DESCRIPTION
OPTICAL FIBER
TECHNICAL FIELD
The present invention relates to an optical fiber that exhibits excellent bending characteristics.
Priority is claimed from Japanese Patent Application No. 2003 - 107760, filed April 11, 2003, from Japanese Patent Application No. 2003 - 199270, filed July 18, 2003, and from Japanese Patent Application No. 2004 - 18514, filed January 27, 2004, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
Japanese Patent No. 2618400 describes an optical fiber that includes a cladding layer provided on the periphery of a center core in which the cladding layer has a refractive index groove that has a lower refractive index. It is suggested that optical fibers having such a structure are expected to exhibit various advantageous effects, such as reducing the dispersion slope and lowering the bending loss. However, to achieve such effects, it is desirable that the value of a2/a1 be between 1.5 and 3.5, where a1 is the radius of the core and a2 is the radius of the inner periphery of the refractive index groove.
Conventionally, transmission systems using WDM (wavelength division multiplexing) and optical fibers therefor have actively been developed in order to increase the transmission capacity of trunk lines or long-distance lines. Optical fibers for WDM transmission are required to have certain characteristics, such as reducing the nonlinear effect and suppressing the dispersion. In recent years, optical fibers that exhibit a
reduced dispersion slope for a system called "metro" in a span of several hundred kilometers and optical fibers that are almost free from loss due to OH have been proposed. When installment of optical fibers into offices and homes (FTTH ; Fiber to the Home) is taken into consideration, characteristics different from those of optical fibers used in transmission are required. That is, when installing optical fibers into
buildings or houses, a very small amount of bending of a bending diameter of 30 mm 9 or 20 mm 9 may be generated. In addition, it is crucial that loss is not increased when a fiber is wound with a small bending diameter to accommodate an excess length. In other words, resistance to a small bending is a crucial characteristic for optical fibers for the FTTH. In addition, good connectivity with optical fibers laid between a base station and buildings or houses (many of which are conventional single-mode fibers for the 1.3 µm band) is also important. Furthermore, low cost is required for such an application.
As optical fibers installed in offices and homes, conventional single-mode fibers for the 1.3 µm band or multimode fibers have been widely used.
However, the allowable smallest bending diameter of such optical fibers is generally about 60 mm 9, and careful attention should be paid to ensure that no bending with a diameter greater than this allowable range is generated when installing the fibers.
Recently, optical fibers having an allowable bending diameter as small as 30 mm 9 have been developed by reducing an MFD (mode field diameter) within the range compliant with ITU-T (International Telecommunication Union - Telecom Standardization) G652, which is an international standard for single-mode fibers (hereinafter, abbreviated as SMFs as appropriate) for the 1.3 µm band.
However, it is desirable for optical fibers that are installed in buildings or houses to have a smaller bending diameter. Although there have been reports about optical fibers with smaller bending diameters, such optical fibers have problems, such as having
increased splice loss compared with conventional optical fiber and increased manufacturing costs.
In addition, the Institute of Electronics, Information and Communication
Engineers Technical Report OFT 2002-81 reports studies on the possibilities of using photonic crystal fibers in houses or buildings. Photonic crystal fibers are optical fibers having a structure in which holes are provided in the vicinity of the center of optical fibers. Although it is expected that photonic crystal fibers may exhibit characteristics that conventional optical fibers do not have, they are inferior in terms of ease of manufacturing.
In addition, it is desirable that conventional optical fibers used for cables have high bending resistance. For example, for cable layout for connecting cables within a closure, using optical fibers exhibiting resistance to smaller bendings, it is possible to enhance the efficiency of connection and accommodation as well as achieving a reduction in the size of the closure. In addition, the installation task may be performed while communication is taking place through fibers other than the fiber to be installed are active for communication. Even in such a situation, it is possible to perform the task without affecting lines used for communication (live lines) due to unintentional contact.
BRIEF SUMMARY OF THE INVENTION
It is an aspect of the present invention to provide an optical fiber that exhibits a low loss due to bending and good connectivity with a common transmission optical fiber, and can be manufactured at a lower cost.
One exemplary embodiment of the present invention provides an optical fiber comprising: a core provided at a center; a first cladding layer provided on a periphery of the core; a second cladding layer provided on a periphery of the first cladding
layer; and a third cladding layer provided on a periphery of the second cladding layer, wherein a maximum refractive index of the core is greater than any of maximum refractive indices of the first cladding layer, the second cladding layer, and the third cladding layer, and the maximum refractive index of the second cladding layer is smaller than any of the maximum refractive indices of the first cladding layer and the third cladding layer, a value of a2/a1 is not less than 2.5 and not more than 4.5 when a radius of the core is a1 and a radius of an outer periphery of the first cladding layer is a2, and a relative refractive index difference of the core with respect to a refractive index of the third cladding layer is not less than 0.20 and not more than 0.70%.
In the optical fiber according to the present invention, it is preferable that a cut-off wavelength be 1260 nm or shorter.
In addition, it is preferable that a refractive index volume V of the second cladding layer expressed by the following Formula (1) be 25%-µm2 or higher.
It is more preferable that the refractive index volume V of the second cladding layer be 50%µm2 or higher.

In the above Formula (1), "r" is a radius,
An(r) is a relative refractive index difference at the radius "r" (with respect to a maximum refractive index of the third cladding layer),
a2 is a radius of an outer periphery of the first cladding layer, and
a3 is a radius of an outer periphery of the second cladding layer.
In the optical fiber according to the present invention, it is preferable that a relative refractive index difference of the first cladding layer with respect to the maximum
refractive index of the third cladding layer be not less than -0.10% and not more than 0.05%.
According to the present invention, it is possible to obtain an optical fiber having a bending loss ratio of 0.4 or less, when assuming an increase in a bending loss at a wavelength of 1550 nm of a single-peak optical fiber that has a single-peak refractive index profile without a second cladding layer and has the same cut-off wavelength when wound ten times around a mandrel having a diameter of 20 mm to be 1, which is a ratio of a value of an increase in bending loss that is measured, in the same manner.
According to the present invention, it is possible to obtain an optical fiber having a bending loss ratio of 0.55 or less, when assuming an increase in a bending loss at a wavelength of 1550 nm of a single-peak optical fiber that has a single-peak refractive index profile without a second cladding layer and has the same cut-off wavelength when wound ten times around a mandrel having a diameter of 15 mm to be 1, which is a ratio of a value of an increase in bending loss that is measured in the same manner.
According to the present invention, it is possible to obtain an optical fiber that exhibits a value of a bending loss at a wavelength of 1550 nm of 0.05 dB or lower per turn when wound in a bending diameter of 20 mm.
According to the present invention, it is possible to obtain an optical fiber that exhibits the value of the bending loss at a wavelength of 1650 nm of 0.05 dB or lower per turn when wound in a bending diameter of 20 mm.
Furthermore, it is possible to obtain an optical fiber having a mode field diameter of 8.3 µm or more at a wavelength of 1550 nm.
According to the present invention, it is possible to obtain an optical fiber that exhibits the value of the bending loss at a wavelength of 1550 nm of 0.05 dB or lower per turn when wound in a bending diameter of 15 mm.
According to the present invention, it is possible to obtain an optical fiber that exhibits the value of the bending loss at a wavelength of 1650 nm of 0.05 dB or lower per turn when wound in a bending diameter of 15 mm.
Furthermore, it is possible to obtain an optical fiber having a mode field diameter of 7.8 µm or more at a wavelength of 1550 nm.
According to the present invention, it is possible to obtain an optical fiber having an MFD value ratio of 0.98 or higher, when assuming a mode field diameter (MFD) at a wavelength of 1550 nm of a single-peak optical fiber that has a single-peak refractive index profile without a second cladding layer and has the same cut-off wavelength to be 1, which is a ratio of a value of an increase in an MFD that is measured in the same manner.
According to the present invention, it is possible to obtain an optical fiber having a mode field diameter of 7.3 µm or more at a wavelength of 1310 nm.
According to the present invention, it is possible to obtain an optical fiber having a mode field diameter of 6.8 µm or more at a wavelength of 1310 nm.
Furthermore, it is possible to obtain an optical fiber that exhibits the value of the bending loss at a wavelength of 1550 nm of 0.05 dB or lower per turn when wound in a bending diameter of 10 mm.
According to the present invention, it is possible to obtain an optical fiber that exhibits the value of the bending loss at a wavelength of 1650 nm of 0.05 dB or lower per turn when wound in a bending diameter of 10 mm.
Furthermore, according to the present invention, it is possible to obtain an optical fiber having a mode field diameter of 7.3 µm or more at a wavelength of 1550 nm.
Furthermore, it is possible to obtain an optical fiber having a mode field diameter of 6.3 µm or more at a wavelength of 1310 nm.
According to the present invention, it is possible to realize an optical fiber that
has a mode field diameter of 7.9 µm or more at a wavelength of 1310 nm, and the value of the bending loss at a wavelength of 1550 nm when wound in a bending diameter of 20 mm of 1 dB or lower per turn.
According to the present invention, it is possible to obtain an optical fiber that exhibits the value of the bending loss at a wavelength of 1550 nm of 0.5 dB or lower per turn when wound in a bending diameter of 20 mm.
Furthermore, it is possible to obtain an optical fiber that has a zero dispersion wavelength of not less than 1300 nm and not more than 1324 nm.
According to the present invention, it is possible to obtain an optical fiber that exhibits low loss and good connectivity with a common transmission optical fiber, and can be manufactured with lower cost.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The aspects of the present invention will become more apparent by describing in detail illustrative, non-limiting embodiments thereof with reference to the accompanying drawings, in which
FIG. 1 is a graph showing a refractive index profile of an optical fiber according to an exemplary embodiment of the present invention ;
FIG. 2 is a graph showing the relationship between the position of a second cladding layer and MFD in Test Example 1 ;
FIG. 3 is a graph showing the relationship between the position of the second cladding layer and bending loss in Test Example 1 ;
FIG. 4 is a graph showing a refractive index profile of an example of an exemplary embodiment of the present invention ;
FIG. 5 is a graph showing a refractive index profile of another example of an exemplary embodiment of the present invention ;
FIG. 6 is a graph showing a refractive index profile of another example of an exemplary embodiment of the present invention ;
FIG. 7 is a graph showing a refractive index, profile of another example of an exemplary embodiment of the present invention ;
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF
THE INVENTION Hereinafter, exemplary embodiments of the invention will be described with reference to the drawings. However, it should not be construed that the present invention is limited to these embodiments; rather, components of these embodiments may be combined if necessary.
A detailed description of the present invention will be described. FIG. 1 illustrates a refractive index profile of an embodiment of an optical fiber according to the present invention.
At the center of the optical fiber according to this embodiment, a core 1 having a radius of a1 and a maximum refractive index of n1 is provided. On the periphery of the core 1, a first cladding layer 2 having an outer periphery radius of a2 and a maximum refractive index of n2 is provided, and on the periphery of the first cladding layer 2, a second cladding layer 3 having an outer periphery radius of a3 and a maximum refractive index of n3 is provided. On the periphery of the second cladding layer 3, a third cladding layer 4 is provided, which is the outermost layer of the optical fiber and has an outer periphery radius of a4 and a maximum refractive index of n4.
As used in this specification, the term "maximum refractive index" means the largest refractive index between an-1 and an where a,, is an outer periphery radius of a layer and an-1 is an outer periphery radius of the next layer on the inner side. In this example, "n" is an integer of one or greater, and a0 is 0 (µm). In a step-shaped refractive index profile as shown in FIG 1, the refractive index remains constant between an-1 and an, and the refractive index is the maximum refractive index. However, as shown in FIGS. 4 to 7,
which will be described later, when each of the layers has a refractive index profile, the maximum refractive index as defined in the above is used.
The optical fiber according to the present invention is designed so that the maximum refractive index n1 of the core 1 is greater than any one of the maximum refractive indices n2, n3, and n4 of the first, second, and third cladding layers 2, 3, and 4, respectively, and the maximum refractive index n3 of the second cladding layer 3 is smaller than any one of the maximum refractive indices n2 and n4 of the first and third cladding layers 2 and 4, respectively.
The refractive index profile of the optical fiber can be formed by doping with dopants, such as germanium, fluorine, or the like. In processes used for manufacturing optical fibers, such as VAD (Vapor-phase Axial Deposition) or CVD (Chemical Vapor Deposition), the refractive index profiles of boundaries between each layer may become blurry due to diffusion of the dopants.
In the optical fiber shown in FIG. 1, the refractive index in the first cladding layer 2 is approximately constant in the direction of the radius, defining a refractive index profile of a nearly perfect step-shaped profile. The refractive index profile of the optical fiber according to the present invention does not necessarily have a perfect step-shaped profile. Even if the refractive index is not a step-shaped profile, it is possible to obtain advantageous effects of the present invention by setting the radius of each of the layers defined by the following formula. Firstly, the radius a1 of the core 1 is defined as the distance between the position in which the relative refractive index difference is one tenth of the maximum relative refractive index difference ?1 within the core 1 and the center. In addition, the outer periphery radii a2 and a3 of the first cladding layer 2 and the second cladding layer 3, respectively, are a respective distance between the portion in which A(r) /dr ("r" represents the radius), the derivative value of the radial profile of the relative
refractive index difference A(r), reaches the extreme value and the center.
It is possible to calculate an equivalent step-shaped refractive index profile exhibiting equivalent characteristics by using the thus calculated radii (hereinafter, this technique is sometimes referred to as "step conversion" as appropriate). In the present invention, even when the actual refractive index profile does not have a step-shaped profile, provided that a refractive index profile calculated using such a step conversion satisfies given refractive index relationships of the present invention, desired advantageous effects of the present invention can be obtained. In Examples in this specification, the relative refractive index differences of equivalent step-shaped profiles that are obtained by performing a step conversion based on the above-described procedures are also shown.
In this specification, the relative refractive index difference ?1 (unit: %) of each of the layers is calculated with respect to the maximum refractive index n4 of the third cladding layer 4 and is expressed by the following Formula (2):

(In the formula, "i" is an integer between 1 and 3, and n1 is the maximum refractive index of the each of the layers.)
As shown in FIG. 1, when the core is made up of a single layer, when the relative refractive index difference ?1 of the core 1 is increased, it is possible to reduce the bending loss but the MFD tends to be decreased. In addition, when ?1 is decreased, a greater MFD can be obtained but the bending loss is increased. The present invention is characterized in that an optical fiber that exhibits excellent bending characteristics can be obtained with an MFD that is comparative to that of a single-peak optical fiber by providing the second cladding layer 3. Although ?1 is not limited to a specific value in the present invention,
by setting ?1 in a range between . 0.20% and 0.70%, more preferably in a
range between 0.25% and ' 0.65%, it is possible to obtain an optical fiber that
exhibits good connection characteristic with a conventional SMF and excellent bending characteristics.
In addition, the relative refractive index difference ?2 of the first cladding layer 2 is preferably 0.05% or less, and is more preferably 0.00% or less. In addition,
the relative refractive index difference ?2 of the first cladding layer 2 is preferably
-0.10% or higher.
As ?2 becomes greater, the cut-off wavelength is increased and a cut-off
wavelength of 1260 nm or shorter becomes impossible. In contrast, the relative refractive
index difference ?2 of the first cladding layer 2 becomes too small, and the containment of
the field due to the first cladding layer 2 becomes significant, which is favorable in
reducing the bending loss but hinders improvement in the connectivity by enlarging the
MFD. For this reason, ?2 is preferably designed to achieve a desired cut-off wavelength,
a satisfactory bending loss, and a desired MFD at the same time. In general, ?2 of -0.10% or higher can provide desired advantageous effects.
In addition, the design range of the relative refractive index difference A3 of the
second cladding layer 3 is specified by the refractive index volume V.
The outer diameter of the outer periphery of the third cladding layer 4 (twice as large as a4), in other words, the outer diameter of the optical fiber is generally 125 µm. In recent years, optical fibers having an outer diameter of about 80 µm have been commercialized for small-sized optical components. Although the optical fiber of the present invention may have an outer diameter in the same range as that of typical optical fibers, the present invention is not limited to the above range.
In addition, although it is possible to control the cut-off wavelength by controlling the radius a1 of the core 1, the bending loss tends to be increased when the cut-off wavelength is reduced in such a manner. Accordingly, the radius a1 of the core 1
is suitably selected according to the required MFD, cut-off wavelength, bending loss, as well as the relative refractive index difference ?1 of the core 1.
The ratio of the outer periphery radius of the first cladding layer 2 with respect to the radius of the core 1 (a2/a1) represents the position of the second cladding layer 3. In the present invention, this value is set to 2.5 or higher, and preferably to 3.0 higher. By
providing the second cladding layer 3 at the position in which a2/a1 falls within the above
range, it is possible to improve the bending loss characteristics while reducing the
variation in the mode field diameter (referred to as MFD in this specification as
appropriate) to a small value, as shown FIGS. 2 to 3, which will be described later.
The effect of reducing the bending loss is expected even when a2/a1 is increased
considerably. However, when a2/a1 is increased, deterioration of optical characteristics
become significant, especially a change in the cut-off wavelength due to a change in ?2,
thereby making the manufacturing thereof difficult. In addition, when a2/a1 is Increased,
the advantageous effects achieved by providing the second cladding layer 3 are reduced, which makes a single-mode transmission difficult. For these reasons, it is desirable to keep the ratio a2/a1 at a value of 4.5 or less.
The outer periphery radius a3 of the second cladding layer 3, as well as the relative refractive index difference ?, is specified by the refractive index volume V as described later.
Optical fibers can be utilized for transmission in a broad wavelength band ranging from the 1300 nm band to the 1600 nm band. Optical fibers for the 1300 nm band are stipulated in G652 by the ITU-T. In general.., the lower-limit wavelength of the 1300 nm band is set to 1260 nm, and a cut-off wavelength of 1260 nm or shorter is stipulated by G652. In order to achieve single-mode transmission in a broad wavelength region ranging from the 1300 nm band to the 1600 nm band, it is desirable that the optical fiber according to the present invention also has a cut-off wavelength of 1260 nm or
shorter. There is a tradeoff between the cut-off wavelength and optical characteristics, such as MFD or bending loss, and the refractive index profile is set according to desired charateristics.
In addition, it has been found that the bending loss ratio shows correlations with the value of a2/a1 and the above-described value of V. More specifically, the bending loss ratio tends to be decreased as V increases, and the relationship between V and the bending loss is determined by the value of a2/a1, i.e., the position of the low refractive index layer. In the present invention, in order to achieve better bending loss characteristics the refractive index volume (V) of the second cladding layer expressed by the above Formula (1) is preferably 25%-µm2 or higher, and more preferably 50%-µm2 or higher. In addition, when single-mode transmission at 1260 nm or higher is taken into consideration, the value of V is preferably 110% µm2 or less.
According to the present invention, provision of the second cladding layer helps to effectively reduce the loss due to bending.
For example, as indicated by Tables 1 to 4, which will be explained later, as for an increase in the bending loss (in this specification, referred to as a "bending loss ratio" as appropriate) when wound ten times around a mandrel having a diameter of 20 mm (20 mm