Optical fiber

The present invention is installed in a part of an optical transmission line in the optical transmission system or on the optical transmission line in an optical transmission system that mainly performs WDM communication in a wavelength band of 1.55 μm. It relates to an optical fiber applicable to a module.

PROBLEM TO BE SOLVED: To perform large-capacity and high-speed optical communication by transmitting a plurality of signal lights in a 1.55 μm wavelength band (1.53 μm to 1.57 μm) in WDM (Wavelength Division Multiplexing) communication. enable. In such an optical transmission system performing WDM communication, it is preferable that the dispersion in the 1.55 μm wavelength band is small so that signal light can be transmitted in a wide wavelength band, and the zero dispersion wavelength is shifted to the 1.55 μm wavelength band. Dispersion Shifted Fiber (DSF) has been used for optical transmission lines.

[0003] However, when the dispersion in the 1.55 μm wavelength band is substantially zero, four-wave mixing, which is a kind of nonlinear optical phenomenon, occurs, and the signal light at the time of reception tends to be deteriorated (for example, H. Taga, See et al., OFC'98, PD13). Therefore, conventionally, a dispersion-shifted optical fiber in which the dispersion at a wavelength of 1.55 μm is set to about -2 ps / nm / km (there is no zero-dispersion wavelength in the signal wavelength band) by further shifting the zero dispersion wavelength to the longer wavelength side (a dispersion-shifted optical fiber). By applying NZ-DSF: Non-zero Dispersion Shifted Fiber) to the optical transmission line, four-wavelength mixing is suppressed. Since the above NZ-DSF has a negative dispersion in the 1.55 μm wavelength band, the optical fiber for dispersion compensation having a positive dispersion in the 1.55 μm wavelength band in order to compensate for this negative dispersion is this NZ-DSF. It may also be applied to optical transmission lines (see, for example, M. Suzuki, et al., OFC'98, PD17).

[0004] As the optical fiber for dispersion compensation, for example, an optical fiber defined by the G652 standard or the G654 standard of ITU-T is known. The G652 standard optical fiber is a standard optical fiber composed of a core region of Ge element-added silica and a clad region of pure silica. This G652 standard optical fiber has a zero dispersion wavelength in the 1.3 μm wavelength band and a dispersion of about 17 ps / nm / km in the 1.55 μm wavelength band. On the other hand, the G654 standard optical fiber has a dispersion of 20 ps / nm / km or less in the 1.55 μm wavelength band. Further, an optical fiber composed of a core region of pure silica and a clad region of silica with F element added and having a dispersion of about 18 ps / nm / km in the 1.55 μm wavelength band is also used as an optical fiber for dispersion compensation.

[0005] Since the conventional optical transmission line composed of the NZ-DSF and the optical fiber for dispersion compensation has a positive dispersion slope as a whole, it is dispersed at one wavelength within the 1.55 μm wavelength band. Is zero, but the dispersion is not zero in other wavelength regions. Therefore, in order to compensate for the residual dispersion in the other wavelength region, each signal light in the other wavelength region is demultiplexed in the base station or the like, and an optical fiber for dispersion compensation of the G652 standard or the G654 standard is used. Dispersion compensation is performed for each signal light. The dispersion slope is given by the slope of the graph showing the wavelength dependence of the dispersion.

[Problems to be Solved by the Invention] As a result of examining the above-mentioned prior art, the inventor has found the following problems. That is, the optical fiber for dispersion compensation of the above G654 standard compensates for the negative dispersion of NZ-DSF in the 1.55 μm wavelength band because the upper limit of the dispersion in the 1.55 μm wavelength band exceeds 20 ps / nm / km. Therefore, it had to be long. Further, in an optical fiber having a simple step-type refractive index profile consisting of a core region and a clad region, the upper limit of dispersion is determined according to the upper limit of the cutoff wavelength, so that the dispersion in the 1.55 μm wavelength band It was difficult to increase.

[0007] The present invention has been made to solve the above-mentioned problems, and has a large positive dispersion in the 1.55 μm wavelength band and compensates for the negative dispersion of NZ-DSF in the 1.55 μm wavelength band. It is an object of the present invention to provide an optical fiber to be used.

[Means for Solving the Problems] The optical fiber according to the present invention includes a core region extending along a predetermined axis and a clad region provided on the outer periphery of the core region. The clad region is a region provided on the outer periphery of the core region, and is an inner clad having a refractive index lower than that of the core region and a region provided on the outer periphery of the inner clad, which is larger than the core region. It has a depressed clad structure composed of an outer clad which is low and has a higher refractive index than the inner clad. Further, in the optical fiber, the difference in the specific refractive index of the core region with respect to the outer clad is 0.30% or more and 0.50% or less, and the difference in the specific refractive index of the inner clad with respect to the outer clad region is -0.50% or more and -0.02% or less. is there. In addition, at a wavelength of 1.55 μm, the optical fiber has a dispersion larger than 18 ps / nm / km and 70 μm.²The above effective cross-sectional area A_effHave.

[0009] The effective cross-sectional area A_effIs given by the following equation (1) as shown in Japanese Patent Application Laid-Open No. 8-248251 (EP 0 724 171 A2).

[0010] [Number 1]<img file="JP4192425B2_D0001.tif" />[0011] Here, E is the electric field associated with the propagating light, and r is the radial distance from the center of the core.

[0012] As described above, since the optical fiber has a large dispersion in the 1.55 μm wavelength band, it is sufficient to compensate for the negative dispersion of the NZ-DSF in the 1.55 μm wavelength band, and the optical fiber has a predetermined diameter. It is preferable that the module can be miniaturized when it is wound around and modularized. Moreover, since the effective cross-sectional area at a wavelength of 1.55 μm is large, the occurrence of nonlinear optical phenomena can be effectively suppressed. Further, in addition to the above characteristics, the optical fiber according to the present invention preferably has a dispersion of 20 ps / nm / km or more at a wavelength of 1.55 μm. Since the optical fiber has a larger dispersion in the 1.55 μm wavelength band, it is possible to further shorten the length when compensating for the negative dispersion of NZ-DSF in the 1.55 μm wavelength band, and the dispersion compensation to which the optical fiber is applied is possible. It will be easier to miniaturize the module. In particular, in order to realize various characteristics at a wavelength of 1.55 μm, all the optical fibers having the above-mentioned configurations have an outer diameter of the core region of 2a (unit: μm) and an outer diameter of the inner clad of 2b (unit: unit). When: μm), it is preferable that the relationship of 2.0 2b / 2a 6.08.3 2a 13.0 is satisfied.

[0013] The optical fiber according to the present invention includes a core region extending along a predetermined axis and having an outer diameter of 9.5 μm or more and 13.0 μm or less, and a clad region having a refractive index lower than that of the core region. It may be a structure. In such a configuration, the difference in the specific refractive index of the core region with respect to the clad region is 0.3% or more and 0.5% or less. The dispersion at a wavelength of 1.55 μm is 20 ps / nm / km or more, and the effective cross-sectional area A at a wavelength of 1.55 μm A._effIs 70 μm²That is all. Since this optical fiber also has a large dispersion in the 1.55 μm wavelength band, it can be short in compensating for the negative dispersion of NZ-DSF in the 1.55 μm wavelength band. Moreover, since the effective cross-sectional area at a wavelength of 1.55 μm is large, the occurrence of nonlinear optical phenomena is effectively suppressed.

[0014] In any of the optical fibers having various structures described above, the transmission loss when wound in a coil shape with a diameter of 60 mm is 0.215 dB / km or less at a wavelength of 1.55 μm, and the wavelength is 1.55. Polarization mode dispersion at μm is 0.25ps km^-1/2It is preferably as follows. In this case, the optical fiber according to the present invention can obtain sufficient characteristics even in a coiled and modularized configuration.

[0015] As a more preferable optical property, the optical fiber according to the present invention is 90 μm.²The above effective cross-sectional area A_effHave. Further, the optical fiber has a cutoff wavelength of 1.4 μm or more in a fiber length of 2 m. Further, the optical fiber has a transmission loss of 0.180 dB / km or less at a wavelength of 1.55 μm.

[0016] The inventors have experimentally confirmed that providing a carbon coating on the surface of the optical fiber according to the present invention is effective in preventing the optical fiber from breaking.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, examples of the optical fiber according to the present invention are shown in FIGS. 1 (a), 1 (b), 2 to 5, 6 (a), and 6 (FIG. 6). This will be described with reference to b), FIG. 7, FIG. 8 (a), FIG. 8 (b), and FIG. The same elements in the drawings are designated by the same reference numerals, and duplicate description will be omitted.

(First Example) FIG. 1 (a) is a diagram showing a cross-sectional structure of an optical fiber according to the first embodiment, and FIG. 1 (b) is a diagram showing the light shown in FIG. 1 (a). The refractive index profile of the fiber. The optical fiber 100 according to the first embodiment extends along a predetermined axis and has a refractive index n.₁A core region 110 having an outer diameter of 2a (μm) and a clad region provided on the outer periphery of the core region 110 are provided. This clad region is a region provided on the outer periphery of the core region 110 in order to further realize the depressed clad structure, and has a refractive index n.₂(&lt;n₁), And a region provided on the outer periphery of the inner clad 120 having an outer diameter of 2b and having a refractive index n.₃(&lt;n₁,&gt; N₂) With the outer clad 130. Therefore, the magnitude relation of the refractive index in each region 110, 120, 130 is n.₁&gt; n₃&gt; n₂Is. A carbon coating 140 is provided on the outer periphery of the optical fiber 100 according to the first embodiment in order to effectively prevent breakage when modularized by being wound in a coil shape.

[0019] The horizontal axis of the refractive index profile 150 shown in FIG. 1 (b) is each portion on the cross section perpendicular to the central axis of the core region 110 along the line L in FIG. 1 (a). Corresponds to. Therefore, in the refractive index profile 150 of FIG. 1 (b), the region 151 is the refractive index at each site on the line L of the core region 110, and the region 152 is the refractive index at each site on the line L of the inner clad 120. Shows the refractive index at each site on the line L of the outer clad region 130.

The optical fiber having such a refractive index profile 150 is a silica-based single-mode optical fiber, for example, Ge element is added to the core region 110, and F element is added to the inner clad 120. It is feasible by doing so. Further, in FIGS. 1 (a) and 1 (b), 2a represents the outer diameter of the core region 110, and 2b represents the outer diameter of the inner clad 120. Δ⁺Is the difference in the index of refraction of the core region 110 with respect to the outer clad 130, Δ^-Represents the difference in the refractive index of the inner clad 120 with respect to the outer clad 130. The difference in the specific refractive index of the core region 110 with respect to the outer clad 130 Δ⁺, And the difference in the refractive index of the inner clad 120 with respect to the outer clad 130 Δ^-Are defined as follows.

[0021] Δ⁺= (n₁-n₃) / N₃Δ^-= (n₂-n₃) / N₃Where n₁Is the index of refraction of the core region 110, n₂Is the index of refraction of the inner clad 120, n₃Is the index of refraction of the outer clad 130. Further, in this specification, the specific refractive index difference Δ is expressed as a percentage, and the refractive indexes of each region in each definition formula are in no particular order. Therefore, a negative value of Δ means that the index of refraction of the corresponding region is lower than the index of refraction of the outer clad 130.

[0022] Then, in the optical fiber 100 according to the first embodiment, the difference in the specific refractive index of the core region 110 with respect to the outer clad 130 Δ⁺Is 0.30% or more and 0.50% or less, and the difference in the refractive index of the inner clad 120 with respect to the outer clad 130 Δ^-Is -0.50% or more and -0.02% or less. In addition, the dispersion at a wavelength of 1.55 μm is larger than 18 ps / nm / km, and the effective cross-sectional area A at a wavelength of 1.55 μm A._effIs 70 μm²That is all.

[0023] FIGS. 2 to 5 are graphs showing the relationship between the outer diameter 2a of the core region 110 of the optical fiber 100 according to the first embodiment and the dispersion at a wavelength of 1.55 μm, respectively. In the graph of FIG. 2, the ratio (2b / 2a) of the outer diameter 2b of the inner clad 120 to the outer diameter 2a of the core region 110 is 4.0, and the specific refractive index difference Δ of the inner clad 120 with respect to the outer clad 130.^-Is fixed at -0.03% respectively. Further, in the graph of FIG. 3, the ratio (2b / 2a) of the outer diameter 2b of the inner clad 120 to the outer diameter 2a of the core region 110 is 4.0, and the specific refractive index difference Δ of the inner clad 120 with respect to the outer clad 130.^-Is fixed at -0.09% respectively. In the graph of FIG. 4, the ratio (2b / 2a) of the outer diameter 2b of the inner clad 120 to the outer diameter 2a of the core region 110 is 4.0, and the specific refractive index difference Δ of the inner clad 120 with respect to the outer clad 130.^-Is fixed at -0.20% respectively. Further, in the graph of FIG. 5, the ratio (2b / 2a) of the outer diameter 2b of the inner clad 120 to the outer diameter 2a of the core region 110 is 4.0, and the specific refractive index difference Δ of the inner clad 120 with respect to the outer clad 130.^-Is fixed at -0.45% respectively.

[0024] In each of FIGS. 2 to 5, G100, G200, and G300 have a specific refractive index difference Δ of the core region 110 with respect to the outer clad 130.⁺Is a graph showing the relationship between the core diameter 2a and the dispersion value at a wavelength of 1.55 μm in each case of 0.30%, 0.40%, and 0.50%. In addition, C1 shows the relationship between the core diameter 2a and the dispersion value at a wavelength of 1.55 μm, where the loss increase (wavelength 1.55 μm) of an optical fiber with a total length of 20 km due to winding with a diameter of 60 mm is 0.01 dB / km. It is a graph. Further, FIGS. 2 to 5 show graphs showing the relationship between the core diameter 2a and the dispersion value at the wavelength of 1.55 μm in each case where the cutoff wavelengths λc are 1.5 μm and 1.6 μm, and the effective cross-sectional area A._effIs 70 μm², 80 μm², 90 μm²Each graph showing the relationship between the core diameter 2a and the dispersion value at a wavelength of 1.55 μm is shown. The cutoff wavelength λc may be allowed up to about 1.60 μm in the case of an optical fiber having a length of several hundred meters, and up to about 1.70 μm in the case of a longer optical fiber. In addition, in each of FIGS. 2 to 5, the cutoff wavelength λc is 1.6 μm or less, and the effective cross-sectional area A._effIs 70 μm²As described above, the range in which the loss increase (wavelength 1.55 μm) of the optical fiber having a total length of 20 km due to being wound with a diameter of 60 mm is 0.01 dB / km or less is shown as a preferable range (shown in each figure). The shaded area).

Judging from FIG. 2, the difference in the specific refractive index of the inner clad 120 with respect to the outer clad 130 Δ^-In the case of an optical fiber with a value of -0.03%, if the outer diameter 2a of the core region 110 is about 8.3 μm or more, the dispersion at a wavelength of 1.55 μm can be about 18 ps / nm / km or more. If the outer diameter 2a of the core region 110 is about 9.2 μm or more, the dispersion at a wavelength of 1.55 μm can be about 20 ps / nm / km or more. Further, if the outer diameter 2a of the core region 110 is about 12.5 μm, the dispersion at a wavelength of 1.55 μm can be increased to about 21.3 ps / nm / km.

Judging from FIG. 3, the difference in the specific refractive index of the inner clad 120 with respect to the outer clad 130 Δ^-In the case of an optical fiber having a value of -0.09%, if the outer diameter 2a of the core region 110 is about 8.3 μm or more, the dispersion at a wavelength of 1.55 μm can be about 18 ps / nm / km or more. If the outer diameter 2a of the core region 110 is about 9.1 μm or more, the dispersion at a wavelength of 1.55 μm can be about 20 ps / nm / km or more. Further, when the outer diameter 2a of the core region 110 is about 12.5 μm, the dispersion at a wavelength of 1.55 μm can be increased to about 21.7 ps / nm / km.

[0027] Judging from FIG. 4, the difference in the specific refractive index of the inner clad 120 with respect to the outer clad 130 Δ^-In the case of an optical fiber with a value of -0.20%, if the outer diameter 2a of the core region 110 is about 9.5 μm or more, the dispersion at a wavelength of 1.55 μm can be about 20.8 ps / nm / km or more. Further, if the outer diameter 2a of the core region 110 is about 12.8 μm, the dispersion at a wavelength of 1.55 μm can be increased to about 22.3 ps / nm / km.

[0028] Further, judging from FIG. 5, the difference in the specific refractive index of the inner layer clad 120 with respect to the outer clad 130 Δ^-In the case of an optical fiber with a value of -0.45%, if the outer diameter 2a of the core region 110 is about 10.5 μm or more, the dispersion at a wavelength of 1.55 μm can be about 23.2 ps / nm / km or more. Further, if the outer diameter 2a of the core region 110 is about 13.0 μm, the dispersion at a wavelength of 1.55 μm can be increased to about 23.5 ps / nm / km.

As can be seen from FIGS. 2 to 5 above, the difference in the specific refractive index of the inner clad 120 with respect to the outer clad 130 Δ^-By making the value smaller (increasing the absolute value), the variance can be increased while maintaining the same value of the cutoff wavelength λc.

Next, a plurality of application examples of the optical fiber according to the first embodiment will be described.

First, the optical fiber 100 according to the first application example has the cross-sectional structure shown in FIG. 1 (a) and the refractive index profile shown in FIG. 1 (b), and has an outer diameter 2a of the core region 110. , Outer diameter 2b of inner clad 120, specific refractive index difference Δ of core region 110 with respect to outer clad 130⁺, The difference in the refractive index of the inner clad 120 with respect to the outer clad 130 Δ^-Are set as follows.

2a (μm): 9.02b (μm): 36.0Δ⁺(%): 0.35Δ^-(%): The optical fiber according to the first application example designed as -0.03 or more has the following optical characteristics as various characteristics having a wavelength of 1.55 μm.

Variance (ps / nm / km): 18.7 Effective cross-sectional area A_eff(μm²): 80.5 Dispersion slope (ps / nm)²/ km): 0.058 Transmission loss when bent to a diameter of 60 mm (dB / km): 0.208 Polarization mode dispersion PMD (ps · km)^-1/2): 0.14 The cutoff wavelength of the optical fiber according to this first application example at a length of 2 m is 1.25 μm. The transmission loss is the sum of the original transmission loss of the optical fiber and the increase in loss due to bending at a diameter of 60 mm.

[0034] The optical fiber according to the second application example also has the cross-sectional structure shown in FIG. 1 (a), and its refractive index profile has the same shape as the refractive index profile shown in FIG. 1 (b). .. The optical fiber of this second application example is designed according to the following specifications.

2a (μm): 10.52b (μm): 42.0Δ⁺(%): 0.35Δ^-(%): The optical fiber according to the second application example designed as -0.03 or more has the following optical characteristics as various characteristics of a wavelength of 1.55 μm.

Variance (ps / nm / km): 20.4 Effective cross-sectional area A_eff(μm²): 93.2 Distributed slope (ps / nm)²/ km): 0.060 Transmission loss when bent to a diameter of 60 mm (dB / km): 0.204 Polarization mode dispersion PMD (ps · km)^-1/2): 0.12 The cutoff wavelength of the optical fiber according to this second application example at a length of 2 m is 1.45 μm. The transmission loss is the sum of the original transmission loss of the optical fiber and the increase in loss due to bending at a diameter of 60 mm.

[0037] The optical fiber according to the third application example is designed according to the following specifications.

2a (μm): 10.52b (μm): 46.0Δ⁺(%): 0.35Δ^-(%): The optical fiber according to the third application example designed as -0.03 or more has the following optical characteristics as various characteristics of a wavelength of 1.55 μm.

Dispersion (ps / nm / km): 21.0 Effective cross-sectional area A_eff(μm²): 103.0 Dispersion slope (ps / nm)²/ km): 0.061 Transmission loss when bent to a diameter of 60 mm (dB / km): 0.202 Polarization mode dispersion PMD (ps · km)^-1/2): 0.12 The cutoff wavelength of the optical fiber according to this third application example at a length of 2 m is 1.59 μm. The transmission loss is the sum of the original transmission loss of the optical fiber and the increase in loss due to bending at a diameter of 60 mm.

[0040] Further, the optical fiber according to the fourth application example is designed according to the following specifications.

2a (μm): 10.02b (μm): 40.0Δ⁺(%): 0.31Δ^-(%): The optical fiber according to the fourth application example designed as -0.03 or more has the following optical characteristics as various characteristics of a wavelength of 1.55 μm.

Variance (ps / nm / km): 19.6 Effective cross-sectional area A_eff(μm²): 98.0 Distributed slope (ps / nm)²/ km): 0.060 Transmission loss when bent to a diameter of 60 mm (dB / km): 0.204 Polarization mode dispersion PMD (ps · km)^-1/2): 0.12 The cutoff wavelength of the optical fiber according to this fourth application example at a length of 2 m is 1.31 μm. The transmission loss is the sum of the original transmission loss of the optical fiber and the increase in loss due to bending at a diameter of 60 mm.

[0043] The optical fiber according to the fifth application example has the cross-sectional structure shown in FIG. 1 (a) and the refractive index profile 160 shown in FIG. 6 (a). As can be seen from the shape of the refractive index profile 160, in this fifth application example, the central portion of the core region 110 is recessed from the periphery, and the hem portion of the core region 110 is inclined (hem portion). Is a shape that extends toward the inner clad 120 side). The horizontal axis of the refractive index profile 160 corresponds to each portion on the cross section perpendicular to the central axis of the core region 110 along the line L in FIG. 1 (a). Therefore, in the refractive index profile 160, the region 161 is the refractive index at each part on the line L of the core region 110 (outer diameter 2a), and the region 162 is the refraction at each part on the line L of the inner clad 120 (outer diameter 2b). The rate and region 163 indicate the refractive index at each site on the line L of the outer clad 130. In this fifth application example, the difference in the specific refractive index of the core region 110 with respect to the outer clad 130 Δ⁺Is the refractive index n of the outer clad₃And the average refractive index n of the core region 110₁Given by, the difference in the index of refraction of the inner clad 120 with respect to the outer clad 130 Δ^-Is the refractive index n of the outer clad₃And the minimum refractive index n of the inner clad 120₂Given by.

[0044] The optical fiber according to the fifth application example is designed based on the following specifications.

2a (μm): 10.02b (μm): 45.4Δ⁺(%): 0.34Δ^-(%): The optical fiber according to the fifth application example designed as -0.03 or more has the following optical characteristics as various characteristics of a wavelength of 1.55 μm.

Variance (ps / nm / km): 19.5 Effective cross-sectional area A_eff(μm²): 105.0 Distributed slope (ps / nm)²/ km): 0.062 Transmission loss when bent to a diameter of 60 mm (dB / km): 0.198 Polarization mode dispersion PMD (ps · km)^-1/2): 0.13 The cutoff wavelength of the optical fiber according to this fifth application example at a length of 2 m is 1.62 μm. The transmission loss is the sum of the original transmission loss of the optical fiber and the increase in loss due to bending at a diameter of 60 mm.

[0047] Further, the sixth application example has the cross-sectional structure shown in FIG. 1 (a) and has the refractive index profile 170 as shown in FIG. 6 (b). This sixth application includes a core region 110 of pure silica and an inner clad 120 and an outer clad 130 of F element-added silica. The horizontal axis of the refractive index profile 170 corresponds to each part on the cross section perpendicular to the central axis of the core region 110 along the line L in FIG. 1 (a). Therefore, in the refractive index profile 170, the region 171 is the refractive index at each part on the line L of the core region 110 (outer diameter 2a), and the region 172 is the refraction at each part on the line L of the inner clad 120 (outer diameter 2b). The rate and region 173 indicate the refractive index at each site on the line L of the outer clad 130.

[0048] The optical fiber according to the sixth application example is designed based on the following specifications.

2a (μm): 11.62b (μm): 46.4Δ⁺(%): 0.31Δ^-(%): The optical fiber according to the sixth application example designed as described above-0.05 has the following optical characteristics as various characteristics having a wavelength of 1.55 μm.

Variance (ps / nm / km): 20.5 Effective cross-sectional area A_eff(μm²): 99 dispersion slope (ps / nm)²/ km): 0.060 Transmission loss when bent to a diameter of 60 mm (dB / km): 0.172 Polarization mode dispersion PMD (ps · km)^-1/2): 0.08 The cutoff wavelength of the optical fiber according to this sixth application example at a length of 2 m is 1.50 μm. The transmission loss is the sum of the original transmission loss of the optical fiber and the increase in loss due to bending at a diameter of 60 mm.

[0051] In addition, when an optical fiber was designed or prototyped under various conditions and its characteristics were evaluated, the optical fiber according to the first embodiment had a sufficiently large dispersion and effective cross-sectional area A at a wavelength of 1.55 μm._effWas found to be obtained. In particular, when the outer diameter of the core region is 2a (unit: μm) and the outer diameter of the inner layer clad region is 2b (unit: μm), it is preferable to satisfy the relational expression of 2.0 2b / 2a 6.0. I also found that there was. Further, in the optical fiber according to the first embodiment, the transmission loss (the sum of the original transmission loss of the optical fiber and the increase in loss due to bending) at a wavelength of 1.55 μm when wound in a coil with a diameter of 60 mm is 0.215 dB / It was also confirmed that the transmission loss was 0.180 dB / km or less, which is the original transmission loss of the optical fiber at a wavelength of 1.55 μm in a more preferable application example. Further, according to the optical fiber according to the first embodiment, the polarization mode dispersion at a wavelength of 1.55 μm is 0.25 ps · km.^-1/2It was also confirmed that the following was achieved.

[0052] The inventors have experimentally confirmed that providing a carbon coating on the surface of the above-mentioned optical fiber is effective in preventing the optical fiber from breaking.

FIG. 7 is a graph showing the experimental results for explaining the fracture prevention effect of the carbon coating, and the graph G400 shows the tensile speed (mm / min) when the carbon-coated optical fiber is fractured. The relationship between and tensile strength (GPa) is shown, and Graph G500 shows the relationship between tensile speed (mm / min) and tensile strength (GPa) when an optical fiber without carbon coating breaks. .. Moreover, while the fatigue index N of the optical fiber coated with carbon exceeds 150, the fatigue index N of the optical fiber without carbon coating is about 25. It is known that the breaking strength (GPa) when an optical fiber breaks is proportional to the 1 / (N + 1) power of the tensile speed (mm / min) that pulls the optical fiber as follows. , N in the following formula is especially called the fatigue index.

(Breaking strength) = α × (tensile speed)^{1 / (N + 1)}As can be seen from Fig. 7, the larger the tensile speed, the smaller the difference in breaking strength due to the presence or absence of the carbon coating (apparently, the faster the pull is, the more difficult it is for scratches to grow and the more it breaks even if the same force is applied. Hateful). However, since the fracture of the actually laid optical fiber is caused by being pulled at a very slow speed, the optical fiber coated with carbon coating having a high fracture strength at a low speed is more preferable.

As described above, since the optical fiber according to the first embodiment has a large positive dispersion in the 1.55 μm wavelength band, when compensating for the negative dispersion of the NZ-DSF in the 1.55 μm wavelength band, It is short and enables miniaturization of the dispersion compensation module to which the optical fiber is applied. In addition, the optical fiber has an effective cross-sectional area A at a wavelength of 1.55 μm._effIs large, so that the occurrence of nonlinear optical phenomena is effectively suppressed. Further, the optical fiber is suitable for modularization because it has a small transmission loss at a wavelength of 1.55 μm when wound in a coil with a diameter of 60 mm and a small polarization mode dispersion at a wavelength of 1.55 μm.

(Second Example) Next, a second example of the optical fiber according to the present invention will be described. FIG. 8 (a) is a diagram showing a cross-sectional structure of the optical fiber according to the second embodiment, and FIG. 8 (b) is a refractive index profile of the optical fiber shown in FIG. 8 (a). The optical fiber 200 according to the second embodiment extends along a predetermined axis and has a refractive index n.₁A core region 210 having a refractive index n and a region provided on the outer periphery of the core region 210.₂(&lt;n₁) With a clad region 220. Therefore, the magnitude relationship of the refractive index in each of the regions 210 and 220 is n.₁&gt; n₂Is. A carbon coating 230 is provided on the outer periphery of the optical fiber 200 according to the second embodiment in order to effectively prevent breakage when modularized by being wound in a coil shape.

The horizontal axis of the refractive index profile 250 shown in FIG. 8 (b) is each portion on the cross section perpendicular to the central axis of the core region 210 along the line L in FIG. 8 (a). Corresponds to. Therefore, in the refractive index profile 250 of FIG. 8 (b), the region 251 shows the refractive index at each part on the line L of the core region 210, and the region 252 shows the refractive index at each part on the line L of the clad region 220. ing.

The optical fiber 200 having such a refractive index profile 250 is a silica-based single-mode optical fiber, which can be realized by adding a Ge element to the core region 210, for example. It can also be realized by using pure silica as the core region 210 and adding the F element to the clad region 220. In FIGS. 8 (a) and 8 (b), 2a indicates the outer diameter of the core region 210, which is Δ.⁺Indicates the difference in the specific refractive index of the core region 210 with respect to the clad region 220.

[0059] Then, in the optical fiber 200 according to the second embodiment, the difference in the specific refractive index of the core region 210 with respect to the clad region 220 Δ⁺(= (n₁-n₂) / N₂) Is 0.3% or more and 0.5% or less, the variance at a wavelength of 1.55 μm is 20 ps / nm / km or more, and the effective cross-sectional area at a wavelength of 1.55 μm is 70 μm.²As described above, the outer diameter of the core region 210 is 9.5 μm or more and 12.0 μm or less.

FIG. 9 is a graph showing the relationship between the outer diameter 2a of the core region 210 of the optical fiber according to the second embodiment and the dispersion at a wavelength of 1.55 μm. In this figure, G100, G200, and G300 have a specific refractive index difference Δ of the core region 210 with respect to the clad region 220.⁺Is a graph showing the relationship between the core diameter 2a and the dispersion value at a wavelength of 1.55 μm in each case of 0.50%, 0.40%, and 0.30%. In addition, C1 shows the relationship between the core diameter 2a and the dispersion value at a wavelength of 1.55 μm, where the loss increase (wavelength 1.55 μm) of an optical fiber with a total length of 20 km due to winding with a diameter of 60 mm is 0.01 dB / km. It is a graph. Further, FIG. 9 shows each graph showing the relationship between the core diameter 2a and the dispersion value at the wavelength of 1.55 μm when the cutoff wavelengths λc are 1.5 μm and 1.6 μm, and the effective cross-sectional area A._effIs 70 μm², 80 μm², 90 μm²Each graph showing the relationship between the core diameter 2a and the dispersion value at a wavelength of 1.55 μm is shown. The cutoff wavelength λc may be allowed up to about 1.60 μm in the case of an optical fiber having a length of several hundred meters, and up to about 1.70 μm in the case of a longer optical fiber. In Fig. 9, the cutoff wavelength λc is 1.6 μm or less, and the effective cross-sectional area A._effIs 70 μm²As mentioned above, the range in which the dispersion at a wavelength of 1.55 μm is 20 ps / nm / km or more and the loss increase (wavelength 1.55 μm) of the optical fiber with a total length of 20 km due to being wound with a diameter of 60 mm is 0.01 dB / km or less. It is shown as a preferable range (shaded area in the figure).

Judging from FIG. 9, if the outer diameter 2a of the core region 210 is about 9.5 μm or more, the dispersion at a wavelength of 1.55 μm can be about 20 ps / nm / km or more. Further, if the outer diameter 2a of the core region 210 is about 12.0 μm, the dispersion at a wavelength of 1.55 μm can be increased to about 20.7 ps / nm / km.

[0062] In the optical fiber 200 according to the second embodiment, the outer diameter 2a of the core region 210 is 11.0 μm, and the difference in the specific refractive index of the core region 210 with respect to the clad region 220 Δ.⁺Is 0.35%. At this time, the cutoff wavelength λc is 1.54 μm, the variance at the wavelength 1.55 μm is 20.3 ps / nm / km, and the effective cross-sectional area A._effIs 100.0 μm², Dispersion slope is 0.060ps / nm²Transmission loss when bent to / km, diameter 60 mm is 0.210 dB / km (0.215 dB / km or less), polarization mode dispersion is 0.10 ps km^-1/2Met.

[0063] Since the optical fiber according to the second embodiment also has a large positive dispersion in the 1.55 μm wavelength band, it can be short in compensating for the negative dispersion of the NZ-DSF in the 1.55 μm wavelength band. It is suitable for miniaturization of dispersion compensation modules to which optical fibers are applied. In addition, the optical fiber has an effective cross-sectional area A at a wavelength of 1.55 μm._effIs large, so that the occurrence of nonlinear optical phenomena is effectively suppressed. Further, the optical fiber is suitable for modularization because the transmission loss (wavelength 1.55 μm) when bent to a diameter of 60 mm is small and the polarization mode dispersion at a wavelength of 1.55 μm is also small.

[0064] The present invention is not limited to the above-described embodiment, and various modifications are possible. For example, six specific application examples are shown as the optical fiber according to the first embodiment, and one specific application example is shown as the optical fiber according to the second embodiment, but the present invention is not limited to these. However, various designs are possible within the above-mentioned appropriate range.

[Effect of the Invention] As described above, according to the optical fiber according to the present invention, the dispersion in the 1.55 μm wavelength band is large, so that when compensating for the negative dispersion of the NZ-DSF in the 1.55 μm wavelength band, It only needs to be short. This facilitates miniaturization of the dispersion compensation module to which the optical fiber according to the present invention is applied. Further, since the optical fiber according to the present invention has a large effective cross-sectional area at a wavelength of 1.55 μm, the occurrence of a nonlinear optical phenomenon is effectively suppressed. Further, the optical fiber according to the present invention has a transmission loss of 0.215 dB / km or less at a wavelength of 1.55 μm when bent to a diameter of 60 mm (more preferably, the original transmission loss of the optical fiber does not include a loss increase due to bending). Is 0.180 dB / km or less), and the polarization mode dispersion at a wavelength of 1.55 μm is 0.25 ps km-^1/2Since it is as follows, it is suitable for modularization.

BRIEF DESCRIPTION OF THE DRAWINGS [Fig. 1] (a) is a diagram showing a cross-sectional structure of a first embodiment of an optical fiber according to the present invention, and (b) is an optical fiber shown in (a). It is a figure which shows the refractive index profile of.

[Figure 2]<img file="JP4192425B2_D0002.tif" />FIG. 3 shows that in the optical fiber according to the first embodiment, the ratio (2b / 2a) of the outer diameter 2b of the inner clad to the outer diameter 2a of the core region is fixed to 4.0, and the difference in the specific refractive index of the inner clad with respect to the outer clad. Δ^-Is a graph showing the relationship between the core diameter (outer diameter of the core region) 2a and the dispersion at a wavelength of 1550 nm when is fixed at -0.09%.

FIG. 4 shows that in the optical fiber according to the first embodiment, the ratio (2b / 2a) of the outer diameter 2b of the inner clad to the outer diameter 2a of the core region is fixed to 4.0, and the difference in the specific refractive index of the inner clad with respect to the outer clad. Δ^-Is a graph showing the relationship between the core diameter (outer diameter of the core region) 2a and the dispersion at a wavelength of 1550 nm when is fixed at -0.20%.

FIG. 5 shows that in the optical fiber according to the first embodiment, the ratio (2b / 2a) of the outer diameter 2b of the inner clad to the outer diameter 2a of the core region is fixed to 4.0, and the difference in the specific refractive index of the inner clad with respect to the outer clad. Δ^-Is a graph showing the relationship between the core diameter (outer diameter of the core region) 2a and the dispersion at a wavelength of 1550 nm when is fixed at -0.45%.

FIG. 6 (a) is a diagram showing a refractive index profile of an application example of an optical fiber according to the first embodiment, and FIG. 6 (b) is a refraction of another application example of the optical fiber according to the first embodiment. It is a figure which shows the rate profile.

FIG. 7 is a graph showing experimental results for explaining the fracture prevention effect of carbon coating.

FIG. 8A is a diagram showing a cross-sectional structure of a second embodiment of an optical fiber according to the present invention, and FIG. 8B is a diagram showing a refractive index profile of the optical fiber shown in FIG. 8A. Is.

FIG. 9 is a graph showing the relationship between the core diameter (outer diameter of the core region) 2a and the dispersion at a wavelength of 1550 nm in the optical fiber according to the second embodiment.