Multi-core optical fiber and optical transmission system

Abstract

A multicore optical fiber (1) is provided with a pair of cores (10) and a cladding (20) covering both of the pair of cores (10). The cladding has a diameter of 124 μm to 126 m inclusive. The center-to-center distance of the pair of cores is less than or equal to 46.5 μm. The crosstalk between opposing propagation cores at the wavelength 1550 nm is less than or equal to -30 dB / 100 km.

Description

[Technical Field]

[0001] This application claims priority to Japanese Application No. 2020-159586, filed on September 24, 2020, and incorporates by reference all of the contents of said Japanese application.

[0002] The present disclosure relates to a multi-core optical fiber and an optical transmission system. [Background technology]

[0003] Uncoupled multi-core optical fibers having a pair of cores and a cladding are known (for example, Non-Patent Document 1 and Non-Patent Document 2). In such multi-core optical fibers, independent optical signals can propagate in each core, and therefore, the transmission capacity per fiber is larger than that of single-core optical fibers having a single core. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Geng, “High-speed, bi-directional dual-core fiber transmission system for high-density, short-reach optical interconnects”, Proc. of SPIE, vol.9390, pp.939009-1-10 (2015). [Non-patent document 2] Tamura, “Low-Loss Uncoupled Two-Core Fiber for Power Efficient Practical Submarine Transmission”, OFC'19, M1E.5 (2019). Summary of the Invention

[0005] The multi-core optical fiber of the present disclosure includes a pair of cores and a cladding covering both of the pair of cores. The diameter of the cladding is 124 μm or more and 126 μm or less. μ The distance between the centers of the pair of cores is 46.5 μm or less. The crosstalk between counter-propagating cores at a wavelength of 1550 nm is -30 dB / 100 km or less. The centers of the pair of cores may be arranged on the same circle centered on the fiber axis and on a line passing through the fiber axis in a cross section perpendicular to the fiber axis.

[0006] An optical transmission system according to an embodiment of the present disclosure includes the above multi-core optical fiber, and an optical transmitter and an optical receiver connected to both ends of each of the pair of cores. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing a cross-sectional configuration of a multi-core optical fiber according to an embodiment. [Figure 2] FIG. 2 is a diagram showing a cross-sectional configuration of a multi-core optical fiber according to a modified example. [Figure 3] FIG. 3 is a diagram illustrating a configuration of an optical transmission system according to the embodiment. [Figure 4] FIG. 4 is a diagram showing the configuration of a measurement system for parallel propagation inter-core crosstalk. [Figure 5] FIG. 5 is a diagram showing the configuration of a measurement system for counter-propagating inter-core crosstalk. [Figure 6] Figure 6 shows an example of calculation of the conditions under which the crosstalk between counter-propagating cores is -30 dB / 100 km at a wavelength of 1550 nm when the effective cross section is 80 μm 2 . [Figure 7] FIG. 7 is a graph showing a calculation example of a region where the cable cutoff wavelength is 1470 nm or less when the relative refractive index difference of the core with respect to the optical cladding is 0.50% and the inter-core distance is 36.5 μm. [Figure 8]Figure 8 shows an example of a calculation of the region where the bending loss at a wavelength of 1625 nm is 0.25 dB / 100 turns or less when the relative refractive index difference of the core with respect to the optical cladding is 0.50% and the core-to-core distance is 36.5 μm, and the fiber is wound into a circular ring with a radius of 30 mm. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Problem to be solved by this disclosure] In a multi-core optical fiber, it is necessary to increase the inter-core distance (inter-core pitch, core spacing), which is the distance between the centers of a pair of cores, in order to reduce crosstalk between signals propagating through each core. In the above Non-Patent Document 1 and Non-Patent Document 2, a large inter-core distance of 46.6 μm or more is ensured. However, when the inter-core distance increases, the distance between the fiber center and the core center also increases, which increases the connection loss due to angle misalignment when splicing multi-core optical fibers.

[0009] An object of the present disclosure is to provide a multi-core optical fiber and an optical transmission system that can suppress crosstalk during actual use while simultaneously suppressing an increase in connection loss.

[0010] [Effects of this disclosure] According to the present disclosure, it is possible to provide a multi-core optical fiber and an optical transmission system that can suppress crosstalk between counter-propagating cores while simultaneously suppressing an increase in connection loss. [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described. A multi-core optical fiber according to one embodiment of the present disclosure includes a pair of cores and a cladding covering both of the pair of cores. The diameter of the cladding is 124 μm or more and 126 μm or less. μThe fiber has a center-to-center distance of 46.5 μm or less. The counter-propagating inter-core crosstalk at a wavelength of 1550 nm is -30 dB / 100 km or less. Here, "counter-propagating inter-core crosstalk" is the logarithm of the ratio of the intensity of light incident on one of the pair of cores at one end to the intensity of light emitted from the other end of the one core that returns to the other end of the pair of cores. Note that the centers of the pair of cores may be located on the same circle centered on the fiber axis and on a straight line passing through the fiber axis in a cross section perpendicular to the fiber axis.

[0011] In this multi-core optical fiber, the distance from the center of gravity of each core to the fiber axis is short, so when two multi-core optical fibers are spliced, if they are ideally aligned so that their centers of gravity coincide with each other, it is possible to suppress an increase in splice loss due to angular misalignment.

[0012] The bending loss at a wavelength of 1625 nm when wound into a circular loop with a radius of 30 mm is 0.25 dB / 100 turns or less, and the cable cutoff wavelength defined in ITU-T G.650.1 may be 1530 nm or less, allowing interconnection with widely used optical fibers conforming to ITU-T G.654A to D.

[0013] The cladding may include an optical cladding covering the pair of cores and a physical cladding surrounding the optical cladding and having a refractive index equal to or greater than that of the optical cladding. In this case, a refractive index step is introduced at the interface between the optical cladding and the physical cladding, which allows the inter-core distance to be further reduced. In addition, the design tolerances for bending loss and cable cutoff wavelength can be expanded.

[0014] The relative refractive index difference of the physical cladding with respect to the optical cladding may be 0.1% or more and 0.4% or less. In this case, the inter-core distance can be reliably reduced. Also, the design tolerances of bending loss and cable cutoff wavelength can be reliably expanded.

[0015] The multi-core optical fiber may further include a low-refractive-index portion disposed between the pair of cores and having a refractive index lower than that of the optical cladding. In this case, the inter-core distance can be further reduced, thereby further suppressing an increase in splice loss due to angular misalignment.

[0016] An optical transmission system according to an embodiment of the present disclosure includes the above multi-core optical fiber, and an optical transmitter and an optical receiver connected to both ends of each of the pair of cores.

[0017] Since this optical transmission system includes the multi-core optical fiber, it is possible to reduce crosstalk and suppress an increase in connection loss.

[0018] [Details of the embodiments of the present disclosure] Specific examples of the multi-core optical fiber and optical transmission system of the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted.

[0019] Fig. 1 is a diagram showing a cross-sectional configuration of a multi-core optical fiber according to an embodiment. As shown in Fig. 1, the MCF 1 includes a pair of cores 10 and a cladding 20. The MCF 1 may further include a resin coating (not shown) that covers the outer peripheral surface of the cladding 20. The MCF 1 is an uncoupled type.

[0020] The pair of cores 10 are arranged point-symmetrically with respect to the fiber axis C in a cross section perpendicular to the fiber axis C of the MCF 1. In the cross section perpendicular to the fiber axis C, the centers of the pair of cores 10 are arranged on the same circle centered on the fiber axis C and on a single line passing through the fiber axis C. The center-to-center distance (core-to-core distance or core spacing) of the pair of cores 10 is 46.5 μm or less. The diameter (core diameter) of each core 10 is, for example, 8 μm or more and 14 μm or less. The core 10 is made of glass, for example, containing silica glass as a main component and doped with an element for refractive index modulation as necessary. In this embodiment, the pair of cores 10 are identical to each other in terms of diameter (shape), material, doped element, and refractive index distribution.

[0021] The pair of cores 10 may be, for example, a combination of a core 10 containing an alkali metal element such as potassium, sodium, or rubidium, and a core 10 not containing an alkali metal element. The pair of cores 10 may be, for example, a combination of a core 10 having a central depression including the central axis of the core 10, and a core 10 not having a central depression.

[0022] The clad 20 is provided around the pair of cores 10 and covers both of the pair of cores 10. The diameter of the clad 20 (clad diameter) is 124 μm or more and 126 μm or less. μ The cladding 20 includes an optical cladding 21 and a physical cladding 22. The optical cladding 21 and the physical cladding 22 are made of glass that contains silica glass as a main component and may contain an element for refractive index modulation added as needed.

[0023] The optical cladding 21 covers both of the pair of cores 10. The optical cladding 21 is provided around the pair of cores 10 and between the pair of cores 10. The optical cladding 21 covers the entire outer circumferential surface of each core 10. The optical cladding 21 is in contact with the outer circumferential surface of each core 10. The diameter of the optical cladding 21 (optical cladding diameter) is, for example, not less than 58 μm and not more than 80 μm.

[0024] The physical cladding 22 is provided around the optical cladding 21 and surrounds the optical cladding 21. The physical cladding 22 covers the optical cladding 21. The physical cladding 22 is in contact with the outer peripheral surface of the optical cladding 21. The diameter of the physical cladding 22 (physical cladding diameter) corresponds to the diameter of the cladding 20.

[0025] The refractive index of the cladding 20 is lower than the refractive index of the core 10. The refractive index of the optical cladding 21 and the refractive index of the physical cladding 22 are both lower than the refractive index of the core 10. The relative refractive index difference of the core 10 with respect to the optical cladding 21 is, for example, not less than 0.30% and not more than 0.60%. The refractive index of the physical cladding 22 is equal to or higher than the refractive index of the optical cladding 21. In other words, the refractive index of the optical cladding 21 is the same as or lower than the refractive index of the physical cladding 22. The relative refractive index difference of the physical cladding 22 with respect to the optical cladding 21 is, for example, not less than 0.1% and not more than 0.4%.

[0026] The relative refractive index difference of the core 10 with respect to pure silica is, for example, −0.10% or more and 0.20% or less. The relative refractive index difference of the optical cladding 21 with respect to pure silica is, for example, −0.50% or more and −0.20% or less. The relative refractive index difference of the physical cladding 22 with respect to pure silica is, for example, −0.30% or more and −0.10% or less.

[0027] Fig. 2 is a diagram showing a cross-sectional configuration of a multi-core optical fiber according to a modified example. As shown in Fig. 2, MCF1A differs from MCF1 (see Fig. 1) in that it further includes a low-refractive-index portion 30, but is the same in other respects. The low-refractive-index portion 30 is disposed between the pair of cores 10 so as to overlap with the fiber axis C. In a cross section perpendicular to the fiber axis C, the center of the low-refractive-index portion 30 coincides with the fiber axis C. The low-refractive-index portion 30 is disposed spaced apart from the pair of cores 10.

[0028] The low refractive index portion 30 has a refractive index lower than that of the optical cladding 21. The relative refractive index difference of the low refractive index portion 30 with respect to the optical cladding 21 is, for example, not less than −0.30% and not more than −0.10%. The low refractive index portion 30 is made of, for example, glass containing silica glass as a main component and doped with an element for refractive index modulation as necessary. The low refractive index portion 30 is doped with, for example, F (fluorine) as an element for refractive index modulation. The diameter of the low refractive index portion 30 is, for example, not less than 0.5 times and not more than 1.0 times the core diameter.

[0029] In the MCF1, 1A, the bending loss at a wavelength of 1625 nm when wound into a circular ring with a radius of 30 mm is 0.25 dB / 100 turns or less. The cable cutoff wavelength λcc is 1530 nm or less. In the MCF1, 1A, the effective cross-sectional area Aeff of light propagating through each core 10 is, for example, 70 μm 2 More than 130 μm 2 The following is the result.

[0030] In the MCF 1, 1A, the pair of cores 10 may propagate a pair of signal light beams in opposite directions or in the same direction. A portion of the incident light beams that enter a core 10 as signal light beams leaks into an adjacent core 10. The ratio of the intensity of the light beams that leak from an adjacent core 10 to the intensity of the light beams that exit the core 10 into which the incident light beams enter is defined as inter-core crosstalk (signal crosstalk). The inter-core crosstalk (counter-propagation inter-core crosstalk) relating to the intensity of the light beams that leak from adjacent cores 10 at the input end at a wavelength of 1550 nm is, for example, -30 dB / 100 km or less. In other words, when the intensity of the incident light beam is 1, the intensity of the leaked light is 1 / 1000 or less. The inter-core crosstalk (parallel-propagation inter-core crosstalk) relating to the intensity of the light beams that leak from adjacent cores 10 at the output end at a wavelength of 1550 nm is, for example, -10 dB / 100 km or less.

[0031] In the case of counterpropagation, inter-core crosstalk can be suppressed compared to co-propagation. In the case of counterpropagation, the inter-core distance required to ensure the same level of inter-core crosstalk as in co-propagation can be reduced. Therefore, in the case of counterpropagation, connection loss due to angle misalignment can be reduced compared to co-propagation.

[0032] In the MCFs 1 and 1A, the optical cladding 21 and the physical cladding 22 have different refractive indices, but the optical cladding 21 and the physical cladding 22 may have the same refractive index. That is, the cladding 20 may have a uniform refractive index.

[0033] Fig. 3 is a diagram showing the configuration of an optical transmission system according to an embodiment. As shown in Fig. 3, the optical transmission system 40 includes an MCF 1 and four optical transceivers (TRx) 41, 42, 43, and 44. One of a pair of cores 10 of the MCF 1 constitutes a transmission line C1, and the other constitutes a transmission line C2. The optical transceivers 41 and 42 are connected to both ends of the transmission line C1 (core 10). The optical transceivers 43 and 44 are connected to both ends of the transmission line C2 (core 10). The optical transceivers 41 and 43 are connected to a fiber end 1a of the MCF 1. The optical transceivers 42 and 44 are connected to a fiber end 1b of the MCF 1.

[0034] The transmission line C1 transmits the signal light 45, and the transmission line C2 transmits the signal light 46. In the optical transmission system 40, the transmission line C1 transmits the signal light 45 bidirectionally, and the transmission line C2 transmits the signal light 46 bidirectionally. That is, one of the optical transceivers 41 and 42 transmits the signal light 45, and the other receives the signal light 45. One of the optical transceivers 43 and 44 transmits the signal light 46, and the other receives the signal light 46.

[0035] The optical transmission system 40 may be configured such that the transmission line C1 transmits the signal light 45 in only one direction. In this case, the optical transmission system 40 may include an optical transmitter and an optical receiver instead of the pair of optical transceivers 41 and 42. Also, the optical transmission system 40 may be configured such that the transmission line C2 transmits the signal light 46 in only one direction. In this case, the optical transmission system 40 may include an optical transmitter and an optical receiver instead of the pair of optical transceivers 43 and 44. The optical transmission system 40 may include an MCF1A instead of the MCF1.

[0036] 4 and 5 are diagrams showing the configuration of a measurement system for parallel-propagating core crosstalk and counter-propagating core crosstalk, respectively.

[0037] 4, in measurement system 50A, an optical transmitter 51 is provided at fiber end 1a of transmission line C1. An optical receiver 52 is provided at fiber end 1b of transmission line C1. An optical transmitter 53 is provided at fiber end 1a of transmission line C2. An optical receiver 54 is provided at fiber end 1b of transmission line C2.

[0038] The optical transmitters 51 and 53 are, for example, laser diodes. The optical receivers 52 and 54 are, for example, photodiodes. The optical transmitter 51 inputs laser light as incident light 55 from fiber end 1a to transmission line C1. The optical receiver 52 measures the intensity of light emitted from transmission line C1 at fiber end 1b. The optical transmitter 53 inputs laser light as incident light 56 from fiber end 1a to transmission line C2. The optical receiver 54 measures the intensity of light emitted from transmission line C2 at fiber end 1b.

[0039] In measuring the parallel propagation inter-core crosstalk using the measurement system 50A, an optical transmitter 51 is used to input incident light 55 from the fiber end 1a into the transmission line C1, and an optical receiver 52 is used to measure the intensity I of the light output from the transmission line C1. 51-52 and the intensity I of the light 55a leaked into the adjacent transmission line C2 using the optical receiver 54. 51-54Furthermore, an optical transmitter 53 is used to input incident light 56 from the fiber end 1a to the transmission line C2, and an optical receiver 54 is used to measure the 2 or The light intensity I emitted from 53-54 and the intensity I of the light 56a leaked into the adjacent transmission line C1 using the optical receiver 52. 53-52 Measure the intensity I 51-52 Strength I 51-54 The logarithm of the ratio log(I 51-54 / I 51-52 ), and intensity I 53-54 Strength I 53-52 The logarithm of the ratio log(I 53-52 / I 53-54 ) is the crosstalk between parallel-propagating cores. Theoretically, the values of the two ratios are the same.

[0040] 5, measurement system 50B differs from measurement system 50A in that the positions of optical transmitter 53 and optical receiver 54 are swapped. That is, in measurement system 50B, optical transmitter 53 inputs laser light as incident light 56 from fiber end 1b into transmission line C2. Optical receiver 54 measures the intensity of light emitted from transmission line C2 at fiber end 1a.

[0041] In the measurement of the counter-propagating inter-core crosstalk by the measurement system 50B, the optical transmitter 51 is used to input the incident light 55 from the fiber end 1a to the transmission line C1, and the optical receiver 52 is used to measure the intensity I of the light output from the transmission line C1. 51-52 and measure the intensity I of the light 55a returned to the adjacent waveguide using the optical receiver 54. 51-54 Furthermore, an optical transmitter 53 is used to input incident light 56 from the fiber end 1b into the transmission line C1, and an optical receiver 54 is used to measure the intensity I of the light output from the transmission line C1. 53-54 and measure the intensity I of the light 56a returned to the adjacent waveguide using the optical receiver 52. 53-52 Measure the intensity I 51-52 Strength I 51-54 The logarithm of the ratio log(I 51-54 / I 51-52 ) and intensity I 53- 54 Strength I53-52 The logarithm of the ratio log(I 53-52 / I 53-54 ) is the counter-propagating core-to-core crosstalk. Theoretically, the values of the two ratios are equal.

[0042] Figure 6 shows the effective cross-sectional area of 80 μm 2 6 is a calculation example of the conditions under which the crosstalk between counter-propagating cores at a wavelength of 1550 nm is -30 dB / 100 km when the effective area is 80 μm. 2 The graph shows the relationship between the inter-core distance (μm) at which the counter-propagating inter-core crosstalk at a wavelength of 1550 nm is -30 dB / 100 km in the case of the above. The horizontal axis shows the relative refractive index difference (%) of the core with respect to the optical cladding. The vertical axis shows the inter-core distance (μm).

[0043] As shown in Figure 6, the greater the relative refractive index difference between the core and the optical cladding, the smaller the inter-core distance can be. 2 In this case, to achieve a core-to-core distance of 46.5 μm or less while suppressing counter-propagating core-to-core crosstalk at a wavelength of 1550 nm to −30 dB / 100 km, the relative refractive index difference of the core with respect to the optical cladding needs to be 0.36% or more. More preferably, the relative refractive index difference of the core with respect to the optical cladding is 0.42% or more. This allows the core-to-core distance to be 41 μm or less, and the connection loss can be stably kept low. Even more preferably, the relative refractive index difference of the core with respect to the optical cladding is 0.48% or more. This allows the core-to-core distance to be 38 μm or less, and the connection loss can be stably kept low. On the other hand, the relative refractive index difference of the core with respect to the optical cladding is preferably 0.60% or less. This prevents an increase in transmission loss due to dopants added to increase the relative refractive index difference, and also prevents an increase in polarization mode dispersion due to asymmetry that may occur during manufacturing.

[0044] FIG. 7 is a graph showing a calculation example of a region where the cable cutoff wavelength is 1470 nm or less when the relative refractive index difference of the core with respect to the optical cladding is 0.50% and the inter-core distance is 36.5 μm. FIG. 7 is a calculation example for a configuration without a low refractive index section, i.e., a configuration corresponding to MCF1. The graph shown in FIG. 7 shows the relationship between the relative refractive index difference (%) of the physical cladding with respect to the optical cladding and the optical cladding diameter (μm) where the cable cutoff wavelength is 1470 nm when the relative refractive index difference of the core with respect to the optical cladding is 0.50% and the inter-core distance is 36.5 μm. The horizontal axis is the relative refractive index difference (%) of the physical cladding with respect to the optical cladding. The vertical axis is the optical cladding diameter (μm) where the cable cutoff wavelength is 1470 nm when the relative refractive index difference of the core with respect to the optical cladding is 0.50% and the inter-core distance is 36.5 μm.

[0045] By manufacturing the cable with a design centered on the structure shown in Figure 7, it is possible to stably suppress the cable cutoff wavelength to 1530 nm or less, even when normal manufacturing variations occur. As a result, it becomes possible to interconnect with optical fibers that comply with the widely used ITU-T G.654 standard.

[0046] Figure 8 shows a calculation example of the region where the bending loss at a wavelength of 1625 nm is 0.25 dB / 100 turns or less when wound into a circular ring with a radius of 30 mm, when the relative refractive index difference of the core with respect to the optical cladding is 0.50% and the inter-core distance is 36.5 μm. Figure 8 shows a calculation example for a configuration without a low refractive index portion, i.e., a configuration corresponding to MCF1. The graph shown in Figure 8 shows the relationship between the relative refractive index difference (%) of the physical cladding with respect to the optical cladding and the optical cladding diameter (μm) at which the bending loss at a wavelength of 1625 nm is 0.25 dB / 100 turns when wound into a circular ring with a radius of 30 mm. The horizontal axis is the relative refractive index difference (%) of the physical cladding with respect to the optical cladding. The vertical axis is the optical cladding diameter (μm) at which the bending loss at a wavelength of 1625 nm is 0.25 dB / 100 turns when wound into a circular ring with a radius of 30 mm.

[0047] By manufacturing the fiber with a design centered on the structure shown in Figure 8, it is possible to stably suppress the bending loss at a bending radius of 30 mm and a wavelength of 1625 nm to 0.50 dB / 100 turns or less, even when normal manufacturing variations occur. As a result, it is possible to stably suppress the cable cutoff wavelength to 1530 nm or less. This makes it possible to interconnect with optical fibers compliant with the widely used ITU-T G.654A to D standards.

[0048] Tables 1 to 4 show calculation examples for the optical properties of MCF1. Table 1 shows calculation examples 1-1 to 1-16. Table 2 shows calculation examples 2-1 to 2-16. Table 3 shows calculation examples 3-1 to 3-16. Table 4 shows calculation examples 4-1 to 4-16. Tables 1 to 4 show the core diameter (μm), the relative refractive index difference of the core with respect to pure silica (%), the inter-core distance (μm), the optical cladding diameter (μm), the relative refractive index difference of the optical cladding with respect to pure silica (%), the relative refractive index difference of the physical cladding with respect to pure silica (%), the diameter of the low-refractive-index section (μm), the counter-propagating inter-core crosstalk XT (dB / 100 km) at a wavelength of 1550 nm, the bending loss (dB / 100 turns) at a wavelength of 1625 nm when wound into a 30 mm radius annular loop, and the cable cutoff wavelength λcc (nm).

[0049] [Table 1] [Table 2] [Table 3] [Table 4]

[0050] Among the calculation examples, calculation examples 3-1, 3-5, 3-9, and 3-13 are comparative examples in terms of counter-propagating core crosstalk, and calculation example 4-8 is comparative example in terms of inter-core distance. The other calculation examples are working examples. From the calculation examples shown in Table 1, it can be seen that by setting the inter-core distance to 35.9 μm or less, more preferably 33.6 μm or less, splicing can be facilitated, and at the same time, the counter-propagating core crosstalk can be kept low, respectively, to -43 dB / 100 km or less and -34 dB / 100 km or less, making it suitable for use in long-distance, large-capacity transmission. Furthermore, by setting the low-refractive-index portion to 0.5 times or more, more preferably 1.0 times or more the core diameter, the inter-core distance can be reduced by an additional 0.8 μm or more, more preferably 2.1 μm or more, compared to a case where no low-refractive-index portion is used, further facilitating splicing.

[0051] From the calculation example shown in Table 2, it can be seen that by setting the inter-core distance to 36.5 μm or less, more preferably 33.3 μm or less, splicing can be made easier, and at the same time the crosstalk between counter-propagating cores can be kept low, respectively, to -45 dB / 100 km or less and -31 dB / 100 km or less, making it suitable for use in long-distance, large-capacity transmission. Furthermore, by setting the low-refractive-index portion to 0.5 times or more, more preferably 1.0 times or more the core diameter, the inter-core distance can be reduced by a further 0.8 μm or more, more preferably 2.1 μm or more, compared to when no low-refractive-index portion is used, making splicing even easier.

[0052] From the calculation example shown in Table 3, it can be seen that by setting the inter-core distance to 33.8 μm or less, splicing can be made easy, and at the same time, the crosstalk between counter-propagating cores can be kept low to -42 dB / 100 km or less, making it suitable for use in long-distance, large-capacity transmission. Furthermore, by setting the low-refractive-index portion to 0.5 times or more, more preferably 1.0 times or more the core diameter, the inter-core distance can be reduced by an additional 0.7 μm or more, more preferably 2.0 μm or more, compared to when no low-refractive-index portion is used, making splicing even easier.

[0053] From the calculation example shown in Table 4, it can be seen that by setting the inter-core distance to 39.1 μm or less, more preferably 35.1 μm or less, splicing can be made easier, and at the same time the crosstalk between counter-propagating cores can be kept low, respectively, to -46 dB / 100 km or less and -30 dB / 100 km or less, making it suitable for use in long-distance, large-capacity transmission. Furthermore, by setting the low-refractive-index portion to 0.5 times or more, more preferably 1.0 times or more the core diameter, the inter-core distance can be reduced by a further 0.7 μm or more, more preferably 2.0 μm or more, compared to when no low-refractive-index portion is used, making splicing even easier.

[0054] As explained above, in MCF1 and 1A, the crosstalk between counter-propagating cores at a wavelength of 1550 nm is reduced to -30 dB / 100 km or less, and more preferably -40 dB / 100 km or less. Generally, when splicing MCFs together, the angular positions around the central axis of the MCFs must be aligned so that all cores face each other. Any angular misalignment causes the central positions of the cores to be spliced to shift, resulting in splice loss (fusion loss).

[0055] The MCFs described in Non-Patent Documents 1 and 2 have a large inter-core distance, which tends to increase splice loss. To suppress fusion loss due to angular misalignment, it is necessary to reduce the inter-core distance. Therefore, in MCFs 1 and 1A, the inter-core distance is set to 46.5 μm or less, more preferably 41 μm or less, and even more preferably 38 μm or less. This reduces the distance from the center of gravity of each core to the fiber axis C to 23.25 μm or less, more preferably 20.5 μm or less, and even more preferably 19 μm or less. By ideally aligning the two MCFs in a direction perpendicular to the fiber axis during splicing, core misalignment due to angular misalignment can be minimized when the centers of gravity of the cores of the two MCFs to be spliced are aligned. Therefore, in MCFs 1 and 1A, the increase in splice loss due to angular misalignment can be suppressed compared to the MCFs described in Non-Patent Documents 1 and 2.

[0056] The optical fiber preform of MCF1 is manufactured, for example, by integrally forming a pair of core portions that will become the pair of cores 10 and an optical cladding portion that will become the optical cladding 21, and then providing a physical cladding portion that will become the physical cladding 22 around the optical cladding portion. In this manufacturing method, it is easier to form a thick physical cladding portion than to form a thick optical cladding portion. This is also true for the manufacturing method of the optical fiber preform of MCF1A. In MCF1, 1A, the inter-core distance is short, so the optical cladding diameter can be made small and the physical cladding 22 can be made thicker accordingly. This improves productivity.

[0057] When wound into a circular loop with a radius of 30 mm, the bending loss of MCF1 and 1A at a wavelength of 1625 nm is 0.50 dB / 100 turns or less, and more preferably 0.25 dB / 100 turns or less. The cable cutoff wavelength is 1530 nm or less, and more preferably 1470 nm or less. This allows interconnection with widely used optical fibers conforming to ITU-T G.654A to D, and more preferably ensures interoperability even with the usual manufacturing variations in MCF.

[0058] The MCFs described in Non-Patent Documents 1 and 2 have a uniform cladding refractive index, which results in a narrow design solution range that satisfies the required characteristics for counter-propagating core-to-core crosstalk, bending loss, and cable cutoff wavelength. In contrast, MCFs 1 and 1A include an optical cladding 21 and a physical cladding 22, whose refractive index is equal to or greater than that of the optical cladding 21, in the cladding 20. This introduces a refractive index step at the interface between the optical cladding 21 and the physical cladding 22, thereby further reducing the inter-core distance. As a result, the design tolerances for bending loss and cable cutoff wavelength can be expanded. In other words, bending loss can be suppressed while improving the controllability of cable cutoff wavelength at the fiber design stage.

[0059] In the MCFs 1 and 1A, the relative refractive index difference of the physical cladding 22 with respect to the optical cladding 21 is 0.1% or more and 0.4% or less. Therefore, in the MCFs 1 and 1A, the inter-core distance can be reliably reduced. This reliably increases the design tolerances for bending loss and cable cutoff wavelength.

[0060] The MCF1A further includes a low refractive index section between the pair of cores 10. This reduces inter-core crosstalk and increases the design margin for crosstalk. By utilizing this margin, the inter-core distance can be further reduced while maintaining low crosstalk. This further suppresses the increase in connection loss due to angular misalignment.

[0061] The optical transmission system 40 includes the MCF1 or MCF1A, and therefore can suppress an increase in connection loss while reducing crosstalk between counter-propagating cores.

[0062] The MCF 1, 1A can be applied, for example, to a branch line branching off from the main line of an optical submarine cable. Because such a branch line has a shorter transmission distance than the main line, the crosstalk characteristics required of the fiber are relaxed. Therefore, it is easier to reduce the inter-core distance of the MCF 1, 1A. In the branch line, a pair of cores 10 allows signals propagating away from the branch point from the main line and signals propagating toward the branch point to propagate in opposite directions. Therefore, crosstalk between counter-propagating cores can be easily suppressed.

[0063] In the MCFs 1 and 1A, the diameter, material, additive element, or refractive index profile of the pair of cores 10 may be different from each other. By making the characteristics of the pair of cores 10 different from each other, the propagation constants of the light waves propagating through each core 10 can be made different. Therefore, crosstalk between counter-propagating cores can be suppressed compared to when the characteristics of the pair of cores 10 are identical. Furthermore, the inter-core distance required to ensure the same level of crosstalk characteristics as when the pair of cores 10 are identical can be reduced. This makes it possible to reduce connection loss due to angular misalignment. [Explanation of symbols]

[0064] 1,1A…MCF 1a, 1b...Fiber end 10...Core 20...Clad 21...Optical cladding 22...Physical Cladding 30...Low refractive index section 40...Optical transmission system 41, 42, 43, 44...Optical transceiver 45,46...Signal light 50A, 50B...Measurement system 51, 53...Optical transmitter 52, 54...Optical receiver 55,56...Incoming light 55a,56a…light C...Fiber axis C1, C2...Transmission path

Claims

1. A pair of cores; a cladding covering both of the pair of cores; a low refractive index portion disposed between the pair of cores, The diameter of the cladding is 124 μm or more and 126 μm or less, the center-to-center distance of the pair of cores is 46.5 μm or less; Counter-propagating core-to-core crosstalk at a wavelength of 1550 nm is −30 dB / 100 km or less; The cladding is an optical cladding covering the pair of cores; a physical cladding surrounding the optical cladding and having a refractive index equal to or greater than the refractive index of the optical cladding; the physical cladding comprises silica glass; the low refractive index portion has a refractive index lower than the refractive index of the optical cladding; The crosstalk between parallel propagating cores at a wavelength of 1550 nm is −10 dB / 100 km or less, The effective cross-sectional area is 70 μm 2 or more and 130 μm 2 or less. Multicore optical fiber.

2. When wound into a circular ring shape with a radius of 30 mm, the bending loss at a wavelength of 1625 nm is 0.25 dB / 100 turns or less. The multi-core optical fiber according to claim 1 .

3. The cable cutoff wavelength is 1530 nm or less. The multi-core optical fiber according to claim 1 or 2.

4. a relative refractive index difference of the physical cladding with respect to the optical cladding is 0.1% or more and 0.4% or less; The multi-core optical fiber according to claim 1 .

5. The multi-core optical fiber according to any one of claims 1 to 4, an optical transmitter and an optical receiver connected to both ends of each of the pair of cores, Optical transmission system.

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