JP5522696B2 - 4-core single-mode optical fiber and optical cable - Google Patents

Description

  The present invention relates to a single mode optical fiber and an optical cable for single mode optical communication.

  With the rapid spread of data communication, the demand for further expansion of transmission capacity is increasing year by year. For this reason, studies have been actively conducted to increase the transmission capacity per optical fiber by using various multiplexing techniques. In addition, an increase in optical communication facilities in an access network, a data center, etc. is also a problem, and the importance of a technique for efficiently processing a large amount of optical fibers is increasing.

  Based on such a background, a multi-core optical fiber technology has been proposed in which a plurality of cores are arranged in the same cladding cross section to improve the spatial multiplexing efficiency per optical fiber (Non-Patent Documents 1 and 2). reference).

  However, in the above-described multi-core optical fiber technology, degradation of transmission quality due to accumulation of crosstalk between adjacent cores becomes a problem. For this reason, Non-Patent Document 1 improves the crosstalk characteristics by arranging a large number of holes to form a core part, and Non-Patent Document 2 improves the crosstalk characteristics by multilayering the refractive index distribution of each core. Techniques to do this are disclosed. However, these prior arts have a problem that the cross-sectional structure of the optical fiber or the refractive index distribution tends to be complicated, and the manufacturing difficulty increases.

  The present invention has been made in view of the background as described above, and its object is to suppress the increase in the number of vacancies in the cross section of the optical fiber and the multilayered refractive index distribution of the core while adjacent to each other. An object of the present invention is to provide a four-core single-mode optical fiber and an optical cable that can improve crosstalk characteristics between cores.

  In the four-core single-mode optical fiber of the present invention, four core parts are arranged in a square lattice shape or a straight line shape in the same cladding section, and in the center between adjacent core sections in the cladding section. By arranging a low refractive index region having an appropriate cross-sectional area and refractive index, the above-described problem is solved.

  According to the four-core single mode optical fiber of the present invention, since the low refractive index region having an appropriate cross-sectional area and refractive index is disposed between the adjacent core portions, the crosstalk characteristic between the adjacent cores is 50% to 1%. There is an effect that it can be reduced to below 10%.

  In addition, the above-described crosstalk characteristics can be improved without increasing the number of holes in the cross section of the optical fiber and making the refractive index distribution of the core multi-layered, so that the productivity of the optical fiber can be dramatically improved. There are also effects such as.

  In addition, since the crosstalk characteristics between adjacent cores can be improved, the multi-efficiency of cores within a limited cross section can be improved.

  Further, in the technology for reducing crosstalk in the four-core single mode optical fiber of the present invention, the ratio of the low refractive index region to the diameter of the core portion is set so that the reduction of the effective cross-sectional area of each core portion is less than 1%. Since the setting is made, there is an effect that it is possible to avoid a decrease in light intensity that can propagate through each core part and a deterioration in connection characteristics between optical fibers.

  Furthermore, the technique for reducing crosstalk in the four-core single-mode optical fiber of the present invention can be applied so as not to cause asymmetry of the electric field distribution in the cross section with respect to the center of each core. Specifically, the symmetry of the electric field distribution of each core with respect to the core center is maintained by arranging three or four low refractive index regions at equal intervals with respect to each core center. Therefore, there is an effect of reducing the accumulation of polarization mode dispersion, which is a problem in high-speed and long-distance transmission.

  In addition, according to the four-core single mode optical fiber of the present invention, the structural conditions of the inter-core distance Λ and the minimum cladding thickness r are derived from the relationship between the normalized frequency V of the core and the mode field diameter. The present invention can also be applied to a four-core single-mode optical fiber using a core having a refractive index distribution.

It is a conceptual diagram which shows the cross-section of the 4-core single mode optical fiber of this invention. It is drawing which shows the dependence with respect to the normalization refractive index of the low refractive index area | region of the standardization power coupling coefficient in the 4-core single mode optical fiber of this invention. It is drawing which shows the dependence with respect to the normalized diameter of the low-refractive-index area | region of the normalized power coupling coefficient in the 4-core single mode optical fiber of this invention as a function of the normalized refractive index. 6 is a drawing describing the minimum cladding thickness r in the four-core single-mode optical fiber of the present invention as a function of the mode field diameter and the normalized frequency V at a wavelength of 1310 nm. 6 is a drawing describing the minimum cladding thickness r in the four-core single mode optical fiber of the present invention as a function of the mode field diameter and the normalized frequency V at a wavelength of 1550 nm. In the four-core single-mode optical fiber of the present invention, the minimum inter-core distance Λ when the normalized refractive index in the low refractive index region is 0.995 is described as a function of the mode field diameter and the normalized frequency V at a wavelength of 1310 nm. It is a drawing. In the four-core single-mode optical fiber of the present invention, the minimum inter-core distance Λ when the normalized refractive index in the low refractive index region is 0.995 is described as a function of the mode field diameter and the normalized frequency V at a wavelength of 1550 nm. It is a drawing. In the four-core single-mode optical fiber of the present invention, the minimum inter-core distance Λ when the normalized refractive index in the low refractive index region is 0.692 is described as a function of the mode field diameter and the normalized frequency V at a wavelength of 1310 nm. It is a drawing. In the four-core single-mode optical fiber of the present invention, the minimum inter-core distance Λ when the normalized refractive index in the low refractive index region is 0.692 is described as a function of the mode field diameter and the normalized frequency V at a wavelength of 1550 nm. It is a drawing. It is drawing which shows the dependence with respect to the normalized refractive index of the relative change of an effective area in the 4 core single mode optical fiber of this invention as a function of the normalized diameter of a low refractive index area | region. In the 4-core single mode optical fiber of this invention, it is a conceptual diagram which shows the cross-sectional structure at the time of setting the low refractive index area | region arrange | positioned at the outer periphery of each core part to three pieces. In the 4-core single mode optical fiber of this invention, it is a conceptual diagram which shows the cross-sectional structure at the time of using four low-refractive-index area | regions arrange | positioned on the outer periphery of each core part. In the 4-core single mode optical fiber of this invention, it is a conceptual diagram which shows another cross-sectional structure at the time of making the low-refractive-index area | region arrange | positioned at the outer periphery of each core part into four pieces. In the 4-core single mode optical fiber of this invention, it is a conceptual diagram which shows the cross-sectional structure at the time of making a cladding structure into a non-circular rectangular structure.

  Embodiments of a four-core single mode optical fiber and an optical cable according to the present invention will be described below with reference to the drawings.

FIG. 1 is a conceptual diagram showing a cross-sectional structure of a four-core single mode optical fiber of the present invention. The four-core single-mode optical fiber of the present invention is adjacent to a cladding portion 1 having a refractive index of n _clad and a diameter of D, and four core portions 2 having a refractive index larger than the refractive index n _clad. It has three or four low refractive index regions 3 having a refractive index n _pit and a diameter d arranged in the center of the core portion 2.

Here, each of the core portions 2 has a square lattice shape (FIG. 1 (a)) or a straight line shape (FIG. 1 (b)) at equal intervals so that the distance between the core centers is Λ in the cross section of the cladding portion 1. The minimum distance from the center of the core portion 2 to the outer periphery of the cladding portion 1 is defined as the minimum cladding thickness r. Further, the refractive index n _clad of the cladding part 1 is larger than the refractive index n _pit of the low refractive index region 3.

  In the embodiment of the present invention, the number of cores in the same clad is limited to four. However, the number of cores included in the same clad includes the inter-core distance Λ, the minimum clad thickness r, and Any number can be set as long as the three structural conditions of the normalized frequency V of the core are satisfied.

  Further, FIG. 1 shows a case where the low refractive index region 3 is circular as an example, but the low refractive index region 3 only needs to satisfy the cross-sectional area condition described later, such as an ellipse, a square, a rectangle, etc. Any shape can be set. In the following embodiments, the core portion 2 of the four-core single-mode optical fiber of the present invention has a step type refractive index distribution having a diameter 2a and a relative refractive index difference with respect to the cladding portion 1 of Δ. Assume that

FIG. 2 shows a four-core single-mode optical fiber according to the present invention, in which the radius a of the core portion 2 is 4 μm, the relative refractive index difference Δ of the core portion 2 with respect to the cladding portion 1 is 0.32%, the inter-core distance Λ is 30 μm, and low refraction. 6 is a _diagram showing the dependence of the normalized power coupling coefficient at a wavelength of 1550 nm on the normalized refractive index n _pit / n _clad of the low refractive index region 3 when the diameter of the refractive index region 3 is d = a. Here, the power coupling coefficient is a coefficient representing the degree of power coupling between adjacent cores, and the vertical axis in FIG. 2 indicates a numerical value normalized by a coefficient when the low refractive index region 3 does not exist.

  FIG. 2 shows that the normalized power coupling coefficient can be reduced to 0.5 or less by setting the normalized refractive index to 0.995 or less. When the low refractive index region 3 is air, the normalized refractive index is about 0.692. Therefore, it can be seen from FIG. 2 that the normalized power coupling coefficient can be reduced to about 12%.

  FIG. 3 is a diagram showing the dependence of the normalized power coupling coefficient on the normalized diameter d / 2a of the low refractive index region 3 as a function of the normalized refractive index in the four-core single mode optical fiber of the present invention. . The normalized power coupling coefficient on the vertical axis indicates the calculation result at a wavelength of 1550 nm. The radius a of the core part 2, the relative refractive index difference Δ of the core part 2, and the inter-core distance Λ are 4 μm as in FIG. , 0.32%, and 30 μm. The solid line and the alternate long and short dash line in the figure indicate the results when the normalized refractive index of the low refractive index region 3 is 0.995 and 0.692, respectively.

  FIG. 3 shows that when the normalized refractive index is 0.995, the normalized power coupling coefficient can be reduced to 0.5 or less by setting the normalized diameter to 0.60 or more. Similarly, when the normalized refractive index is 0.692, the normalized power coupling coefficient can be reduced to 0.5 or less by setting the normalized diameter to 0.12 or more, and by setting the normalized diameter to 0.54 or more, It can be seen that the normalized power coupling coefficient can be reduced to 0.1 or less.

  Here, the reduction of the power coupling coefficient is caused by the low refractive index region 3 suppressing the spread of the electric field distribution of each core part 2 and reducing the amount of overlap of the electric field distribution between adjacent core parts. The effect of suppressing the spread of the electric field distribution is substantially proportional to a coefficient obtained by integrating the amount of change in the refractive index with respect to the cladding portion 1 in the low refractive index region 3 with respect to the entire low refractive index region 3. Therefore, in order to obtain the effect of reducing the power coupling coefficient shown in FIG. 2 and FIG. 3, it is only necessary to apply an equivalent refractive index change over an equivalent area. For this reason, the low refractive index region 3 in the four-core single-mode optical fiber of the present invention can take any shape other than a circle.

  FIG. 4 shows, as a function of the mode field diameter and the normalized frequency V, at a wavelength of 1310 nm, the minimum cladding thickness r for which the confinement loss at a wavelength of 1625 nm is 0.001 dB / km or less in the four-core single mode optical fiber of the present invention. The four dash-dot lines in the figure show the results when the minimum cladding thickness r is 36 to 48 μm.

The normalized frequency V at the wavelength λ is obtained by using the core radius a and the relative refractive index difference Δ,
V≡ (2πa / λ) n _core (2Δ) ^1/2 (1)
Defined by Here, n _core is the refractive index of the core 2. A confinement loss of 0.001 dB / km corresponds to a cumulative amount of confinement loss after 100 km transmission being 0.1 dB or less.

  The solid line in FIG. 4 shows the condition that the bending loss at a wavelength of 1625 nm and a bending radius of 30 mm is 0.1 dB / 100 turns, and a bending loss characteristic of 0.1 dB / 100 turns or less can be realized in the region on the right side of the solid line. Also, the broken line in FIG. 4 shows the condition that the effective cutoff wavelength is 1260 nm, and an effective cutoff wavelength of 1260 nm or less can be realized in the region on the left side of the broken line. These bending loss characteristics and effective cut-off wavelength characteristics are equal to the values recommended in Non-Patent Document 3 as standard values for conventional general-purpose single-mode optical fibers.

  According to Non-Patent Document 3, the minimum value of the mode field diameter at a wavelength of 1310 nm of a conventional general-purpose single mode optical fiber is recommended as 8 μm. As shown in FIG. 4, by setting the normalized frequency V at a wavelength of 1310 nm in the range of 2.00 to 2.73 and the minimum cladding thickness r in the range of 36 to 44 μm, the bending loss characteristics and effectiveness equivalent to those of a conventional general-purpose single mode optical fiber are obtained. It can be seen that a cut-off wavelength characteristic can be realized and a mode field diameter of 8.0 to 11.0 μm at a wavelength of 1310 nm can be realized.

  FIG. 5 shows the minimum cladding thickness r with a confinement loss at a wavelength of 1625 nm of 0.001 dB / km or less as a function of the mode field diameter and the normalized frequency V at a wavelength of 1550 nm in the four-core single mode optical fiber of the present invention. Three dash-dot lines in the figure show the results when the minimum cladding thickness r is 32 to 40 μm. In addition, the solid line and the broken line in the figure indicate the conditions of the bending loss and the effective cutoff wavelength, respectively, as in FIG. However, in FIG. 5, the effective cut-off wavelength was considered as 1500 nm.

  From FIG. 5, by setting the normalized frequency V at a wavelength of 1550 nm in the range of 2.00 to 2.66 and the minimum cladding thickness r in the range of 33 to 40 μm, the bending loss characteristic equivalent to that of the conventional general-purpose single mode optical fiber can be obtained. It can be seen that an effective cutoff wavelength characteristic of 1500 nm or less and a mode field diameter of 9.0 to 11.0 μm at a wavelength of 1550 nm can be realized.

  In FIG. 6, in the four-core single-mode optical fiber of the present invention, the minimum inter-core distance Λ when the normalized refractive index of the low refractive index region 3 is 0.995 is the mode field diameter and the normalized frequency V at a wavelength of 1310 nm. Shown as a function. The six dot-dash lines in the figure show the results when the minimum inter-core distance Λ is 42 to 62 μm. The standardized diameter of the low refractive index region 3 is 0.6, and in this embodiment, the minimum inter-core distance Λ is derived as a condition that the crosstalk after transmission of 100 km is −30 dB. The solid line and the broken line in the figure are the bending loss characteristic and the effective cutoff wavelength characteristic, respectively, and indicate the same characteristic conditions as in FIG.

  From FIG. 6, by setting the normalized frequency V at a wavelength of 1310 nm in the range of 2.00 to 2.73 and the minimum inter-core distance Λ in the range of 43 to 58 μm, the bending loss characteristics equivalent to those of conventional general-purpose single-mode optical fibers are obtained. It can also be seen that an effective cut-off wavelength characteristic can be realized and a mode field diameter of 8.0 to 11.0 μm can be realized at a wavelength of 1310 nm.

  Therefore, from FIG. 4 and FIG. 6, the normalized frequency V at 2.00 to 2.73, the minimum cladding thickness r is 36 to 44 μm, the minimum inter-core distance Λ is 43 to 58 μm, and the normalized refractive index of the low refractive index region 3 is Is set to 0.995 or less, and the standardized diameter of the low refractive index region 3 is set to a range of 0.6 or more, so that the bending loss characteristic and the effective cutoff wavelength characteristic equivalent to those of the conventional general-purpose single mode optical fiber are realized, and the wavelength is 1310 nm. It can be seen that a four-core single-mode optical fiber with a mode field diameter of 8.0 to 11.0 μm can be realized. Further, the cladding outer diameter D at this time is 133 to 170 μm when a square lattice arrangement is used, and 201 to 262 μm when a linear arrangement is used.

  In the four-core single-mode optical fiber of the present invention shown in FIG. 7, the minimum inter-core distance Λ when the normalized refractive index of the low refractive index region 3 is 0.995 is the mode field diameter and the normalized frequency V at a wavelength of 1550 nm. Shown as a function. The five dash-dot lines in the figure show the results when the minimum inter-core distance Λ is 36 to 52 μm. The standardized diameter of the low refractive index region 3 is 0.6, and in this embodiment, the minimum inter-core distance Λ is derived as a condition that the crosstalk after transmission of 100 km is −30 dB. The solid line and the broken line in the figure are the bending loss characteristic and the effective cutoff wavelength characteristic, respectively, and indicate the same characteristic conditions as in FIG.

  From Fig. 7, the bending loss characteristics equivalent to those of conventional general-purpose single-mode optical fibers are obtained by setting the normalized frequency V at a wavelength of 1550 nm to a range of 2.00 to 2.66 and the minimum inter-core distance Λ to a range of 38 to 52 μm. And an effective cutoff wavelength characteristic of a wavelength of 1500 nm or less and a mode field diameter of 9.0 to 11.0 μm at a wavelength of 1550 nm.

  Therefore, from FIG. 5 and FIG. 7, the normalized frequency V at a wavelength of 1550 nm is 2.00 to 2.66, the minimum cladding thickness r is 33 to 40 μm, the minimum inter-core distance Λ is 38 to 52 μm, and the normalized refractive index of the low refractive index region 3 Is set to 0.995 or less, and the standardized diameter of the low refractive index region 3 is set to a range of 0.6 or more. This realizes bending loss characteristics equivalent to conventional general-purpose single-mode optical fibers and an effective cutoff wavelength of 1500 nm or less. It can be seen that characteristics and a mode field diameter of 9.0 to 11.0 μm at a wavelength of 1550 nm can be realized. The cladding outer diameter D at this time is 120 to 154 μm when a square lattice arrangement is used, and 180 to 236 μm when a linear arrangement is used.

  In the four-core single-mode optical fiber of the present invention shown in FIG. 8, the minimum inter-core distance Λ when the normalized refractive index of the low refractive index region 3 is 0.692 is expressed as the mode field diameter and the normalized frequency V at a wavelength of 1310 nm. Shown as a function. The five dash-dot lines in the figure show the results when the minimum inter-core distance Λ is 40 to 56 μm. The standardized diameter of the low refractive index region 3 is 0.55, and in this embodiment, the minimum inter-core distance Λ is derived as a condition that the crosstalk after transmission of 100 km is −30 dB. The solid line and the broken line in the figure are the bending loss characteristic and the effective cutoff wavelength characteristic, respectively, and indicate the same characteristic conditions as in FIG.

  From Fig. 8, the bending loss characteristics equivalent to those of conventional general-purpose single-mode optical fibers can be obtained by setting the normalized frequency V at a wavelength of 1310 nm in the range of 2.00 to 2.73 and the minimum inter-core distance Λ in the range of 41 to 56 µm. It can also be seen that an effective cut-off wavelength characteristic can be realized and a mode field diameter of 8.0 to 11.0 μm can be realized at a wavelength of 1310 nm.

  Therefore, from FIG. 4 and FIG. 8, the normalized frequency V at 2.00 to 2.73, the minimum cladding thickness r is 36 to 44 μm, the minimum inter-core distance Λ is 41 to 56 μm, and the normalized refractive index of the low refractive index region 3 is Is set to 0.692 or less, and the standardized diameter of the low refractive index region 3 is set to a range of 0.55 or more. This realizes bending loss characteristics and effective cutoff wavelength characteristics equivalent to those of conventional general-purpose single-mode optical fibers, and a wavelength of 1310 nm. It can be seen that a four-core single-mode optical fiber with a mode field diameter of 8.0 to 11.0 μm can be realized. Further, the cladding outer diameter D at this time is 130 to 168 μm when a square lattice arrangement is used, and 195 to 256 μm when a linear arrangement is used.

  In FIG. 9, in the four-core single-mode optical fiber of the present invention, the minimum inter-core distance Λ when the normalized refractive index of the low refractive index region 3 is 0.692 is expressed as the mode field diameter and the normalized frequency V at a wavelength of 1550 nm. Shown as a function. The four dash-dot lines in the figure show the results when the minimum inter-core distance Λ is 36 to 48 μm. The standardized diameter of the low refractive index region 3 is 0.55, and in this embodiment, the minimum inter-core distance Λ is derived as a condition that the crosstalk after transmission of 100 km is −30 dB. The solid line and the broken line in the figure are the bending loss characteristic and the effective cutoff wavelength characteristic, respectively, and indicate the same characteristic conditions as in FIG.

  From Fig. 9, by setting the normalized frequency V at a wavelength of 1550 nm in the range of 2.00 to 2.66 and the minimum inter-core distance in the range of 36 to 48 μm, the bending loss characteristics equivalent to those of conventional general-purpose single mode optical fibers can be obtained. It can be seen that an effective cutoff wavelength characteristic of 1500 nm or less and a mode field diameter of 9.0 to 11.0 μm at a wavelength of 1550 nm can be realized.

  Therefore, from FIG. 5 and FIG. 9, the normalized frequency V at a wavelength of 1550 nm is 2.00 to 2.66, the minimum cladding thickness r is 33 to 40 μm, the minimum inter-core distance Λ is 36 to 48 μm, and the normalized refractive index of the low refractive index region 3 Is set to 0.692 or less, and the standardized diameter of the low refractive index region 3 is set to a range of 0.55 or more. This realizes bending loss characteristics equivalent to those of conventional general-purpose single-mode optical fibers and effective cutoff wavelength characteristics of 1500 nm or less. And a mode field diameter of 9.0 to 11.0 μm at a wavelength of 1550 nm can be realized. Further, the cladding outer diameter D at this time is 117 to 148 μm when a square lattice array is used, and 174 to 224 μm when a linear array is used.

  FIG. 10 shows the dependence of the relative change of the effective area on the normalized refractive index in the four-core single mode optical fiber of the present invention as a function of the normalized diameter of the low refractive index region 3. The calculated wavelength was 1550 nm, and the relative change in the vertical axis was calculated based on the effective area when the low refractive index region 3 did not exist.

  From FIG. 10, it can be seen that the amount of reduction in the effective area due to the provision of the low refractive index region 3 is less than 1%, which is a sufficiently small value. In general, it is considered that the light intensity capable of propagating in an optical fiber increases in proportion to the effective area of the optical fiber. Further, it is known that the connection loss of the optical fiber is mainly caused by the amount of axial deviation of the central axis of the optical fiber and the mismatch of the mode field diameter. In the four-core single mode optical fiber of the present invention, the reduction in effective area and mode field diameter accompanying the provision of the low refractive index region 3 is almost negligible. It is considered that the decrease in light intensity and the deterioration in connection loss are almost negligible.

  FIG. 11 shows a conceptual diagram of a cross-sectional structure when the number of low refractive index regions 3 arranged on the outer periphery of each core portion 2 is three in the four-core single mode optical fiber of the present invention. FIG. 13 shows a cross-sectional structure of the four-core single-mode optical fiber of the present invention when the number of low refractive index regions 3 arranged on the outer periphery of each core portion 2 is four.

  In long-distance / high-speed optical transmission, degradation of transmission characteristics due to accumulation of polarization mode dispersion becomes a problem. In general, in an optical fiber having symmetry with respect to the central axis, an increase in polarization mode dispersion due to an increase in stress strain in the cross section of the optical fiber can be reduced. Therefore, as shown in FIGS. 11 to 13, in the four-core single mode optical fiber of the present invention, the low refractive index regions 3 arranged on the outer periphery of each core portion 2 are equidistantly spaced from the core center. By arranging three or four in the optical fiber, it is possible to realize a four-core single mode optical fiber that suppresses an increase in polarization mode dispersion.

  FIG. 14 shows a cross-sectional structure of the four-core single-mode optical fiber of the present invention when the cladding structure is a non-circular rectangular structure. In the above-described embodiments, the clad portion 1 has a circular shape with a diameter D. However, the four-core single-mode optical fiber of the present embodiment has the normalized frequency V and the inter-core distance of the core portion 2 described above. This can be realized if all structural conditions of Λ and the minimum cladding thickness r are satisfied.

  Therefore, as shown in FIG. 14, for example, in the rectangular clad part 1, four core parts 2 are linearly arranged on the clad part 1, and the rectangular major axis becomes the clad diameter D, so that the rectangular If the minor axis is set to be twice or more the minimum cladding thickness r, it is possible to obtain the same operation and effect as in the previous embodiments.

Further, according to Non-Patent Document 4, the normalized frequency V and mode field diameter 2W of a step-type core having a core diameter 2a and a core relative refractive index difference Δ are:
W / a = 0.65 + 1.619V ^-1.5 + 2.879V ^-6 (2)
It is disclosed that it can be described by.

Further, according to Non-Patent Document 5, the standardized frequency V in the above-described step-type refractive index distribution is the extended standardized frequency T for an arbitrary refractive index distribution,
T ² = 2 (2π / λ) ² ∫ {n ² (r) −n ² (∞)} rdr (3)
It is disclosed that it can be rewritten.

  Here, n (r) represents the refractive index at the point of radius r, and n (∞) represents the refractive index of the cladding. Accordingly, the relationship between the mode field diameter and the minimum core distance Λ with respect to the normalized frequency V and the relationship between the mode field diameter and the normalized frequency V with respect to the minimum cladding thickness r shown in the embodiment of the present invention are arbitrary refractions. The present invention can also be applied to the core portion 2 having a rate distribution.

  As described above, according to the four-core single-mode optical fiber of the present invention, a long-distance / high-speed light can be obtained by arranging a low refractive index region having an appropriate cross-sectional area and refractive index between adjacent cores. It is possible to reduce the crosstalk characteristics between adjacent cores to 50% to 10% or less while maintaining transmission characteristics and connection characteristics suitable for transmission.

  1: Clad portion, 2: Core portion, 3: Low refractive index region.

"Multi-core holey fibers for the long-distance (> 100km) ultra large capacity transmission", K. Imamura, et al., In Proc. OFC'09, OTuC3 (2009). "Length Dependence of Cutoff Wavelength of Trench-Assisted Multi-Core Fiber", Y. Arakawa, et al., In Proc. OECC'11, 6C2-5 (2011). "Characteristics of a single-mode optical fiber and cable", ITU-T, Recommendation G.652. "Loss analysis of single-mode fiber splices", D. Marcuse, Bell Sys. Tech. J, vol. 56, no. 5, p. 703 (1976). "Optical fibers and fiber-type devices", Kawakami, et al., Baifukan, (1996).

Claims (6)

A clad portion having a refractive index of n _clad and a diameter of D, four core portions having a normalized frequency of V and a diameter of 2a, and a low refractive index region having a refractive index of n _pit and a diameter of d. Have
The core portions are arranged in a square lattice shape or a straight line shape so that the inter-core distances Λ are equally spaced in the cross section of the clad portion, and the low refractive index regions are adjacent to the cores in the cross section of the clad portion. Place in the center between the parts,
The normalized frequency V at a wavelength of 1310 nm is in the range of 2.00 to 2.73, the minimum cladding thickness r from the center of the core to the outer periphery of the cladding is in the range of 36 to 44 μm, and the inter-core distance Λ is in the range of 43 to 58 μm. The ratio n _pit / n _clad of the refractive index n _{pit to} the refractive index n _{clad is in} the range of 0.995 or less, the ratio d / 2a of the diameter d to the diameter 2a is in the range of 0.6 or more, and the diameter D is 133 to 262 μm. set in the range of, respectively,
Bending loss characteristics of 0.1 dB / 100 or less at a wavelength of 1625 nm, a bending radius of 30 mm, an effective cutoff wavelength characteristic of 1260 nm or less, and a mode field diameter characteristic of 8.0 to 11.0 μm at a wavelength of 1310 nm,
And the change in effective area at a wavelength of 1550 nm due to the presence or absence of the low refractive index region is less than 1%,
Further , a four-core single mode optical fiber characterized in that the confinement loss at a wavelength of 1625 nm is 0.001 dB / km or less and the crosstalk after 100 km transmission at a wavelength of 1550 nm is -30 dB or less . A clad portion having a refractive index of n _clad and a diameter of D, four core portions having a normalized frequency of V and a diameter of 2a, and a low refractive index region having a refractive index of n _pit and a diameter of d. Have
The core portions are arranged in a square lattice shape or a straight line shape so that the inter-core distances Λ are equally spaced in the cross section of the clad portion, and the low refractive index regions are adjacent to the cores in the cross section of the clad portion. Place in the center between the parts,
The normalized frequency V at a wavelength of 1550 nm is in the range of 2.00 to 2.66, the minimum cladding thickness r from the center of the core to the outer periphery of the cladding is in the range of 33 to 40 μm, and the inter-core distance Λ is in the range of 38 to 52 μm. The ratio n _pit / n _clad of the refractive index n _{pit to} the refractive index n _{clad is in} the range of 0.995 or less, the ratio d / 2a of the diameter d to the diameter 2a is in the range of 0.6 or more, and the diameter D is 120 to 236 μm. set in the range of, respectively,
Bending loss characteristics of 0.1 dB / 100 or less at a wavelength of 1625 nm, a bending radius of 30 mm, an effective cutoff wavelength characteristic of 1500 nm or less, and a mode field diameter characteristic of 9.0 to 11.0 μm at a wavelength of 1550 nm,
And the change in effective area at a wavelength of 1550 nm due to the presence or absence of the low refractive index region is less than 1%,
Further , a four-core single mode optical fiber characterized in that the confinement loss at a wavelength of 1625 nm is 0.001 dB / km or less and the crosstalk after 100 km transmission at a wavelength of 1550 nm is -30 dB or less . A clad portion having a refractive index of n _clad and a diameter of D, four core portions having a normalized frequency of V and a diameter of 2a, and a low refractive index region having a refractive index of n _pit and a diameter of d. Have
The core portions are arranged in a square lattice shape or a straight line shape so that the inter-core distances Λ are equally spaced in the cross section of the clad portion, and the low refractive index regions are adjacent to the cores in the cross section of the clad portion. Place in the center between the parts,
The normalized frequency V at a wavelength of 1310 nm is in the range of 2.00 to 2.73, the minimum cladding thickness r from the center of the core to the outer periphery of the cladding is in the range of 36 to 44 μm, and the inter-core distance Λ is in the range of 41 to 56 μm. The ratio n _pit / n _clad of the refractive index n _{pit to} the refractive index n _{clad is in} the range of 0.692 or less, the ratio d / 2a of the diameter d to the diameter 2a is in the range of 0.55 or more, and the diameter D is 130 to 256 μm. set in the range of, respectively,
Bending loss characteristics of 0.1 dB / 100 or less at a wavelength of 1625 nm, a bending radius of 30 mm, an effective cutoff wavelength characteristic of 1260 nm or less, and a mode field diameter characteristic of 8.0 to 11.0 μm at a wavelength of 1310 nm,
And the change in effective area at a wavelength of 1550 nm due to the presence or absence of the low refractive index region is less than 1%,
Further , a four-core single mode optical fiber characterized in that the confinement loss at a wavelength of 1625 nm is 0.001 dB / km or less and the crosstalk after 100 km transmission at a wavelength of 1550 nm is -30 dB or less . A clad portion having a refractive index of n _clad and a diameter of D, four core portions having a normalized frequency of V and a diameter of 2a, and a low refractive index region having a refractive index of n _pit and a diameter of d. Have
The core portions are arranged in a square lattice shape or a straight line shape so that the inter-core distances Λ are equally spaced in the cross section of the clad portion, and the low refractive index regions are adjacent to the cores in the cross section of the clad portion. Place in the center between the parts,
The normalized frequency V at a wavelength of 1550 nm is in the range of 2.00 to 2.66, the minimum cladding thickness r from the center of the core to the outer periphery of the cladding is in the range of 33 to 40 μm, and the inter-core distance Λ is in the range of 36 to 48 μm. The ratio n _pit / n _clad of the refractive index n _{pit to} the refractive index n _{clad is in} the range of 0.692 or less, the ratio d / 2a of the diameter d to the diameter 2a is in the range of 0.55 or more, and the diameter D is 117 to 224 μm. set in the range of, respectively,
Bending loss characteristics of 0.1 dB / 100 or less at a wavelength of 1625 nm, a bending radius of 30 mm, an effective cutoff wavelength characteristic of 1500 nm or less, and a mode field diameter characteristic of 9.0 to 11.0 μm at a wavelength of 1550 nm,
And the change in effective area at a wavelength of 1550 nm due to the presence or absence of the low refractive index region is less than 1%,
Further , a four-core single mode optical fiber characterized in that the confinement loss at a wavelength of 1625 nm is 0.001 dB / km or less and the crosstalk after 100 km transmission at a wavelength of 1550 nm is -30 dB or less . The low refractive index region, the 4-core single-mode optical according to any one of claims 1 to 4, characterized in three or be four equally spaced concentric position of each of the core portions fiber. An optical cable using at least one four-core single-mode optical fiber according to any one of claims 1 to 5 .

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