrier-to-noise ratio), so that the minimum signal
light power of a light receiving unit needs to be greater than that in digital transmission
used for communications.
[0003]
As described above, when analog transmission using intensity modulation is
performed for video transmission, high power transmission is needed to compensate for a
distribution loss and guarantee a high CNR. However, there was a problem in that only
a predetermined intensity of light (SBS threshold power) can be transmitted through the
optical fiber although higher power is intended to be transmitted, and the remaining light
becomes backscattered light and returns to the incident side, so that transmissible signal
light power was limited.
[0004]
As means for suppressing SBS, there is a technique for changing a dopant
concentration and a residual stress in a longitudinal direction (for example, see Patent
Document 1). In this technique, by changing the dopant concentration or the residual
stress in the longitudinal direction, it is possible to enlarge the Brillouin spectrum and
suppress occurrences of SBS. In addition, techniques for giving such a refractive index
profile for an optical fiber that SBS can be suppressed, are proposed (for example, see
Patent Documents 2 to 5, and 7).
Patent Document 1: Japanese Patent No. 2584151

Patent Document 2: WO 2004/100406
Patent Document 3: U.S. Patent No. 7082243
Patent Document 4: Japanese Unexamined Patent Application Publication No.
2006-154707
Patent Document 5: Japanese Unexamined Patent Application Publication No.
2006-184534
Patent Document 6: Japanese Unexamined Patent Application Publication No.
2006-133314
Patent Document 7: Japanese Unexamined Patent Application Publication No.
2006-154713
Non-Patent Document 1: "Design concept for optical fibers with enhanced SBS
threshold" Optics Express, Vol. 13, Issue 14, p. 5338 (July 2005), Andrey
Non-Patent Document 2: "Nonlinear Optical Fibers with Increased SBS
Thresholds" OFC/NFOEC 2006, OtyA3, Scott Bickham, Andrey Kobyakov, Shenping Li
Disclosure of the Invention
Problems that the Invention is to Solve
[0005]
As a technique for suppressing SBS, as described above, the technique for
changing a dopant concentration or a residual stress in a longitudinal direction is reported
(Patent Document 1). However, in this technique, optical characteristics are changed in
the longitudinal, so that this technique is not preferable for practical use.
[0006]
In addition, techniques for suppressing SBS by giving a refractive index profile
for an optical fiber (Patent Documents 2 to 5 and 7) are reported. In theses techniques,

a change in optical characteristics in a longitudinal direction does not occur, However,
a structure having a refractive index profile suitable for desired characteristics is needed.
[0007]
In Patent Documents 2, 3, and 5, for an optical fiber described which has a
three-layer structured refractive index profile, the refractive index profile is set to a
suitable condition, thereby suppressing SBS and obtaining the same optical
characteristics as ITU-T Recommendation G.652 (hereinafter, referred to as G652).
However, not all structures described in Patent Documents 2, 3, and 5 satisfy the same
optical characteristics as G652, and when manufacturing is practically performed on the
basis of these conditions, a suitable design value for each condition is needed.
[0008]
In Patent Document 4, a uniform bending loss becomes worse, and in
consideration of handling of the optical fiber, this phenomenon is not preferable.
In Patent Document 6, fluorine needs to be added at an intentional position, so
that manufacturing a preform using a VAD method is difficult.
In Patent Document 7, only the shape of a refractive index profile is described,
and detailed parameters and the like are not mentioned.
Means for Solving the Problems
[0009]
The invention is designed to solve the above-mentioned problems. An object
of the invention is to provide an optical fiber and an optical fiber preform which have
stable characteristics in a longitudinal direction, compatibility with G652, excellent
manufacturability, and suppressed SBS, by giving a structure design value for a suitable
refractive index profile.

[0010]
According to a first aspect of the invention, there is provided an optical fiber
having a refractive index profile constituted by: a three-layer structured core which
includes, in the central portion of the core, a first core having a substantially uniform
positive relative refractive index difference Δl in a region from the center of the core to a
radius of Rl urn, a second core which surrounds and comes in contact with the first core
and has a substantially uniform positive relative refractive index difference Δ2 in a region
from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and
comes in contact with the second core and has a substantially uniform positive relative
refractive index difference Δ3 in a region from the radius of R2 µm to a radius of R3 µm;
and a cladding which surrounds and comes in contact with the three-layer structured core
and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%,
Δl, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1, when Δl, Δ2, and Δ3
have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y
satisfy 0.25% Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and
Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, X and Y satisfy 0.25% Δ2, Δ3> Δ2, and
Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4%
is satisfied, and X and Y satisfy 0.25% Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and
Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%+3 dB.
FIG 9 is a view showing a relationship between Δ2+Δ3 and R2/R1 satisfying
MFD=7.9tol0.2um.
FIG. 10 is a view showing a relationship between A3-A1 and SBSeff.
FIG. 11 is a view showing a relationship between Δ1-Δ2 and SBSeff.
FIG. 12 is a view showing a relationship between Δ1-Δ2 and SBSeff.
FIG. 13 is a view showing relationships between SBS thresholds and MFDs of
Examples 1 and la to lg.
FIG. 14 is a view showing relationships between SBS thresholds and MFDs of
Examples 1 and la to lv.
FIG 15 is a view showing relationships between SBS thresholds and MFDs of
Examples 2a to 2f.

FIG. 16 is a view showing relationships between SBS thresholds and MFDs of
Examples 2g to 2m.
FIG. 17 is a view showing a refractive index profile of an optical fiber of
Example 3.
FIG. 18 is a view showing a refractive index profile of an optical fiber preform
of Example 5.
FIG. 19 is a view showing relationships between SBS thresholds and MFDs of
Example 5.
FIG. 20 is a view showing a refractive index profile of an optical fiber preform
of Example 6.
FIG. 21 is a view showing relationships between SBS thresholds and MFDs of
Example 6.
FIG. 22 is a view showing a refractive index profile of an optical fiber preform
of Example 7.
Description of Symbols
[0012]
1: LIGHT SOURCE AT WAVELENGTH OF 1.32 µm
2: LIGHT SOURCE AT WAVELENGTH OF 1.55 µm
3: EDFA
4: POWER METER FOR MEASURING BACKSCATTERED LIGHT POWER
5: 9:1 COUPLER
6: POWER METER FOR MEASURING INCOMING LIGHT POWER
7: POWER METER FOR MEASURING TRANSMITTED LIGHT POWER
8: OPTICAL FIBER TO BE MEASURED

Best Mode for Carrying Out the Invention
[0013]
An optical fiber of the invention has a refractive index profile constituted by: a
three-layer structured core which includes, in the central portion of the core, a first core
having a substantially uniform positive relative refractive index difference Δ1 in a region
from the center of the core to a radius of Rl µm, a second core which surrounds and
comes in contact with the first core and has a substantially uniform positive relative
refractive index difference Δ2 in a region from the radius of Rl urn to a radius of R2 µm,
and a third core which surrounds and comes in contact with the second core and has a
substantially uniform positive relative refractive index difference Δ3 in a region from the
radius of R2 urn to a radius of R3 µm; and a cladding which surrounds and comes in
contact with the three-layer structured core and has a substantially uniform refractive
index, or has a refractive index profile constituted by: a three-layer structured core which
includes, in the central portion of the core, a first core having a maximum relative
refractive index difference Δ1 in a region from the center of the core to a radius of Rl µm,
a second core which surrounds and comes in contact with the first core and has a
minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to
a radius of R2 µm, and a third core which surrounds and comes in contact with the
second core and has a maximum relative refractive index difference Δ3 in a region from
the radius of R2 µm to a radius of R3 µm; and a cladding which surrounds and comes in
contact with the three-layer structured core and has a substantially uniform refractive
index.
In the optical fiber, Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have

relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1, when Δ1, Δ2, and Δ3 have relationships of
Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% Δ2 and Δ3> Δ2.
[0021]
The refractive index profile of the optical fiber of the invention, as illustrated in
FIG. 6, may allow relative refractive index differences not to have uniform values. FIG.
6 is a view illustrating a second example of a refractive index profile of the optical fiber
of the invention. The refractive index profile is constituted by: a three-layer structured
core which includes, in the central portion of the core, a first core having a maximum
relative refractive index difference Δ1 in a region from the center of the core to a radius
of Rl µm, a second core which surrounds and comes in contact with the first core and
has a minimum relative refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 (am, and a third core which surrounds and comes in contact with the
second core and has a maximum relative refractive index difference Δ3 in a region from
the radius of R2 urn to a radius of R3 µm; and a cladding which surrounds and comes in
contact with the three-layer structured core and has a substantially uniform refractive
index, wherein Δ1> Δ2 and Δ3> Δ2.
[0022]
For the optical fiber having the refractive index profile constituted by the
three-layer structured core and the one-layer structured cladding for surrounding the core,
in order to obtain an optical fiber which has the above-mentioned optical characteristics,
that is, characteristics compatible with G652 and an SBS threshold equal to or higher
than two times that of (by +3 db) the SMF having the same MFD as the optical fiber of
the invention, detailed examinations were repeated. As a result, it was found that

relationships between Δ1, Δ2, A3, Rl, R2, and R3 were limited.
[0023]
FIG. 7 is a view showing a relationship of a SBS suppression effect SBSeff on
an SMF when Δ1 -A2=X and Δ3-Δ2=Y. Here, SBSeff is defined by the following
expression.
SBSeff=SBS threshold of the optical fiber of the invention-SBS threshold of the
SMF having the same MFD as the optical fiber of the invention
[0024]
As can be seen from FIG. 7, by allowing X+Y to be greater than 0.4%, it is
possible to improve SBSeff to be equal to or higher than +3 dB and increase the SBS
threshold. However, by only using the condition, in some cases, an optical fiber having
the optical characteristics compatible with the G652 cannot be obtained.
Specifically, in order to obtain an optical fiber having a zero-dispersion
wavelength in the range of 1300 to 1324 nm, it is preferable that X and Y have
relationships of X<0.6%, 0.1 % Δ1.
FIG. 10 is a view showing a relationship between A3-A1 and SBSeff. As
shown in FIG. 10, when A3-A1 is negative, SBSeff significantly changes with a slight
change in relative refractive index difference. On the other hand, when A3-A1 is
positive, the change rate of SBSeff with respect to the change in relative refractive index
difference is small. In addition, when SBSeff is approximated to a quadratic equation of
A3-A1, the approximation curve is a parabola which opens down, and the inflection point
is A3- Δ1>0. It can be seen that the effect of SBSeff caused by the change in relative
refractive index difference is small when A3-A1 is positive.
When an optical fiber preform is manufactured, in some cases, due to fluctuation
in dopant concentration, the relative refractive index difference is changed from a target
by about ±0.05%. In this case, there is a possibility that SBSeff is decreased to be lower
than a target. When A3-A1 is positive, the change rate of SBSeff caused by the change
in relative refractive index difference is small. Therefore, SBSeff does not significantly
change with respect to the change in relative refractive index difference caused by the
fluctuation in dopant concentration, and SBS characteristics that are always stable can be
obtained.
In addition, in the refractive index profile of FIG. 10, Δ1 is 0.5%, Δ2 is 0.22%,

A3 is in the range of 0.40 to 0.65% at an interval of 0.025%, and R2/R1 is 2.2.
However, as shown in Table 1, in other combinations of Δ1, Δ2, A3, Rl, R2, and R3, the
inflection point is A3- Δ1>0, and it can be seen that SBS characteristics which are always
stable can be obtained when A3-A1 is positive.
It is preferable that Δ1 -A2 be equal to or greater than 0.25%. FIG. 11 is a view
showing a relationship between Al-A2 and SBSeff. As shown in FIG. 11, when Δ1 is
greater than Δ2 by 0.25% or higher, SBSeff can be significantly increased as compared
with the case where the difference between Δ1 and Δ2 is equal to or smaller than 0.25%,
thereby obtaining a higher SBS suppression effect. In addition, when Δ1 -A2 is equal to
or greater than 0.25%, the effect of the change in relative refractive index difference
caused by fluctuation in dopant concentration during manufacturing of the optical fiber
preform is reduced. Therefore, even when Δ1-Δ2 is changed, SBSeff does not change
significantly, and degradation in yield can be prevented.
In the refractive index profile of FIG 11, Δ1 is in the range of 0.44% to 0.56% at
an interval of 0.03%, Δ2 is 0.24%, Δ3 is 0.55%, and R2/R1 is 2.2. However, as shown
in FIG. 12, in a refractive index profile using other combinations of Δ1, Δ2, A3, Rl, R2,
and R3, that is, when Δ1 is in the range of 0.44% to 0.56%, Δ2 is in the range of 0.18%
to 0.26%, Δ3 is in the range of 0.45% to 0.65%, and R2/R1 is in the range of 1.8 to 2.6,
the same tendency can be obtained. Particularly, in a refractive index profile in which
Δ1-Δ2 is equal to or greater than 0.25%, a higher SBS suppression effect can be obtained,
and dependence of SBSeff on a change in Δ1-Δ2 is reduced.
Table 1



In addition, by disposing the third core as described above, it is possible to
obtain the SBS threshold higher than that of a conventional optical fiber by +3 dB or
higher and obtain characteristics compatible with the G652.
Examples
[0029]
Example 1 and Comparative Example 1
In Table 2, structural parameters and optical characteristics of an optical fiber of
Example 1 which has the refractive index profile of FIG. 5 are shown. In addition,
structural parameters and optical characteristics of an optical fiber of Comparative
Example 1 are shown. The optical fiber of Comparative Example 1 is an SMF having a
step-index profile as illustrated in FIG. 4.
[0030]
Table 2



As shown in Table 2, the optical fiber having the structural parameters of
Example 1 related to the invention had an SBS threshold of 12.2 dBm for a length of 20
km, and could obtain a higher suppression effect than the optical fiber of Comparative
Example 1 which had the same MFD by +3.5 dB. In addition, the optical fiber of
Example 1 had the same optical characteristics as the SMF of Comparative Example 1
and satisfied the G652 standard.
[0032]
Examples 1a to 1g
Table 3 shows results in the case where structural parameters of Example 1 are
represented by using X, Y, and R2/R1 described above.
[0033]

[0034]
The optical fibers having the structural parameters of Examples la to lg shown
in Table 3 had SBS thresholds of 12.4 to 13.3 dBm for a length of 20 km as shown in FIG.
13 and could obtain suppression effects higher than the SMF having the same MFD by
+3.7 to +4,6 dB. In addition, the optical characteristics of the optical fibers of
Examples la to lg all satisfied the G652 standard.
Examples lh to lv
Tables 4 and 5 show results in the case where structural parameters of Example
1 are represented by using X, Y, and R2/R1 described above. The optical fibers having
the structural parameters of Examples la to lg shown in Table 3 and Examples lh to lv
shown in Tables 4 and 5 had SBS thresholds of 10.9 to 13.8 dBm for a length of 20 km as
shown in FIG. 14 and could obtain suppression effects higher than the SMF having the
same MFD by +3.1 to +4.5 dB. In addition, the optical characteristics of the optical
fibers of Examples lh to lv all satisfied the G652 standard.

[0035]
Examples 2a to 2f
Table 6 shows optical characteristics in the case where structural parameters of a
refractive index profile of the optical fiber having the refractive index profile of FIG. 6
are represented by using X, Y, and R2/R1 described above.
[0036]

[0037]
The optical fibers having the structural parameters of Examples 2a to 2f shown
in Table 6 had SBS thresholds of 12.0 to 13.7 dBm for a length of 20 km as shown in FIG.
15 and could obtain suppression effects higher than the SMF having the same MFD by
+3.3 to +5.0 dB. In addition, the optical characteristics of the optical fibers of
Examples 2a to 2f all satisfied the G652 standard.
In addition, by changing the refractive index of the third core as illustrated in
FIG. 6, the amount of dopant GeO2 in the core can be reduced, so that it is possible to
reduce a loss in the optical fiber.
Examples 2g to 2m
Table 7 shows optical characteristics in the case where structural parameters of a
refractive index profile of the optical fiber having the refractive index profile of FIG. 6
are represented by using X, Y, and R2/R1 described above. The optical fibers having
the structural parameters of Examples 2a to 2f shown in Table 6 and Examples 2g to 2m
shown in Table 7 had SBS thresholds of 10.8 to 14.3 dBm for a length of 20 km as shown
in FIG 16 and could obtain suppression effects higher than the SMF having the same
MFD by +3.2 to +4.7 dB. In addition, the optical characteristics of the optical fibers of
Examples 2g to 2m all satisfied the G652 standard.

[0038]
Examples 3 and 4
FIG. 17 shows a refractive index profile of an optical fiber of Example 3 related
to the invention. The refractive index profile in Example 3, as shown in FIG. 17, is
constituted by: a three-layer structured core which includes, in the central portion of the
core, a first core in a region from the center of the core to a radius of Rl µm, a second
core which surrounds and comes in contact with the first core in a region from the radius
of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact
with the second core in a region from the radius of R2 µm to a radius of R3 µm; and a
cladding which surrounds and comes in contact with the three-layer structured core.
However, unlike in Examples 1 and 2, the refractive index profile of the core smoothly
changes, and the boundary thereof is obscure. Therefore, by using the change rate
(dA/dr) of the relative refractive index difference in the radial direction, the diameter of
each layer was determined. The relative refractive index difference Δ1 of the first core,
as represented by the following equation (1), is defined as A which becomes equivalently
uniform in the region from the center of the core to the radius Rl, the relative refractive
index difference Δ2 of the second core is defined as a relative refractive index difference
that becomes a minimum value in the region between the radii Rl and R2 µm, and the
relative refractive index difference Δ3 of the third core is defined as a relative refractive
index difference that becomes a maximum value in the region between the radii R2 and
R3 µm.
[0039]
Equation 1


[0040]
The structural parameters of the optical fiber of Example 3 defined as described
above and the optical characteristics thereof are shown in Table 8. In addition, in Table
8, the structural parameters of the optical fiber of Example 4 having the same refractive
index profile as Example 3 and the optical characteristics thereof are shown.
[0041]


[0042]
As shown in Table 8, the optical fibers of Examples 3 and 4 had SBS thresholds
of 12.2 to 12.7 dBm for a length of 20 km, and could obtain higher suppression effects
than the SMF having the same MFD by +3.5 to +4.0 dB. In addition, the optical fibers
of Examples 3 and 4 all satisfied the G652 standard.
[0043]
Example 5
FIG. 18 is a refractive index profile of an optical fiber preform of Example 5.
The optical fiber preform in this example, as shown in FIG. 18, is constituted by: a
three-layer structured core which includes, in the central portion of the core, a first core
having a substantially uniform positive relative refractive index difference Δ1 in a region
from the center of the core to a radius of Rl µm, a second core which surrounds and
comes in contact with the first core and has a substantially uniform positive relative
refractive index difference Δ2 in a region from the radius of Rl µm to a radius of R2 µm,
and a third core which surrounds and comes in contact with the second core and has a
substantially uniform positive relative refractive index difference Δ3 in a region from the
radius of R2 µm to a radius of R3µm. Like Examples 1 and 2, the three-layer
structured core is included.
Structural parameters of the optical fiber preform of Example 5 are represented
in Tables 9 and 10 by using X, Y, and R2/R1 described above, and optical characteristics
that were exhibited when the preform was drawn into an optical fiber are shown. The
optical fiber drawn from the optical fiber preform of Example 5 had an SBS threshold of
10.9 to 13.8 dBm for a length of 20 km as shown in FIG. 19, and obtained a suppression
effect higher than the SMF having the same MFD by +3.1 to +4.5 dB, so that the G652

standard was further satisfied.

[0044]
Example 6
FIG. 20 shows a refractive index profile of an optical fiber preform of Example 6.
The optical fiber preform in this example, as shown in FIG. 20, is constituted by: a
three-layer structured core which includes, in the central portion of the core, a first core
having a maximum relative refractive index difference Δ1 in a region from the center of
the core to a radius of Rl µm, a second core which surrounds and comes in contact with
the first core and has a minimum relative refractive index difference Δ2 in a region from
the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in
contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 urn to a radius of R3 jam. Like Examples 1 and 2, the
three-layer structured core is included.
Structural parameters of the optical fiber preform of Example 6 are represented
in Tables 11 and 12 by using X, Y, and R2/R1 described above, and optical characteristics
that were exhibited when the preform was drawn into an optical fiber are shown. The
optical fiber drawn from the optical fiber preform of Example 6 had an SBS threshold of
10.8 to 14.3 dBm for a length of 20 km as shown in FIG. 21, and obtained a suppression
effect higher than the SMF having the same MFD by +3.2 to +4.7 dB, so that the G652
standard was further satisfied.

[0045]
Example 7
FIG. 22 shows a refractive index profile of an optical fiber preform of Example 7.
The optical fiber preform in this example, as shown in FIG. 22, is constituted by: a
three-layer structured core which includes, a first core disposed in a region from the
center of the core to a radius of Rl, a second core which surrounds and comes in contact
with the first core and is disposed in a region from the radius of Rl µm to a radius of R2 µm, and a third core which surrounds and comes in contact with the second core and is
disposed in a region from the radius of R2 µm to a radius of R3 um. Like Examples 1,
2, 5, and 6, the three-layer structured core is included. However, unlike in Examples 1,
2, 5, and 6, the refractive index profile smoothly changes, and the definition of the
boundary thereof is the same as in Examples 3 and 4.
Structural parameters of the optical fiber preform of Example 7 are represented
in Table 13, and optical characteristics that were exhibit when the preform was drawn
into an optical fiber are shown. The optical fiber drawn from the optical fiber preform
of Example 7 had an SBS threshold of 12.6 dBm for a length of 20 km and obtained a
suppression effect higher than the SMF having the same MFD by +3.8 dB, and the G652
standard was satisfied.



Industrial Applicability
[0046]
According to the invention, for the optical fiber having a segment-core type
refractive index profile, the relationships between the relative refractive index differences
Δ1, Δ2, and Δ3 of the layers are suitably designed, and the position of the third core is
suitably determined. Therefore, it is possible to increase an SBS threshold by +3 dB or
higher as compared with the SMF having the same MFD while maintaining the optical
characteristics described in G652.
In addition, by allowing the relative refractive index difference of the third core
to be greater than that of the first core, it is possible to improve manufacturability of the
optical fiber preform.

CLAIMS
1. An optical fiber having a refractive index profile constituted by:
a three-layer structured core which includes, in the central portion of the core, a
first core having a substantially uniform positive relative refractive index difference Δ1
in a region from the center of the core to a radius of Rl µm, a second core which
surrounds and comes in contact with the first core and has a substantially uniform
positive relative refractive index difference Δ2 in a region from the radius of Rl urn to a
radius of R2 µm, and a third core which surrounds and comes in contact with the second
core and has a substantially uniform positive relative refractive index difference Δ3 in a
region from the radius of R2 µm to a radius of R3 µm; and
a cladding which surrounds and comes in contact with the three-layer structured
core and has a substantially uniform refractive index,
wherein
A2 is equal to or less than 0.4%,
Δ1, Δ2, and Δ3 have relationships of Δ1> Δ2, Δ3> Δ2, and Δ3> Δ1,
when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y,
(X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% Δ2, Δ3> Δ2, and Δ3> Δ1,
when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2==Y,
(X+Y)>0.4% is satisfied,
X and Y satisfy 0.25% Δ2, Δ3> Δ2, and Δ3> Δ1,
when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y,
(X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% Δ2, Δ3> Δ2, and Δ3> Δ1,
when Δ1, Δ2, and Δ3 have relationships of Δ1-Δ2=X and Δ3-Δ2=Y,
(X+Y)>0.4% is satisfied, and X and Y satisfy 0.25% Δ2, Δ3> Δ2, and Δ3> Δ1, when
Δ1-Δ2=X and Δ3-Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%