The present invention relates in general to dispersion-shifted
optical fiber of nearly zero chromatic dispersion within a 1.55 µm wavelength
band, while achieving reduced non linear effects and low bending loss, and relates
in particular to an optical fiber whose dispersion slope is reduced sufficiently.
Dispersion-shifted optical fiber (referred to as DS-fiber
hereinbelow) is an optical fiber whose chromatic dispersion value is almost zero
in a 1.55 µm wavelength band where the transmission loss is minimal for quartz
group optical fiber. For example, a DS-fiber having a staircase type refractive
index distribution (refractive index profile) is well known.
The DS-fiber having such a refractive index profile is
characterized by having smaller bending loss compared with other types of DS-fiber,
such as step-profile type or triangular-profile type fibers, and somewhat larger
mode field diameter (hereinbelow referred to as MFD); however, relative to the normal
single-mode fiber for 1.3 µm band, the MFD is relatively small at about 8 µm
When MFD is small, transmission problems are encountered
because not only splice losses are increased but, for applications requiring high
power density within the fiber such as optical amplifier applications for example,
non linear effects become high and transmission characteristics become seriously
A quantitative measure of non linear effects is n2/Aeff
where n2 is a non linear refractive index for the fiber, and Aeff is
the effective cross section area of the fiber. Because n2 is approximately
constant for a given optical material, Aeff must be made large to decrease non linear
effects in the fiber
On the other hand, Aeff and MFD in DS-fibers are related
by the following expression:
where k is a correction factor.
Here, when the core diameter changed in DS-fibers, there
are not less than two radius values for zero chromatic dispersion in the 1.55 wavelength
Of these solution values, the smallest value is referred
to as the small-diameter solution, and the next smallest value is referred to as
the large-diameter solution. Generally, a DS-fiber having staircase type refractive
index profile adopts the large-diameter solution.
It has been reported that the correction factor k for a
DS-fiber having the normal staircase-type refractive index profile with a large-diameter
solution is about 0.944 and remains unchanged regardless of processing parameters
used on the fiber.
Therefore, to increase the Aeff, it is necessary to increase
However, the normal DS-fiber having the staircase type
refractive index profile based on the large-diameter solution has a constant MFD
value of approximately 8 µm, and therefore, it can not increase the Aeff and
enable reduction in non linear effects.
To resolve such problem, the present inventors have proposed
a DS-fiber having small-diameter solution in a
Japanese Patent Application, First Publication No. Hei8-220362
(Application date Heisei 7, February 10).
In this invention, a small-diameter solution is adopted
for DS-fiber having a staircase type refractive index profile, thereby increasing
the correction factor to about 0.95~0.97, and MFD to about 7.8~8.6 µm. The
result is that Aeff is increased and non linear effects have been reduced.
However, in this invention, although an advantage is gained
that Aeff is increased by adopting the small-diameter solution, there remained a
difficulty that bending loss and dispersion slope are increased.
Furthermore, for wavelength division multiplexing (WDM)
transmission systems, which have been under active development in recent years,
even greater reduction in non linear effects is demanded However, it is difficult
for DS-fiber with staircase type refractive index profile to meet such a challenge,
because of its limited ability to increase Aeff.
The present inventors have submitted a Japanese Patent
Application, Fist Publication, No Hei10-62640 (application date Heisei 8, August
15), and disclosed a DS-fiber with emphasis on increasing the Aeff.
The DS-fiber disclosed in JPA, First Publication No. Hei10-62640
has a ring-structured refractive index profile, and is comprised by a center core
section having a high refractive index, and a ring core section provided separately
from the center core section having a low refractive index, cladding provided on
the outer periphery of the ring core section, and an intermediate layer disposed
between the center core section and the ring core section.
As disclosed in JPA, First Publication, No. Hei8-220362,
in DS-fibers having the staircase refractive index profile, it was known that Aeff
can be increased by adopting the small-diameter solution Therefore, in this invention,
the small-diameter solution is adopted with a primary objective of increasing Aeff.
The ring-structured DS-fiber (presented in JPA, First Publication
No. Hei10-62640) shows almost zero chromatic dispersion in the 1.55 µm band,
and its Aeff is higher than that of DS-fiber having the staircase type refractive
index profile, thereby resulting in decreased non linear effects as well as low
bending loss. Therefore, this type of DS-fiber met two of the important requirements.
However, such a DS-fiber still left a problem that the
dispersion slope increases because of the increase in Aeff. High dispersion slope
is not desirable in wavelength multiplexing transmission systems because it causes
inconsistencies in the transmission of the plural wavelengths.
Accordingly, although increasing in Aeff has been a priority
topic, in the past, to reduce non linear effects in DS-fiber for use in wavelength
multiplexing system, in recent years, there have been a DS-fiber with achieving
sufficient reduction in dispersion slope, rather than with increasing Aeff, to meet
the needs of recent system
Therefore, one of the topics of study in the present invention
is to develop a DS-fiber, whose Aeff would be high enough for use in wavelength
division multiplexing system to decrease non linear effects, with high priority
placed on decreasing its dispersion slope
It is a distinguishing feature of the present invention
to provide a DS-fiber based on large-diameter solution.
In the DS-fiber having the ring-structured refractive index
profile, presented in the previous invention, small-diameter solution was used because
the emphasis was primarily to increase the Aeff. For this reason, it was not possible
to reduce its dispersion slope sufficiently.
In the study that led to the present invention, the emphasis
was placed on reducing the dispersion slope, and it was discovered that, by using
the large-diameter solution to design a DS-fiber, its dispersion slope can be made
sufficiently small while increasing its Aeff more than that in DS-fiber having the
staircase refractive index profile, so that the resulting DS-fiber can be used in
wavelength multiplexing transmission system, by having both low non linear effects
and small bending loss.
Figure 1 is a drawing showing an example of the refractive
index profile of the DS-fiber of the present invention.
The DS-fiber of the present invention exhibits the following
characteristics. The chromatic dispersion in the 1.55 µm wavelength band is
nearly zero but not zero, the effective cross section area is 45~70 µm2,
bending loss is 0.1~100 dB/m, the dispersion slope is 0.05~0.08 ps/km/nm2.
It has a cutoff wavelength which answers for a single mode propagation in the 1.55
In the present invention, the operational band of 1.55
µm wavelength (the 1.55 µm wavelength band) means a range of wavelengths
between 1520 to 1580 nm.
Also, chromatic dispersion nearly zero means that, within
the operational band, chromatic dispersion value is in a range between -5~+5 ps/nm.km
However, it is necessary that chromatic dispersion value does not actually become
0 ps/nm·km This is because, if chromatic dispersion value is 0 ps/nm.km, non
linear effects such as four-optical-mixing become undesirable large.
Also, the effective cross section area Aeff is defined
by the following relation.
where r is a radius of the fiber, E(r) is the electric field strength at radius
Bending loss is a value measured with a wavelength of 1.55
µm in a fiber bent at a bend radius (2R) of 20 mm. Cutoff wavelength is a value
measured according to a method of Japanese Industrial Standards (JIS) or CCITT 2m
protocol or a value measured in actual use.
Also, dispersion slope relates to wavelength-dependence
of chromatic dispersion, and is obtained as a slope of a curve in a graph of wavelength
(nm) on x-axis and chromatic dispersion (ps/nm·km) on y-axis.
The primary feature of DS-fiber of the present invention
is that dispersion slope is in a range 0.05~0.08 ps/km/nm2 and is made
At the same time, the effective cross section area Aeff
is 45~70 µm2, which is large compared with the staircase type refractive
index profile, and is able to suppress non linear effects to a level sufficient
for use in the WDM transmission systems.
In other words, an optical fiber having the properties
described above will be a DS-fiber with sufficiently reduced non linear effects
so as to be applicable to WDM transmission systems, and have small bending loss
and low dispersion slope.
If Aeff is less than 45 µm2, suppression
of non linear effects is insufficient If it is in excess of 70 µm2,
the large-diameter solution does not exist in low bending loss region so that it
is difficult to satisfy the needs for the dispersion slope
Also, bending loss in excess of 100 dB/m is not desirable,
because the transmission loss becomes high even with a slight curvature in the fiber.
Also, because the large-diameter solution is adopted, it
is possible to realize a sufficiently small range, 0.05~0.08 ps/km/nm2,
of dispersion slope. This range is chosen, because it is difficult to produce dispersion
slope of less than 0.05 ps/km/nm2, and if the dispersion slope exceeds
0.08 ps/km/nm2, such a DS-fiber does not satisfy the needs of reduced
dispersion slope as defined in the present invention.
Further, DS-fiber is usually a single-mode fiber, and must
always provide single-mode transmission within the operational band To do this,
the cutoff wavelength must be a value to guarantee single-mode transmission.
For the present DS-fiber to possess the properties described
above, the first necessary condition is that the fiber must have a ring-structured
refractive index profile such as the one shown in Figure 1.
Figure 1 shows a concentric structure of DS-fiber comprised
by a center core section 1, a first ring section 2 surrounding the center core section
1, a second ring section 3 surrounding the first ring section 2, and a third ring
section 4 surrounding the second ring section 3, followed by a cladding surrounding
the third ring section 4. The core section 1, first ring section 2, second ring
section 3 and the third ring section 4 are arranged in the state of a concentric
Refractive indexes n0, n1, n2,
and n3, for the core section 1, first ring section 2, second ring section
3 and the third ring section 4, respectively, are related as follows: n0>n2,
n2>n1, n2> n3, n3≤n4.
As will be described later, cladding 5 is made of either
pure silica or F-doped silica, therefore, n4 is not limited to the refractive
index of pure silica
In the refractive index profile shown in Figure 1, based
on a reference of zero refractive index for cladding 5, the relative refractive
index differences designated as follows &Dgr;1 for the difference between cladding
5 and center core section 1; &Dgr;2 between cladding 5 and first ring section
2; &Dgr;3 between cladding 5 and second ring section 3; and &Dgr;4 between cladding
5 and third ring section 4 The actual values are in the following ranges: +0.5~+0
8 % for &Dgr;1; -0 1~+0 1 % for &Dgr;2, +0 05~+0 5 % for &Dgr;3, and -0.2~+0.0
% for &Dgr;4.
However, a problem with such a DS-fiber is that, with increasing
Aeff, there is a tendency for increase in the cutoff wavelength. Therefore, it is
desirable to shift the cutoff wavelength to a lower value by selecting a low value
for &Dgr;4 such as the range described above.
Therefore, a second necessary condition for the DS-fiber
is to adopt the large-diameter solution for the core diameter.
To accomplish this requirement, referring to Figure 1,
the outer diameters of center core section 1, first ring section 2, second ring
section 3 and third ring section 4, respectively, designated as 2a, 2b, 2c, 2d,
should be chosen such that b/a is not less than 1.5, and preferably between 1.5∼4.0
Also, the actual values for "a" is 2.0∼4.0 µm,
and "b-a" is 1.0∼5.0 µm, "c-b" is 1.0~12 µm; and "d-c" is 0.0∼20
When b/a is less than 1.5, it is not possible to increase
Aeff sufficiently, on the other hand, it is not desirable to exceed 4.0, because
of the difficulty in controlling the properties of manufactured fiber.
When a is less than 2.0 µm or exceeds 4 µm, no
large-diameter solution exist to satisfy the properties of the DS-fiber of the present
When b-a is less than 1.0 µm, there is no benefit
of providing the first ring section 2, and if it exceeds 5.0 µm, the fiber
may suffer from problems of manufacture as well as some properties such as cutoff
wavelength and bending loss.
When c-b is less than 1.0 µm, there is no benefit
of providing the second ring section 3, and if it exceeds 12 µm, the fiber
may suffer from problems of manufacture as well as some properties such as increase
in the cutoff wavelength.
In overall summary, therefore, by selecting the values
of the parameters (&Dgr;1, &Dgr;2, &Dgr;3, &Dgr;4, b/a, a, b-a, c-b, d-c)
in the range described above, and by adopting the large-diameter solution, a DS-fiber
having the present property values will be obtained.
Table 1 summarizes the properties of DS-fiber, exemplified
by cutoff wavelength (&lgr;c), Aeff, bending loss (BL), Dispersion Slope (DS),
and the various combinations of parameters to meet the first and second requirements.
As can be understood from Table 1, by combining parameters
from a wide range of values, it is possible to obtain DS-fiber having the targeted
It is clear that such characteristics have not been attainable
in conventional DS-fibers.
The DS-fiber in the prevent invention is based on adopting
the large-diameter solution from the two core sizes available for zero chromatic
dispersion in the 1.55 mm band, thereby attaining a low bending loss as well as
a relatively high Aeff, and enabling to lower its dispersion slope to not more than
The present DS-fiber can be produced by normal fiber manufacturing
method such as vapor-phase axial deposition (VAD) method, so that, in the present
case, the center core section 1 and the second ring section 3 were made from Ge-doped
silica or pure silica, and the first ring section 2, third ring section 4 and cladding
5 were made from pure silica or F-doped silica.
In a fiber having the profile shown in Figure 1, the distribution
of electric field strength produced by propagating light is shaped in such a way
to leave a long tail in the cladding 5 because of the presence of the second ring
section 3, therefore, it is preferable that, when manufacturing mother material
for the optical fiber, fair section of soot for cladding should be made at the same
time as the soot for the center core.
Determination of the refractive index profile by a method
such as Refracted Near-Field Profiling (RNFP) method on the DS-fiber fibers produced
in this study showed that corners were found to be rounded and smooth shaped, compared
with the schematic profile shown in Figure 1.
The values should be chosen such that peak values are selected
for parameters such as &Dgr;1, &Dgr;2, &Dgr;3, &Dgr;4 and half value full
width half peak value should be selected for a~d
Typical properties of the test fiber produced in this study
are summarized in Table 2, which shows that the produced fiber meets the properties
required in the present invention
Aeff at 1550 nm (µm2)
MFD at 1550 nm (µm)
&lgr;c for 2 m fiber (µm)
Bending Loss at 1550 nm, 20 &PHgr; (dB/m)
Zero-dispersion wavelength (µm)
Dispersion Value at 1550 nm (ps/km/nm)
Dispersion Slope at 1550 nm (ps/km/nm2)
Transmission Loss at 1550 nm (dB/km)
Polarization dispersion (ps/√km)