Multicore optical fiber

Abstract

This multicore optical fiber comprises n (n is an integer of 2 or more) cores 11, and a cladding region 22 surrounding each of the n cores and having a refractive index lower than that of the cores 11. A relative refractive index difference Δ between the cores and a cladding and a core spacing Λ are determined such that a space utilization efficiency SEMCF is greater than or equal to n times a space utilization efficiency SEref of a single core optical fiber serving as a reference.

Description

Multi-core optical fiber

[0001] The present disclosure relates to multi-core optical fibers.

[0002] Multi-core optical fibers (MCFs) with multiple core regions are being actively investigated to dramatically increase transmission capacity using space division multiplexing technology. In particular, MCFs with a standard cladding diameter (125±1μm diameter of the cladding region) are attracting attention due to their manufacturability and compatibility with existing standard technologies.

[0003] Non-Patent Document 1 discloses a two-core MCF that maintains conventional low loss while exhibiting low crosstalk characteristics suitable for long-distance submarine systems. Patent Document 1 discloses a four-core MCF that exhibits low crosstalk characteristics suitable for long-distance submarine systems. Patent Document 2 discloses a four-core MCF in which each core has a step-index structure, taking into consideration ease of optical fiber manufacture and mass productivity. All of the MCFs in Non-Patent Document 1, Patent Documents 1 and 2 employ standard cladding diameters.

[0004] Patent No. 6560806 Patent No. 7172634

[0005] Y. Tamura, T. Hayashi, T. Nakanishi, and T. Hasegawa, "Low-Loss Uncoupled Two-Core Fiber for Power Efficient Practical Submarine Transmission," 2019 Optical Fiber Communications Conference and Exhibition (OFC), 2019, M1E.5, DOI: 10.1364 / OFC.2019.M1E.5J. M. Gene and P. J. Winzer, "A Universal Specification for Multicore Fiber Crosstalk," IEEE Photonics Technology Letters, vol. 31, no. 9, pp. 673-676, 2019, DOI: 10.1109 / LPT.2019.2903717A. Carena, V. Curri, G. Bosco, R. Cigliutti, E. Torrengo, P. Poggiolini, A. Nespola, D. Zeolla, and F. Forghieri, "Novel figure of merit to compare fibers in coherent detection systems with uncompensated links," Optics Express vol. 20, no. 1, 2012, pp. 339-346, DOI: 10.1364 / OE.20.000339V. Curri A. Carena, G. Bosco, P. Poggiolini, M. Hirano, Y. Yamamoto, and F.Forghieri, "Fiber figure of merit based on maximum reach," 2013 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference (OFC / NFOEC), Anaheim, CA, USA, 2013, OTh3G.2, DOI: 10.1364 / OFC.2013.OTh3G.2.

[0006] There is a trade-off between improving the optical properties of the core and reducing the amount of crosstalk, and there has been an issue in that even if multi-core optical fibers are used, the transmission performance does not necessarily increase by several times compared to conventional optical fibers. For example, a two-core MCF has twice the optical path compared to a single-core optical fiber, so it is expected that the communication capacity per optical fiber will double, but it does not necessarily achieve double the transmission performance.

[0007] The present disclosure has been made in view of the above, and has an object to provide a multi-core optical fiber that can obtain desired transmission performance.

[0008] A multi-core optical fiber according to one aspect of the present disclosure comprises n (n is an integer of 2 or greater) cores and claddings that surround each of the n cores and have a lower refractive index than the cores, and the relative refractive index difference between the cores and the cladding and the core spacing are determined so that the spatial utilization efficiency is n times or more that of a standard single-core optical fiber.

[0009] According to the present disclosure, it is possible to provide a multi-core optical fiber that can achieve desired transmission performance.

[0010] FIG. 1 is a cross-sectional view showing an example of the configuration of a multi-core optical fiber. FIG. 2 is a view showing an example of a refractive index profile in a near-core region. FIG. 3 is a cross-sectional view showing an example of the configuration of a multi-core optical fiber. FIG. 4 is a view showing an example of a refractive index profile in a near-core region. FIG. 5 is a cross-sectional view showing an example of the configuration of a multi-core optical fiber. FIG. 6 is a view showing the relationship between the ratio of effective areas and the figure of merit. FIG. 7 is a view showing the upper limit of crosstalk at which the figure of merit becomes a multiple of the number of cores. FIG. 8 is a view showing an example of crosstalk due to differences in the propagation direction of light. FIG. 9 is a view showing core spacing. FIG. 10 is a view showing structural conditions necessary to increase the figure of merit by two or more times in a two-core multi-core optical fiber. FIG. 11 is a view showing structural conditions necessary to increase the figure of merit by four or more times in a four-core multi-core optical fiber. FIG. 12 is a view showing structural conditions necessary to increase the figure of merit by two or more times in a two-core multi-core optical fiber. FIG. 13 is a view showing structural conditions necessary to increase the figure of merit by four or more times in a four-core multi-core optical fiber. FIG. 14 is a view showing the relationship between the core spacing and the increase in splice loss due to rotational misalignment.

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0012] [Configuration of multi-core optical fiber] An example of the configuration of the MCF of this embodiment will be described with reference to the cross-sectional view of Fig. 1. The left side of Fig. 1 is a cross-sectional view of a two-core MCF, and the right side of Fig. 1 is a cross-sectional view of a four-core MCF.

[0013] The MCF shown in Fig. 1 comprises multiple cores 11 arranged along the longitudinal direction of the MCF (depth direction in the figure), cladding regions 22 surrounding each core 11, and cladding regions 21 surrounding all of the cladding regions 22. In other words, the MCF shown in Fig. 1 comprises multiple cores 11 within cladding region 21, each surrounded by cladding region 22. All of the MCFs shown in Fig. 1 have a cladding region 21 with a diameter of 125±1 µm, i.e., a standard cladding diameter.

[0014] Figure 2 shows the refractive index distribution in the region near each core 11. The horizontal axis represents the position along the cross-sectional direction of the region near the core, and the vertical axis represents the relative refractive index difference. The radius of the core 11 is a, and the radius of the cladding region 22, which has the lowest refractive index, is a1.

[0015] The core 11 has the highest refractive index, and the cladding region 22 surrounding the core 11 has a lower refractive index than the core 11. The cladding region 21 has a higher refractive index than the cladding region 22, but a lower refractive index than the core 11. The relative refractive index difference between the core 11 and the cladding region 22, which has the lowest refractive index, is designated as Δ. If the refractive index of the core 11 is taken as the reference (0), Δ takes a negative value. Each of the cores 11 has a W-shaped refractive index profile as shown in FIG. 2 or a refractive index profile equivalent thereto.

[0016] An example of the configuration of an MCF according to this embodiment will be described with reference to the cross-sectional view of Fig. 3. The left side of Fig. 3 is a cross-sectional view of a two-core MCF, and the right side of Fig. 3 is a cross-sectional view of a four-core MCF. The MCF in Fig. 3 differs from the MCF in Fig. 1 in that each core 11 is surrounded by cladding regions 23 and 22.

[0017] The MCF shown in Fig. 3 comprises a plurality of cores 11 arranged along the longitudinal direction of the MCF, cladding regions 23 and 22 surrounding each core 11, and cladding region 21 surrounding all of the cladding regions 22. In other words, the MCF shown in Fig. 3 comprises a plurality of cores 11 within cladding region 21, each surrounded by cladding regions 23 and 22. All of the MCFs shown in Fig. 3 have a cladding region 21 with a diameter of 125±1 µm, i.e., a standard cladding diameter.

[0018] Figure 4 shows the refractive index distribution in the region near each core 11. The horizontal axis represents the position along the cross-sectional direction of the region near the core, and the vertical axis represents the relative refractive index difference. The radius of the core 11 is a, and the radius of the cladding region 22 with the lowest refractive index is a1.

[0019] The core 11 has the highest refractive index, and the cladding region 23 surrounding the core 11 and the cladding region 21 have lower refractive indices than the core 11. The refractive indices of the cladding region 23 and the cladding region 21 may be the same or different. The refractive index of the cladding region 22 is lower than the cladding regions 23 and 21. The relative refractive index difference between the core 11 and the cladding region 22 with the lowest refractive index is designated as Δ. If the refractive index of the core 11 is taken as the reference (0), Δ takes a negative value. Each of the cores 11 has a trench-type refractive index profile as shown in FIG. 4 or a refractive index profile equivalent thereto.

[0020] FIG. 5 shows an example of a cross-sectional view of an MCF including a coating. Optical fibers typically have a resin coating 31 as a protective layer around the cladding region 21. As shown in FIG. 5( a), the diameter of the coating 31 for an optical fiber with a standard cladding diameter is 245±15 μm. As shown in FIG. 5( b), a thin coating 31 with a diameter of 200±20 μm can also be applied to an optical fiber with a standard cladding diameter. Since the MCF of this embodiment also has a standard cladding diameter, either type of coating 31 can be applied. A thin coating 31 reduces the physical space occupied by the optical fiber, enabling higher-density optical fiber packaging.

[0021] Although FIG. 5 shows a two-core MCF having a W-type refractive index profile, a four-core MCF and an MCF having a trench-type refractive index profile can also be used.

[0022] [Transmission Performance of Optical Fiber] Next, the transmission performance of the optical fiber will be described.

[0023] As shown in Non-Patent Document 2, the spatial efficiency SE, which indicates the transmission performance potential of MCF, is MCF is expressed by the following equation (1) taking into consideration the nonlinear effect, noise of the optical amplifier, and crosstalk (XT).

[0024]

[0025] where N is the number of spatial channels, η TRX is a constant that represents the degradation of transmission performance due to the transponder, N s is the number of spans, χ is a constant representing the performance degradation due to nonlinear effects, P ASEis the ASE noise power in the optical amplifier, and κ is the amount of XT per span.

[0026] As shown in Non-Patent Document 3, under ideal system conditions, that is, when the optical amplifier completely compensates for the span loss, P ASE is calculated by the following equation (2).

[0027]

[0028] As shown in Non-Patent Document 4, the performance degradation χ due to nonlinear effects in each span depends on the transmission characteristic parameters of the optical fiber, and is expressed as (L eff / γ)(α / D), where L eff is the nonlinear interaction length, γ is the nonlinear coefficient, α and D are the transmission loss and chromatic dispersion per km, all of which are parameters that depend on the structure and manufacturing conditions of the optical fiber. MCF is expressed by the following equation (3).

[0029]

[0030] where k0 and n2 are the wave number and nonlinear refractive index in a vacuum, respectively, and are constants determined by the wavelength and the substance and amount added to the glass. Aeff is the effective cross-sectional area.

[0031] The Figure of Merit (FOM), which represents the transmission performance of optical fiber, is the maximum space utilization efficiency SE obtained from Shannon's law above. MCF The spatial efficiency SE of a single-core optical fiber is ref That is, it can be defined by the following equation (4).

[0032]

[0033] If the FOM is more than the number of cores (two times for two cores, four times for four cores), the transmission performance of the MCF is considered sufficient compared to the reference single-core optical fiber. For example, if each core of a two-core MCF has the same optical properties as the reference single-core optical fiber, the transmission performance of the MCF can be fully demonstrated by designing the MCF so that the FOM is twice as large.

[0034] To obtain an accurate FOM, we evaluate the spatial efficiency of a single-core optical fiber and an MCF under the same transmission conditions. Specifically, we install a transmitter that transmits an optical signal at one end of the single-core optical fiber and a receiver that receives the optical signal at the other end, and propagate the optical signal through the single-core optical fiber to measure the spatial efficiency SE of the single-core optical fiber. ref The same transmitter and receiver are placed at both ends of the MCF core, and an optical signal is propagated through the MCF core to evaluate the spatial efficiency SE of the MCF. MCF The remaining cores are also equipped with separate transmitters and receivers to transmit optical signals. MCF To evaluate the SE, we use the measurement results of the same transmitter and receiver as for the single-core optical fiber. MCF SE ref You can find the FOM by dividing by

[0035] The reference single-core optical fiber should be an ITU-T G.654 compatible optical fiber for submarine long-distance optical fiber and an ITU-T G.652 compatible optical fiber for terrestrial long-distance optical fiber. The loss, chromatic dispersion, crosstalk, and effective area of ​​the single-core optical fiber and MCF are evaluated, and the FOM is calculated numerically.

[0036] In this disclosure, the FOM was calculated using the following system conditions: a span length of 80 km, an optical amplifier noise figure of 5 dB, and a frequency grid of 50 GHz. This is within the range of system conditions used in typical long-distance optical communication systems (e.g., a span length of approximately 60-90 km, a noise figure of approximately 3-6 dB, and a frequency grid of approximately 25-200 GHz). Digital coherent systems use technologies such as Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM), which perform multilevel modulation by combining two Amplitude-Shift Keying (ASK) signals with orthogonal phases. A similar FOM can also be used in this case.

[0037] 6 shows the relationship between the ratio of effective areas and FOM for two-core MCFs with and without inter-core crosstalk. The horizontal axis is the ratio of the effective area Aeff of each core of the MCF to the effective area Aeff of a single-core optical fiber. ref The vertical axis is FOM. In Fig. 6, the relationship between the ratio of effective area and FOM when there is no influence of XT is shown by a solid line, and the relationship between the ratio of effective area and FOM when there is an XT of -40 dB per span (80 km) is shown by a dashed line. Here, the reference single-core optical fiber is a general low-loss optical fiber conforming to ITU-T G.654, and has an effective area Aeff at a wavelength of 1.55 µm. ref 110 μm 2 It was decided.

[0038] As can be seen from the solid line, if Aeff is increased (the horizontal axis is 1.0 or more), the FOM is doubled (3 dB) or more due to the performance improvement caused by the increase in Aeff. As can be seen from the dashed line, the FOM decreases when XT occurs. In this disclosure, Aeff ≧ Aeff ref Then, the upper limit of XT (allowable XT) at which the FOM is 3 dB or more is determined, and the relative refractive index difference between the core and cladding and the core spacing are set so that the XT is equal to or less than the allowable XT. This makes it possible to provide an MCF with transmission performance several times higher than that of a standard single-core optical fiber.

[0039] In the case of a four-core MCF, the value is twice as high (3dB) as that of an equivalent core structure and XT, and can be interpreted as a value obtained by adding an additional 3dB to the FOM in the figure.

[0040] Figure 7 shows the upper limit of XT (allowable XT) per span (80 km) where the FOM is the number of cores times the effective area. The horizontal axis is the ratio of the effective area to the single-core optical fiber, and the vertical axis is the allowable XT. In Figure 7, the allowable XT of optical fibers for long-distance submarine systems is shown by a solid line, and the allowable XT of optical fibers for long-distance terrestrial systems is shown by a dashed line. Aeff of the reference single-core optical fiber ref The total distance is Aeff, taking into account the long-distance undersea system conforming to ITU-T G.654. ref = 110 μm 2and 10,000 km, and Aeff is set considering the long-distance terrestrial system conforming to ITU-T G.652. ref = 80 μm 2 and 1,000km.

[0041] The allowable XT for a long-distance undersea system conforming to ITU-T G.654 can be approximated by the following equation (5).

[0042]

[0043] For a long-distance terrestrial system conforming to ITU-T G.652, the allowable XT can be approximated by the following equation (6).

[0044]

[0045] When optical transmission is performed using an MCF, it is known that the XT between cores can be significantly reduced by configuring the system so that light propagates in opposite directions between adjacent cores. In the four-core configuration shown in Figures 1 and 3, adjacent cores refer to the cores that are closest to each other. In a four-core MCF, when light propagates in opposite directions between adjacent cores, the light propagates in the same direction in each vertically aligned core, and in the same direction in each horizontally aligned core, but the light propagates in opposite directions between the vertically aligned core and the horizontally aligned core. For example, in the four-core MCF shown in Figures 1 and 3, light propagates toward the back of the vertically aligned core and toward the front of the horizontally aligned core.

[0046] Figure 8 shows an example of the XT per span (80 km) due to differences in the propagation direction of light between adjacent cores of an MCF. In Figure 8, the solid line shows the XT characteristics when the propagation direction of light is the same between adjacent cores, and the dashed line shows the XT characteristics when the propagation direction of light is different between adjacent cores. Figure 8 shows that there is an XT difference of approximately 25 dB over one span (80 km) between the solid and dashed lines. The dashed line in Figure 8 represents the difference ΔXT (right scale) between XT during co-propagation and counter-propagation, and is expressed by the following equation (7):

[0047]

[0048] When defining the XT of an MCF, considering counter-propagation between cores, XT varies nonlinearly with the length of the optical fiber, so it is easier to handle the XT during co-propagation as an optical fiber specification. Therefore, it is preferable to use Equation (5) or (6) for the allowable XT to make the FOM at least several times the number of cores during co-propagation, and to use the value obtained by adding Equation (7) to Equation (5) or (6) for counter-propagation.

[0049] Next, we will explain the structural conditions of two-core and four-core MCFs suitable for long-distance submarine and terrestrial systems. The relative refractive index difference between the core and cladding and the core spacing are set so that the FOM is several times or more that of a standard single-core optical fiber.

[0050] Figure 9(a) shows the core spacing of a two-core MCF, and Figure 9(b) shows the core spacing of a four-core MCF. As shown in the figure, the core spacing is the distance between the centers of adjacent cores 11. In the case of four cores 11 arranged in a square lattice, the core spacing is the distance between the centers of the closest cores 11.

[0051] The relative refractive index difference between the core and the cladding is based on the refractive index of the core as shown in Figures 2 and 4. Since the refractive index of the cladding is lower than that of the core, the relative refractive index difference will be expressed as a negative value in the following explanation.

[0052] First, we will explain the structural conditions of a two-core MCF suitable for long-distance submarine systems. The reference single-core optical fiber is an optical fiber conforming to ITU-T G.654, which is commonly used in submarine systems. The transmission distance is assumed to be 10,000 km, and the cutoff condition is set to 1.53 μm or less, which is equivalent to that of an optical fiber conforming to ITU-T G.654. ​​In addition, Aeff ref is 110 μm at a wavelength of 1.55 μm 2 It was decided.

[0053] Figure 10 shows the structural conditions required for a two-core MCF to double (3 dB) the FOM. The horizontal axis is the relative refractive index difference Δ (%) between the core and cladding, and the vertical axis is the core spacing Λ (μm). The solid line indicates the lower limit of the core spacing at which the FOM is 3 dB or more during co-propagation, and the dashed line indicates the lower limit of the core spacing at which the FOM is 3 dB or more during counter-propagation. In the region below the solid or dashed line, the FOM becomes less than 3 dB due to an increase in XT. The core structure of the MCF is such that the ratio of the cladding radius a1 to the core radius a is a1 / a = 2.5, and the ratio of the effective area Aeff / Aeff ref ≧1.0, and the cutoff wavelength was set to 1.53 μm or less.

[0054] As the core spacing narrows, the FOM becomes less than double (3 dB) due to the increase in XT. The lower limit of the core spacing Λ at which the FOM is 3 dB or more is given by the following equation (8) when the light propagation direction between adjacent cores is the same, and by the following equation (9) when the light propagation direction between adjacent cores is opposite.

[0055]

[0056]

[0057] In FIG. 10, the region that satisfies the condition of formula (8) is the region above the solid line in FIG. 10, and the region that satisfies the condition of formula (9) is the region above the dashed line in FIG.

[0058] As the core spacing increases, loss increases due to leakage of light from the cores (excess loss in design). To sufficiently reduce excess loss, it is preferable to design the fiber so that the excess loss at a wavelength of 1.625 μm is 0.01 dB / km or less. Specifically, the core spacing Λ condition for excess loss to be 0.01 dB / km or less at a wavelength of 1.625 μm is expressed by the following equation (10):

[0059]

[0060] The region that satisfies the condition of formula (10) is the region below the dashed line in FIG.

[0061] Therefore, to obtain more than double the FOM with a two-core MCF that is optically compatible with ITU-T G.654, it is necessary to simultaneously satisfy at least equations (9) and (10). In other words, when using an optical fiber compliant with ITU-T G.654 as the standard, the two-core MCF is designed so that the relative refractive index difference Δ between the core and cladding and the core spacing Λ are in the range below the dashed line and above the dotted line in Figure 10.

[0062] Next, we will explain the structural requirements of a four-core MCF suitable for long-distance undersea systems. The reference single-core optical fiber is the same as the two-core case.

[0063] Figure 11 shows the structural conditions required for a four-core MCF to increase the FOM by four times (6 dB) or more. The horizontal axis is the relative refractive index difference Δ (%) between the core and cladding, and the vertical axis is the core spacing Λ (μm). The solid line indicates the lower limit of the core spacing at which the FOM is 6 dB or more during co-propagation, and the dashed line indicates the lower limit of the core spacing at which the FOM is 6 dB or more during counter-propagation. In the region below the solid or dashed line, the FOM becomes less than 6 dB due to an increase in XT. The core structure of the MCF is such that the ratio of the cladding radius a1 to the core radius a is a1 / a = 2.5, and the ratio of the effective area Aeff / Aeff ref ≧1.0, and the cutoff wavelength was set to 1.53 μm or less.

[0064] The lower limit of the core spacing Λ at which the FOM is 6 dB or more is given by the following equation (11) when the light propagation direction between adjacent cores is the same, and by the following equation (12) when the light propagation direction between adjacent cores is opposite.

[0065]

[0066]

[0067] In FIG. 11, the region that satisfies the condition of formula (11) is the region above the solid line in FIG. 11, and the region that satisfies the condition of formula (12) is the region above the dashed line in FIG.

[0068] The condition for the core spacing Λ that results in an excess loss of 0.01 dB / km or less when the wavelength is 1.625 μm is given by the following equation (13).

[0069]

[0070] The region that satisfies the condition of equation (13) is the region below the dashed line in FIG.

[0071] Therefore, to obtain an FOM of four or more times with a four-core MCF that is optically compatible with ITU-T G.654, it is necessary to simultaneously satisfy at least equations (12) and (13). In other words, when using an optical fiber compliant with ITU-T G.654 as the reference, the four-core MCF is designed so that the relative refractive index difference Δ between the core and cladding and the core spacing Λ are in the region below the dashed-dotted line and above the broken line in Figure 11.

[0072] Next, we will explain the structural conditions of a two-core MCF suitable for long-distance terrestrial systems. The reference single-core optical fiber is an optical fiber conforming to ITU-T G.652, which is commonly used in terrestrial systems. The transmission distance is assumed to be 1,000 km, and the cutoff condition is set to 1.26 μm or less, which is equivalent to that of an optical fiber conforming to ITU-T G.652. In addition, Aeff ref is 80 μm at a wavelength of 1.55 μm 2 It was decided.

[0073] Figure 12 shows the structural conditions required for a two-core MCF to double (3 dB) the FOM. The horizontal axis is the relative refractive index difference Δ (%) between the core and cladding, and the vertical axis is the core spacing Λ (μm). The solid line indicates the lower limit of the core spacing at which the FOM is 3 dB or more during co-propagation, and the dashed line indicates the lower limit of the core spacing at which the FOM is 3 dB or more during counter-propagation. In the region below the solid or dashed line, the FOM becomes less than 3 dB due to an increase in XT. The core structure of the MCF is such that the ratio of the cladding radius a1 to the core radius a is a1 / a = 2.5, and the ratio of the effective area Aeff / Aeff ref ≧1.0, and the cutoff wavelength was set to 1.26 μm or less.

[0074] The lower limit of the core spacing Λ at which the FOM is 3 dB or more is given by the following equation (14) when the light propagation direction between adjacent cores is the same, and by the following equation (15) when the light propagation direction between adjacent cores is opposite.

[0075]

[0076]

[0077] In FIG. 12, the region that satisfies the condition of equation (14) is the region above the solid line in FIG. 12, and the region that satisfies the condition of equation (15) is the region above the dashed line in FIG.

[0078] The condition for the core spacing Λ that results in an excess loss of 0.01 dB / km or less when the wavelength is 1.625 μm is given by the following equation (13).

[0079]

[0080] The region that satisfies the condition of equation (16) is the region below the dashed line in FIG.

[0081] Therefore, to obtain more than double the FOM with a two-core MCF that is optically compatible with ITU-T G.652, it is necessary to simultaneously satisfy at least equations (15) and (16). In other words, when using an optical fiber compliant with ITU-T G.652 as a reference, the two-core MCF is designed so that the relative refractive index difference Δ between the core and cladding and the core spacing Λ are in the region below the dashed line and above the dotted line in Figure 12.

[0082] Next, we will explain the structural requirements of a four-core MCF suitable for long-distance terrestrial systems. The reference single-core optical fiber is the same as the two-core case.

[0083] Figure 13 shows the structural conditions required for a four-core MCF to increase the FOM by four times (6 dB) or more. The horizontal axis is the relative refractive index difference Δ (%) between the core and cladding, and the vertical axis is the core spacing Λ (μm). The solid line indicates the lower limit of the core spacing at which the FOM is 6 dB or more during co-propagation, and the dashed line indicates the lower limit of the core spacing at which the FOM is 6 dB or more during counter-propagation. In the region below the solid or dashed line, the FOM becomes less than 6 dB due to an increase in XT. The core structure of the MCF is such that the ratio of the cladding radius a1 to the core radius a is a1 / a = 2.5, and the ratio of the effective area Aeff / Aeff is ref ≧1.0, and the cutoff wavelength was set to 1.26 μm or less.

[0084] The lower limit of the core spacing Λ at which the FOM is 6 dB or more is given by the following equation (17) when the light propagation direction between adjacent cores is the same, and by the following equation (18) when the light propagation direction between adjacent cores is opposite.

[0085]

[0086]

[0087] In FIG. 13, the region that satisfies the condition of equation (17) is the region above the solid line in FIG. 13, and the region that satisfies the condition of equation (18) is the region above the dashed line in FIG.

[0088] The condition for the core spacing Λ that results in an excess loss of 0.01 dB / km or less when the wavelength is 1.625 μm is given by the following equation (19).

[0089]

[0090] The region that satisfies the condition of equation (19) is the region below the dashed line in FIG.

[0091] Therefore, to obtain an FOM of four or more times greater with a four-core MCF that is optically compatible with ITU-T G.652, it is necessary to simultaneously satisfy at least equations (18) and (19). In other words, when using an optical fiber compliant with ITU-T G.652 as a reference, the four-core MCF is designed so that the relative refractive index difference Δ between the core and cladding and the core spacing Λ are in the region below the dashed-dotted line and above the broken line in Figure 13.

[0092] [Splice loss and core spacing of multi-core optical fiber] There are several methods for splicing optical fibers, including fusing together fibers by melting the glass at the fiber end surface, and connecting connectors attached to the ends of the fibers. Fusion splicing is often used outdoors. In single-core optical fibers, the core is positioned with high precision at the center of the cladding. This makes it easy to align the cores when fusion splicing, and empirically the increase in loss due to fusion splicing is about 0.02 dB. Since MCFs have cores other than the center of the optical fiber, when fusion splicing, it is necessary to align the central axes of both fibers and then align the cores. Misalignment of the cores increases splice loss.

[0093] Figure 14 shows the relationship between core spacing and the increase in splice loss due to rotational misalignment. The horizontal axis is the core spacing, and the vertical axis is the increase in splice loss due to rotational misalignment, i.e., the loss increase relative to a rotational misalignment of 0°. Here, the number of cores is set to 2, the mode feed diameter is set to 12 μm, and an axial misalignment of 0.6 μm is assumed, taking into account the core position deviation of a standard optical fiber.

[0094] 14, it can be seen that the wider the core spacing and the larger the rotational misalignment, the greater the splice loss. Therefore, in order to reduce splice loss in an MCF, it is preferable to make the core spacing as narrow as possible.

[0095] Considering that the increase in splice loss in a single-core optical fiber is 0.02 dB, it is reasonable to target a splice loss increase of 0.02 dB. As shown in Figure 14, the core spacing at which the increase in splice loss is 0.02 dB or less for a rotational misalignment of 1° is 48 μm. With a core spacing of 48 μm, the increase in splice loss for a rotational misalignment of 0.5° is 0.005 dB or less, which is sufficiently small. Therefore, considering the splice loss of MCFs, it is preferable to set the core spacing at 48 μm or less.

[0096] As described above, the multi-core optical fiber of this embodiment includes n (n is an integer of 2 or more) cores 11 and cladding regions 22 that surround each of the n cores and have a lower refractive index than the cores 11. The relative refractive index difference Δ between the cores and the cladding and the core spacing Λ are related to the space utilization efficiency SE MCF The spatial efficiency SE of a single-core optical fiber is ref This ensures an FOM that is several times larger than that of commonly used G.652 or G.654 compliant single-core optical fibers, improving system performance. As a result, the distance of one span of MCF can be extended, reducing the number of repeaters and reducing costs.

[0097] 11 Core 21, 22, 23 Cladding region 31 Coating

Claims

1. A multi-core optical fiber comprising n (n is an integer of 2 or more) cores and cladding surrounding each of the n cores and having a lower refractive index than the cores, wherein the relative refractive index difference between the cores and the cladding and the core spacing are determined so that the spatial utilization efficiency is n times or more that of a standard single-core optical fiber.

2. A multi-core optical fiber according to claim 1, wherein the reference single-core optical fiber is an optical fiber conforming to ITU-T G.654, the number of cores n is 2, and the relative refractive index difference Δ between the core and the cladding and the core spacing Λ (μm) are A multi-core optical fiber comprising:

3. A multi-core optical fiber according to claim 1, wherein the reference single-core optical fiber is an optical fiber conforming to ITU-T G.654, the number of cores n is 4, and the relative refractive index difference Δ between the core and the cladding and the core spacing Λ (μm) are A multi-core optical fiber comprising:

4. A multi-core optical fiber according to claim 1, wherein the reference single-core optical fiber is an optical fiber conforming to ITU-T G.652, the number of cores n is 2, and the relative refractive index difference Δ between the core and the cladding and the core spacing Λ (μm) are A multi-core optical fiber comprising:

5. A multi-core optical fiber according to claim 1, wherein the reference single-core optical fiber is an optical fiber conforming to ITU-T G.652, the number of cores n is 4, and the relative refractive index difference Δ between the core and the cladding and the core spacing Λ (μm) are A multi-core optical fiber comprising:

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