Patent Application
20150226915
The present invention relates to an optical fiber.
In order to improve the signal-to-noise ratio (SN ratio) in an optical transmission system, an optical fiber is required to have low attenuation and low nonlinearity. To reduce the nonlinearity of an optical fiber, it is effective to enlarge the effective area of the optical fiber. However, if the diameter of a core is enlarged in order to expand the effective area of the optical fiber, a higher order mode will be propagated. Therefore, in order to prevent signal degradation due to interference between propagation modes, the effective cutoff wavelength of LP 11 mode of an optical fiber is needed to be not more than the signal light wavelength, that is, 1530 nm or less in the case where signal light is transmitted in the C-band (1530 to 1565 nm), for example.
W-type refractive-index profile is known as a refractive-index profile of an optical fiber which can not only expand an effective area but also make a single mode effectively at the wavelength of 1530 nm or more. The optical fiber having a W-type refractive-index profile has a core, an inner cladding which encloses the core and which has a refractive index smaller than the refractive index of the core, and an outer cladding which encloses the inner cladding and which has a refractive index smaller than the refractive index of the core and larger than the refractive index of the inner cladding.
When actually using an optical fiber, it is necessary to bend and store the excess length of the optical fiber in a repeater, transceiving equipment, a cable joint box, and the like. In such case, the bend radius is about 25 mm, for example.
In the past, however, in most cases the bend loss of an optical fiber has been discussed at the value in terms of bend loss available with a bend radius of 10 mm, and there has been little examination for bend loss in the case of a bend radius in which an optical fiber is actually used. For example, Japanese Patent Application Laid-Open No. 2011-197667 proposes a design method for refractive-index profile of an optical fiber in order to reduce the bend loss at a bend radius of 10 mm, although no investigation has been done with respect to the bend radius of 25 mm, for example, in which an optical fiber is actually used.
An object of the present invention is to provide an optical fiber having a W-type refractive-index profile in which the bend loss at a practically used bend radius can be reduced.
In order to solve the object, an optical fiber of the invention comprises: a core including the central axis, an inner cladding enclosing the core and having a refractive index smaller than a refractive index of the core, and an outer cladding enclosing the inner cladding and having a refractive index which is smaller than the refractive index of the core and larger than the refractive index of the inner cladding, wherein the optical fiber has a bend loss α(R) at a bend radius R and
becomes minimum at the bend radius of 25 mm or less.
The optical fiber of the present invention may have an effective area of 110 μm2 or more and 170 μm2 or less, and have a cutoff wavelength of 1600 nm or less. The effective area preferably be 110 μm2 or more and 160 μm2 or less, and more preferably, be 120 μm2 or more and 140 μm2 or less. In these cases, the cutoff wavelength may be 1500 nm or less. Here, the cutoff wavelength is the fiber cutoff wavelength defined in ITU-T G.650.1.
In an optical fiber of the present invention, regarding a radius “a” of the core and an outside radius “b” of the inner cladding, a ratio b/a may be 2.0 or more and 4.5 or less, and a relative refractive index difference of the outer cladding relative to the inner cladding, Δ2=100×(n3−n2)/n3 [%], may be 0.01% or more and 0.15% or less. In such case, the core may have a diameter of 12.0 μm or more and 15.5 μm or less, and a relative refractive index difference of the core relative to the inner cladding, Δ1=100×(n1−n2)/n1 [%], may be 0.25% or more and 0.35% or less. Preferably, the diameter be 12.0 μm or more and 14.5 μm or less, and the relative refractive index difference Δ1 be 0.25% or more and 0.35% or less.
In the optical fiber of the present invention, the core may have a depressed portion existing at the center of the core and having a refractive index lower than that of a circumference of the depressed portion. In such case, the core may have a diameter of 12.0 μm or more and 15.5 μm or less, and regarding a radius “d” of the depressed portion, a ratio a/d may be 2.2 or more and 4.0 or less, and the depressed portion may have a refractive-index depth, Δ1′=100×(n1−n0)/n1 [%], of 0.01% or more and 0.15% or less. Preferably, the diameter be 12.0 μm or more and 14.0 μm or less, the ratio a/d be 2.2 or more and 4.0 or less, and the refractive-index depth be 0.01% or more and 0.15% or less.
Another aspect of the invention is an optical fiber transmission line which is equipped with an optical fiber of the present invention as an optical transmission line.
According to the present invention, it is possible to provide an optical fiber having a W-type refractive-index profile in which the bend loss at an actually used bend radius is reduced.
Hereafter, with reference to accompanying drawings, examples for carrying out the present invention will be described in detail. The present invention is not limited to the examples and is shown by the claims, including all modifications equivalent to or within the scope of a claim.
The optical fiber having a W-type refractive-index profile has an inner cladding and an outer cladding. In a case where the bend radius is small, the equivalent refractive-index becomes equal to the effective refractive index of the LP01 mode in the region of the inner cladding. When the bend radius becomes larger, the equivalent refractive-index becomes equal to the effective refractive index of the LPOI mode at the interface between the inner cladding and the outer cladding. Furthermore, when the bend radius becomes further larger, the equivalent refractive-index becomes equal to the effective refractive index of the LP01 mode in the region of the outer cladding. The optical fiber having a W-type refractive-index profile will exhibit significant change of bend-loss characteristics at a boundary of the bend radius at which the effective refractive index of the LP01 mode and the equivalent refractive index of the outer cladding become equal.
where α(R) [dB/m] is a bend loss at the bend radius R [mm]. The bend radius Rt at which
becomes the minimum is 24 mm. The radius Rt is determined by the relation between the effective refractive index of the fundamental mode and the equivalent refractive-index profile. Therefore, by appropriately designing the structure of an optical fiber, it is made possible to reduce Rt to equal to or less than a bend radius which is actually used and to decrease the bend loss at the actually used bend radius.
The effective refractive index of LPOI mode of an optical fiber is determined by the refractive-index profile of the optical fiber. It is suitable to set b/a to 4.5 or less, because if b/a is made smaller, Rt can be made smaller since the interface between the inner cladding and the outer cladding becomes closer to the center core, thereby lessening the bend radius at which the effective refractive index of LP01 mode and the equivalent refractive index of the outer cladding become equal. Also, it is preferable to set Δ2 to 0.15% or less, since Rt can be similarly made smaller by lessening the refractive-index difference between the inner cladding and the outer cladding.
Table I summarizes the profile parameters and characteristic values of optical fibers of Examples 1 to 18, and shows core diameter 2a, ratio b/a between core radius and outside radius of inner cladding, on the basis of the refractive index of the inner cladding, relative refractive index difference Δ1 of a core, relative refractive index difference Δ2 of outer cladding, effective area Aeff at the wavelength of 1550 nm, cutoff wavelength λc, Rt, bend loss αb10 at the wavelength of 1550 nm at the bend radius of 10 mm, and bend loss αb25 at the wavelength of 1550 nm at the bend radius of 25 mm. These optical fibers have the refractive-index profile shown in
Table II summarizes the profile parameters and characteristic values of optical fibers of Examples 21 to 38, and shows core diameter 2a, relative refractive index difference Δ1′ of a depressed portion, relative refractive index difference Δ1 of a core, ratio a/d between core radius and radius of the depressed portion, ratio b/a between the core radius and the radius of an inner cladding, relative refractive index difference Δ2 of an outer cladding, effective area Aeff at the wavelength of 1550 nm, cutoff wavelength λc, Rt, bend loss αb10 at the wavelength of 1550 nm at the bend radius of 10 mm, and bend loss αb25 at the wavelength of 1550 nm at the bend radius of 25 mm. These optical fibers have the refractive-index profile shown in
In order to reduce the nonlinearity of an optical fiber, it is effective to enlarge an effective area. However, when the effective area becomes large, in the case where optical fibers in which the effective area is thus enlarged are connected with optical fibers which are already installed as an optical transmission line or used in a transmission apparatus, such as ordinary single-mode fibers based on ITU-T G.652 series, dispersion shift fibers based on ITU-T G.653 series, nonzero dispersion shift fibers based on the ITU-T G.655 and G.656 series, the connection loss may become large, and as a result OSNR may become low. Therefore, it is suitable that the effective area at the wavelength of 1550 nm be 110 μm2 or more and 160 μm2 or less. The cutoff wavelength is preferably 1600 nm or less. This will enable guarantee for effective single mode transmission at the wavelength of 1530 nm or more. The effective area is preferably 120 μm2 or more and 140 μm2 or less (Examples 3 to 13, and Examples 23 to 32), and the cutoff wavelength is preferably 1500 nm or less (Example 1, Examples 4 to 15, Example 21, and Examples 24 to 35).
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-024168 | Feb 2014 | JP | national |
