Abstract
The optical fiber according to the present invention includes, in a cross section of the optical fiber, one core region (11) and a cladding region (12) that is arranged on an outer periphery of the core region. The cladding region is a medium that has a lower refractive index than that of the core region and also has a smaller refractive index wavelength dispersion than that of the core region. The optical fiber has a solid core and therefore, allows more reduction in the Rayleigh scattering loss compared to an optical fiber having a hollow core. In addition, since the optical fiber adopts, for the cladding region, a medium that has a smaller refractive index wavelength dispersion than that of the core region, it allows a reduction in the wavelength dispersion of n.sub.eff.
Claims
1. An optical fiber, comprising, in a cross section of the optical fiber, one core region and a cladding region that is arranged on an outer periphery of the core region, wherein the cladding region is a medium having a lower refractive index n than that of the core region and having a smaller refractive index wavelength dispersion than that of the core region, in which the refractive index n satisfies, at a wavelength of 1.55 m,
1.444>n>0.0343.sub.d+3.771, provided that .sub.d is an Abbe number.
2. The optical fiber according to claim 1, further comprising, in the cross section of the optical fiber, a physical cladding region that is arranged on an outer periphery of the cladding region and comprises a fluorine-doped glass, wherein the physical cladding region has a higher refractive index than that of the cladding region, and a medium of each of the core region and the physical cladding region is silica glass.
Description
BRIEF DESCRIPTION OF DRAWINGS
(1) FIG. 1 is a characteristics diagram that describes the wavelength dispersion of an effective refractive index n.sub.eff in an SI-type pure silica core fiber.
(2) FIG. 2 is a characteristics diagram in which the effective refractive indices of an optical fiber according to the present invention and a conventional optical fiber are compared.
(3) FIG. 3 is a characteristics diagram that illustrates a relationship between the refractive index n and the Abbe number .sub.d of a medium that can be used for a cladding region of the optical fiber according to the present invention.
(4) FIG. 4 is a diagram that describes the optical fiber according to the present invention. FIG. 4(A) is a diagram that describes a refractive index dispersion in a cross section. FIG. 4(B) is a diagram that describes a cross-section structure.
(5) FIG. 5 is a characteristics diagram that describes a wavelength dependence of a relative refractive index difference of the optical fiber according to the present invention.
(6) FIG. 6 is a characteristics table in which the group delay times of the optical fiber according to the present invention and a fluorine-doped core optical fiber are compared.
(7) FIG. 7 is a diagram that describes an optical fiber according to the present invention. FIG. 7(A) is a diagram that describes a refractive index dispersion in a cross section. FIG. 7(B) is a diagram that describes a cross-section structure.
DESCRIPTION OF EMBODIMENTS
(8) Embodiments of the present invention will be described with reference to attached drawings. The embodiments described below are examples of the present invention and the present invention is not limited to the embodiments described below. It should be noted that in this description and the drawings, the components denoted by the same reference numeral are the same as each other.
Overview of the Invention
(9) FIG. 1 is a diagram that describes a relationship between a wavelength (lateral axis) and an effective refractive index (vertical axis) in a step-index (SI) type pure silica core optical fiber. The pure silica core optical fiber has a core radius of a=4.2 m that is equal to that of a general-purpose single mode optical fiber. Three straight lines illustrated in FIG. 1 show the wavelength dispersion of the effective refractive indices n.sub.eff when a relative refractive index difference of the cladding to the core of the pure silica core optical fiber is 0.3%, 0.35%, and 0.4%. As illustrated in FIG. 1, n.sub.eff always decreases on a longer wavelength side in any structure, due to the refractive index wavelength dispersion of pure silica glass. In addition, with a decrease of , that is, a decrease in the refractive index of the cladding, the effective refractive indices n.sub.eff uniformly decrease.
(10) Here, by employing a waveguide structure in which the relative refractive index difference increases with an increase in an operating wavelength, the wavelength dispersion of n.sub.eff decreases, allowing a reduction in a group delay time. FIG. 2 is a diagram that describes that the group delay time can be reduced by making the refractive index wavelength dispersion of the cladding smaller than the refractive index wavelength dispersion of the core. Data A and data B are examples of the refractive index wavelength dispersion of a core and a cladding in a conventional optical fiber and the optical fiber according to the present invention, respectively. The dashed double line of the data A shows the refractive index wavelength dispersion of the core of the conventional optical fiber, the dashed line of the data A shows the refractive index wavelength dispersion of the cladding of the conventional optical fiber, the double solid line of the data B shows the refractive index wavelength dispersion of the core of the optical fiber of the present invention, and the solid line of the data B shows the refractive index wavelength dispersion of the cladding of the optical fiber of the present invention.
(11) Since the conventional optical fiber uses glass having the same wavelength dispersion for the core and cladding, each relative refractive index difference is .sub.1=.sub.2=.sub.3 even at different operating wavelengths. On the other hand, the optical fiber according to the present invention uses, for the cladding, glass having a smaller refractive index wavelength dispersion than in the core and thereby obtains .sub.1>.sub.2>.sub.3 with an increase in the operating wavelength. By selecting, for the cladding, a medium with which the relative refractive index becomes smaller on the longer wavelength side as in the data B, the slope of the straight line shown in FIG. 1 can be moderated. That is, this means that the wavelength dispersion of n.sub.eff is reduced.
(12) Thus, the present invention can provide an optical fiber that can reduce both the group delay time and the Rayleigh scattering loss by adopting, for a cladding region of the optical fiber, a medium having a smaller refractive index wavelength dispersion than that of a core region.
First Embodiment
(13) In an optical fiber of this embodiment, the medium of a cladding region has a larger Abbe number than that of the medium of a core region.
(14) In general, the refractive index wavelength dispersion of an optical medium is represented by the Abbe number .sub.d. The Abbe number is defined by .sub.d=(n.sub.d1)/(n.sub.Fn.sub.C) using a refractive index n.sub.d at the wavelength of 587.56 nm, a refractive index n.sub.F at the wavelength of 486.1 nm, and a refractive index n.sub.C at the wavelength of 656.3 nm; and in general, the larger .sub.d is, the smaller the refractive index wavelength dispersion is.
(15) Therefore, it is only required to use, for the cladding region, a material that has a larger Abbe number and a smaller refractive index compared to the core region. In FIG. 3, a relationship between the refractive index n and the Abbe number .sub.d at the wavelength of 1.55 m of a fluorine-doped silica glass is indicated by a solid line. Note that as for pure silica glass, n=1.444. As the refractive index decreases due to doping of fluorine, .sub.d increases. A relationship between n and .sub.d of the fluorine-doped glass is represented by the following expression:
[Math. 2]
n=0.0343.sub.d+3.771(2)
(16) More specifically, an optical medium with which the relationship between n and .sub.d falls in the gray region of FIG. 3 (region where the Abbe number is larger and the refractive index is smaller compared to a core material of pure silica glass), that is, an optical medium in which
[Math. 3]
1.444>n>0.0343.sub.d+3.771(3)
(17) is obtained is used for the cladding; and thereby the group delay time .sub.d (the second term of the expression (1)) can be reduced. Thus, the optical fiber of this embodiment can moderate a tradeoff relationship between a decrease in the group delay time and an increase in loss.
(18) FIG. 4 is a diagram that describes an example of the optical fiber of this embodiment. FIG. 4(A) is a diagram that describes a refractive index profile in a cross section of the optical fiber. FIG. 4(B) is a diagram that describes a cross-section structure of the optical fiber. In the optical fiber, silica glass is used for a core region 11 and S-FPL53 (Abbe number .sub.d=94.93, Non-Patent Literature 5) that satisfies the relationship in the expression (3) is used for a cladding region 12. At this point, fluorine is doped into the core region 11 so that the relative refractive index difference between the core region 11 and the cladding region 12 at the wavelength of 1.55 m is 0.35%. This allows a reduction of the refractive index of the cladding region 12 by 0.67% compared to pure silica glass, achieving a single mode operation.
(19) FIG. 5 shows the wavelength dependence (solid line) of the relative refractive index difference in the optical fiber described with reference to FIG. 4. Here, the radius of the core is 4.2 m as with a general-purpose SMF. In addition, for comparison, the wavelength dependence (dashed line) of the relative refractive index difference in the SI-type fluorine core optical fiber (comparison example) is also shown. The SI-type fluorine core optical fiber has a core and cladding formed of fluorine-doped glass, in which the core radius is 4.2 m, a core refractive index is lower by 0.67% compared to that of pure silica glass, and the relative refractive index difference is 0.35%. In this SI-type fluorine core fiber, the relative refractive index difference is independent of the operating wavelength; however, in the optical fiber of this embodiment, the relative refractive index difference increases with an increase of the operating wavelength.
(20) FIG. 6 is a diagram that describes results of calculating a group delay time in each of the optical fiber of this embodiment and the SI-type fluorine core optical fiber that is a comparative example in FIG. 5. The group delay times indicate , .sub.n, and .sub.d that are described for the expression (1). The group delay times of the optical fiber of this embodiment and the SI-type fluorine core optical fiber are 4.84 s/km and 4.85 s/km, respectively. The optical fiber of this embodiment achieves a lower delay by 0.17% compared to the SI-type fluorine core optical fiber. The .sub.n that is the first term of the expression (1) is 4.78 s/km for both. This is because both have the same core refractive index. On the other hand, .sub.d that is the second term of the expression (1) is 0.06 s/km for the optical fiber of this embodiment and is 0.07 s/km for the SI-type fluorine core optical fiber; it is reduced in the optical fiber of this embodiment. This is because the wavelength dispersion of n.sub.eff is reduced.
(21) As described above, the optical fiber of the present invention can be expected not only to reduce delay due to a reduction of n.sub.eff of a fluorine-doped core but also to further reduce the delay due to a reduction in the wavelength dispersion of n.sub.eff. In addition, as for an optical loss of an optical fiber in which light is sufficiently confined, a loss in a core is dominant. Therefore, as in the present invention, by using glass having a large Abbe number .sub.d in the cladding region, a tradeoff between a reduction of delay and an increase in the Rayleigh scattering loss can be moderated.
Second Embodiment
(22) An optical fiber of this embodiment has a W-type refractive index distribution. More specifically, the optical fiber further includes a physical cladding region 13 that is arranged on an outer periphery of the cladding region 12, as compared to the optical fiber described in the first embodiment, in which the physical cladding region 13 has a higher refractive index than that of the cladding region 12, and the medium of each of the core region 11 and the physical cladding region 13 is silica glass.
(23) FIG. 7 is a diagram that describes a refractive index distribution structure of the optical fiber. The optical fiber uses silica-based glass such as fluorine-doped glass for the core region 11 as with the optical fiber of the first embodiment; has a low refractive index layer (cladding region 12) of glass with a large Abbe number .sub.d coaxially arranged on an outside thereof; and has silica-based glass (physical cladding region 13) such as fluorine-doped glass arranged on an outside of the low refractive index layer.
(24) The optical fiber is preferable in that, since heating conditions in spinning are determined by a silica-based glass cladding in the outermost layer having a large area, the manufacturing technique for a general-purpose optical fiber can be applied. In addition, by adopting the existing W-type structure described in Non-Patent Literature 6, optical characteristics equivalent to that of a general-purpose optical fiber can be obtained.
INDUSTRIAL APPLICABILITY
(25) The present invention is applicable to an inter-terminal communication in an optical communication system.
REFERENCE SIGNS LIST
(26) 11 Core region 12 Cladding region 13 Physical cladding region
