Multicore optical fiber

Abstract

This multicore optical fiber comprises: four cores each extending along the central axis of the multicore optical fiber; a common cladding surrounding the four cores and having a lower refractive index than the refractive index of each of the four cores; and a costing resin surrounding the common cladding. The diameter of the common cladding is 124.5-125.5 μm inclusive. In each of the four cores, Aeff satisfies expression (1) and expression (2), and λcc satisfies expression (1) and expression (3).　The four cores are disposed such that Dc satisfies expression (4) with respect to Aeff and λcc of each of the first core and the second core, and OCT, Aeff, and λcc of the first core satisfy expression (5).

Description

[Technical Field]

[0001] This disclosure relates to a multi-core optical fiber. This application claims priority to Japanese Application No. 2022-192004, filed on November 30, 2022, and incorporates by reference all the contents of said Japanese application. [Background technology]

[0002] Patent Document 1 and Non-Patent Document 1 describe a step-index multi-core optical fiber (hereinafter, MCF) having four cores and a common cladding. Patent Document 2 describes an MCF in which crosstalk (hereinafter, XT) between cores is suppressed by providing a first cladding region with a low refractive index between the cores and the common cladding. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-88458 [Patent Document 2] Japanese Patent Publication No. 2020-86054 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-509171 [Non-patent literature]

[0004] [Non-Patent Document 1] T. Matsui et al., “STEP-INDEX PROFILE MULTI-CORE FIBRE WITH STANDARD 125μM CLADDING TO FULL-BAND APPLICATION”, ECOC 2019, M. 1. D. 3 [Non-patent document 2] Y. Sagae et al., “Ultra-Low-XT Multi-Core Fiber with Standard 125-μm Cladding for Long-Haul Transmission”, OECC 2019, TuC3-4 [Non-patent document 3] T. Hayashi et al., “Uncoupled Multi-core Fiber Design for Practical Bidirectional Optical Communications,” OFC, M1E. 1 [Non-patent document 4] RJ Black and C. Pask, J. Opt. Soc. Am. A, JOSAA 1(11), p.1129-1131, 1984 [Non-patent document 5] Y. Kobayashi and T. Hayashi, “Behavior and measurement method of inter-core crosstalk in multicore fibers with core-dependent loss,” Opt. Express 31(1), pp.502-508 (2023). Summary of the Invention

[0005] The MCF of the present disclosure comprises four cores each extending along the central axis of the MCF, a common cladding surrounding the four cores and having a refractive index lower than that of each of the four cores, and a coating resin surrounding the common cladding. The diameter of the common cladding is 124.5 μm or more and 125.5 μm or less. The effective area of each of the four cores at a wavelength of 1550 nm is defined as Aeff [μm 2 ] and the cable cutoff wavelength is λcc [μm], Aeff satisfies the formula (1) and formula (2), and λcc satisfies the formula (1) and formula (3). JPEG2024116792000011.jpg8149 JPEG2024116792000012.jpg8149 JPEG2024116792000013.jpg8149 The four cores are arranged so that, in a cross section perpendicular to the central axis, if the center-to-center distance between a first core, which is one of the four cores, and the second core located closest to the first core is Dc [μm] and the shortest distance between the interface between the common cladding and the coating resin and the center of the first core is OCT [μm], Dc satisfies equation (4) for both Aeff and λcc of the first core and the second core, and the OCT, Aeff, and λcc of the first core satisfy equation (5). JPEG2024116792000014.jpg14149 JPEG2024116792000015.jpg14149 [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a cross-sectional view perpendicular to the central axis of an MCF according to an embodiment. [Figure 2] Figure 2 shows the relationship between the minimum value of the adjacent core spacing where XT is 10-4 / (100km)2 or less during counter propagation at a wavelength of 1625nm and the effective cross-sectional area, plotted for multiple cable cutoff wavelengths. [Figure 3] FIG. 3 is a plot of the relationship between the lower limit of the outer cladding thickness at which the leakage loss is 0.001 dB / km or less at a wavelength of 1625 nm and the effective cross-sectional area for a number of cable cutoff wavelengths. [Figure 4] FIG. 4 is a plot of the relationship between the lower limit of the outer cladding thickness at which the leakage loss is 0.0005 dB / km or less at a wavelength of 1625 nm and the effective cross-sectional area for a number of cable cutoff wavelengths. [Figure 5]Figure 5 is a graph showing the relationship between the upper limit of the effective cross-sectional area and the cable cutoff wavelength, for a number of cladding diameters, where there exists a central design core pitch value for which XT at a wavelength of 1625 nm satisfies -40 dB / km or less and leakage loss is 0.001 dB / km or less, even when the core pitch varies by ±1 μm from the central design value. [Figure 6] Figure 6 is a graph showing the relationship between the upper limit of the effective cross-sectional area and the cable cutoff wavelength for a range of cladding diameters, where the core pitch design center value satisfies XT of 10 -4 / km or less at a wavelength of 1625 nm and the leakage loss is 0.0005 dB / km or less, even when the core pitch varies by ±1 μm from the design center value. [Figure 7] FIG. 7 is a cross-sectional view perpendicular to the central axis of the MCF according to the first modification. [Figure 8] FIG. 8 is a cross-sectional view perpendicular to the central axis of the MCF according to the second modification. [Figure 9] FIG. 9 is a diagram showing a refractive index profile around the core that can be applied to the MCF of the present disclosure. [Figure 10] FIG. 10 is a diagram showing a refractive index profile around the core that can be applied to the MCF of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0007] [Problem to be solved by this disclosure] Non-Patent Document 2 describes that the MCF described in Patent Document 2 experiences increased loss in the region of 1580 nm or higher. As such, conventional technologies have not been able to fully suppress leakage loss at a wavelength of 1625 nm. Therefore, when transmitting over long distances using the L-band (wavelengths from 1565 nm to 1625 nm), transmission loss increases, resulting in degradation of signal transmission quality.

[0008] The present disclosure provides an MCF that can suppress degradation of signal transmission quality in the wavelength range of 1530 nm or more and 1625 nm or less.

[0009] [Effects of this disclosure] The MCF of the present disclosure can suppress degradation of signal transmission quality in the wavelength range of 1530 nm or more and 1625 nm or less.

[0010] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described. (1) An MCF according to one embodiment of the present disclosure comprises four cores each extending along a central axis of the MCF, a common cladding surrounding the four cores and having a refractive index lower than that of each of the four cores, and a coating resin surrounding the common cladding, the diameter of the common cladding being 124.5 μm or more and 125.5 μm or less, and each of the four cores has an effective cross-sectional area Aeff [μm 2 ] and the cable cutoff wavelength is λcc [μm], Aeff satisfies the formula (1) and formula (2), and λcc satisfies the formula (1) and formula (3). The four cores are arranged so that, in a cross section perpendicular to the central axis, if the center-to-center distance between a first core, which is one of the four cores, and the second core located closest to the first core is Dc [μm] and the shortest distance between the interface between the common cladding and the coating resin and the center of the first core is OCT [μm], Dc satisfies equation (4) for both Aeff and λcc of the first core and the second core, and the OCT, Aeff, and λcc of the first core satisfy equation (5). JPEG2024116792000016.jpg8149 JPEG2024116792000017.jpg8149 JPEG2024116792000018.jpg8149 JPEG2024116792000019.jpg14149 JPEG2024116792000020.jpg14149 In this MCF, the XT during parallel propagation between the first and second cores at a wavelength of 1625 nm is 10 -4 / km or less. Furthermore, the leakage loss at a wavelength of 1625 nm is 0.001 dB / km or less. Therefore, degradation of signal transmission quality at the wavelength of 1625 nm can be suppressed. With this MCF, degradation of signal transmission quality can be suppressed in long-distance transmission by counter propagation at least in the wavelength range of 1530 nm to 1625 nm.

[0011] (2) In the above (1), Aeff may satisfy the formula (6) and λcc may satisfy the formula (7) in each of the four cores. JPEG2024116792000021.jpg8149 JPEG2024116792000022.jpg8149 In this case, degradation of signal transmission quality can be suppressed in long-distance counter-propagation transmission at least in the wavelength range of 1460 nm to 1625 nm, i.e., in the wavelength range of 1460 nm to 1625 nm or in the wavelength range of 1530 nm to 1625 nm.

[0012] (3) In the above (1) or (2), λcc may satisfy equation (8) in each of the four cores. JPEG2024116792000023.jpg8149 In this case, the confinement in the core can be strengthened, and therefore the XT and leakage loss can be further suppressed.

[0013] (4) In any one of the above (1) to (3), the four cores may be arranged so that the OCT, Aeff, and λcc of the first core satisfy formula (9). JPEG2024116792000024.jpg14149 In this case, the leakage loss at a wavelength of 1625 nm is 0.0005 dB / km or less.

[0014] (5) In any one of (1) to (4) above, in the cross section, the centers of the four cores may be arranged at the four vertices of a square with a side length of Dc. In this case, the four cores are arranged symmetrically, and therefore the optical properties of the four cores can be made uniform.

[0015] (6) In any one of (1) to (4) above, in the cross section, the centers of the four cores may be located at the four vertices of an isosceles trapezoid with three sides of length Dc and one side longer than Dc. In this case, the cores can be identified without providing a marker.

[0016] (7) In any one of the above (1) to (6), the common cladding may be provided in contact with the outer peripheral surfaces of the four cores. In this case, a complex refractive index structure is not used, thereby improving manufacturability.

[0017] (8) In the above (7), the relative refractive index difference of each of the four cores with respect to the refractive index of the common cladding may be 0.50% or less. In this case, manufacturability can be improved by not using a deep depressed cladding or a refractive index trench. In addition, transmission loss can be reduced by not making the refractive index difference between the cores and the common cladding excessive.

[0018] (9) Any one of the MCFs (1) to (6) above may further include four individual claddings surrounding each of the four cores inside the common cladding, and when the relative refractive index difference of each of the four individual claddings based on the refractive index of the common cladding is Δic [%], Δic may satisfy equation (10). JPEG2024116792000025.jpg8149 In this case, manufacturability can be improved by not using deep depressed cladding or index trenches.

[0019] (10) Any one of the MCFs (1) to (6) above may further include four individual claddings surrounding each of the four cores inside the common cladding, and the relative refractive index difference of the cores inside the individual claddings based on the refractive index of each individual cladding may be 0.50% or less. In this case, manufacturability can be improved by not using a deep depressed cladding or a refractive index trench. Furthermore, transmission loss can be reduced by not making the refractive index difference between the cores and the individual claddings excessive.

[0020] [Details of the embodiments of the present disclosure] Specific examples of MCFs according to the present embodiment will be described with reference to the drawings as necessary. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. In the description of the drawings, identical elements are given the same reference numerals, and redundant explanations will be omitted.

[0021] Fig. 1 is a cross-sectional view perpendicular to the central axis of an MCF according to an embodiment. As shown in Fig. 1, the MCF 1 according to the embodiment is a four-core fiber including four cores 2, a common cladding 3, and a coating resin 4. The cores 2 are made of glass containing silica as a main component. In the cross-section perpendicular to the central axis AX, the four cores 2 have the same circular shape. Each of the four cores 2 extends along the central axis AX of the MCF 1.

[0022] In a cross section perpendicular to the central axis AX, if the center-to-center distance between a first core, which is one of the four cores 2, and the second core located closest to the first core is Dc [μm], Dc has the same value regardless of which of the four cores 2 is the first core. In other words, the centers of the four cores 2 are located at the four vertices of a square with a side length of Dc [μm]. The first core and the second core are adjacent cores, and Dc is the center-to-center distance between the adjacent cores. In the MCF1, the four cores 2 are arranged symmetrically, which allows for uniform optical properties among the four cores 2.

[0023] For each of the four cores 2, the effective cross section at a wavelength of 1550 nm is defined as Aeff [μm 2 ] and the cable cutoff wavelength is λcc [μm], Aeff satisfies formula (1) and formula (2), and λcc satisfies formula (1) and formula (3). Note that the Aeff of the four cores 2 may be the same as or different from one another. Also, the λcc of the four cores 2 may be the same as or different from one another. JPEG2024116792000026.jpg8149 JPEG2024116792000027.jpg8149 JPEG2024116792000028.jpg8149 Aeff is 70 μm 2 By satisfying the above, it is possible to suppress the deterioration of signal transmission quality due to nonlinear interference. 2 In order for λcc to be equal to or greater than 1270 nm, λcc must be equal to or greater than 1270 nm. By keeping λcc equal to or less than 1530 nm, single mode operation can be achieved in the C band (wavelengths from 1530 nm to 1565 nm) and the L band (wavelengths from 1565 nm to 1625 nm). This makes it possible to realize an optical fiber suitable for optical signal transmission in the C band and the L band. In formula (1), in order for λcc to be equal to or less than 1530 nm, Aeff must be equal to or greater than 101.2 μm. 2 It must be the following:

[0024] In each of the four cores 2, Aeff may satisfy equation (6), and λcc may satisfy equation (7). In each of the four cores 2, λcc may further satisfy equation (8). JPEG2024116792000029.jpg8149 JPEG2024116792000030.jpg8149 JPEG2024116792000031.jpg8149 In formula (1), in order for λcc to be 1460 nm or less, Aeff must be 93.0 μm 2 It must be the following:

[0025] The four cores 2 are such that Dc is equal to Aeff and λcc of any of the first core and the second core. are arranged so as to satisfy the formula (4). JPEG2024116792000032.jpg14149 This resulted in an XT of 10 for parallel propagation between adjacent cores at a wavelength of 1625 nm. -4 Therefore, the XT (opposite XT) during counter propagation between adjacent cores at a wavelength of 1625 nm is 10 -4 / (100km) 2 This makes it possible to sufficiently suppress the degradation of signal quality caused by XT during counter propagation. Note that in parallel propagation, the transmission direction of optical signals is the same between adjacent cores. In counter propagation, the transmission direction of optical signals is different between adjacent cores.

[0026] In particular, in long-distance transmission using counter propagation, the transmission loss (span loss) of the optical fiber between adjacent amplifier repeaters (spans) is small. Therefore, based on Non-Patent Document 3, the dominant XT that is allowable from the perspective of suppressing signal quality degradation is indirect XT, where parallel-propagating optical signals become XT via adjacent cores. Within each span, indirect XT is proportional to the square of the span length. Therefore, here, 1 / (100 km) 2 The unit is 10. -4 / (100km) 2 is 10 -8 / km 2 The accumulation of indirect XT over multiple spans is linear, i.e., a simple sum of the indirect XT at each span. Therefore, when counterpropagation XT is 10 -4 / (100km) 2 If the XT ratio is less than 10, the counter-propagation multi-core optical fiber transmission system with an average span length of 100 km or less can achieve a counter-propagation XT ratio of approximately 10 -4 / km or less. For the entire transmission system, signal quality degradation due to XT can be suppressed regardless of the optical fiber length (or number of spans). In other words, noise caused by XT can be suppressed more effectively than noise caused by optical amplifiers or nonlinear interference.

[0027] Regarding XT during parallel propagation, the level at which the optical signal transmission quality deteriorates, i.e., 10 -4 / km or less, the XT during counter propagation is set to a level that suppresses degradation of optical signal transmission quality. This makes it possible to achieve a value that suppresses leakage loss for OCT, which will be described later.

[0028] Figure 2 shows the XT of 10 in counter-propagation at a wavelength of 1625 nm. -4 / (100km) 2 The relationship between the lower limit of Dc (Dmin) and Aeff is plotted for a number of λccs, where λcc is 1.26 μm, 1.36 μm, 1.46 μm, and 1.53 μm. The horizontal axis of FIG. 2 is Aeff [μm 2 ], and the vertical axis represents Dmin [μm]. For example, when λcc = 1.53 μm, the relationship between Dmin and Aeff must be in the range above the bottom curve in Figure 2.

[0029] The relationship shown in Figure 2 is summarized in Equation (4). Figure 2 was obtained by creating multiple combinations of Aeff, λcc, and Dmin. Specifically, the radius ra of core 2 and the core 2 By varying the relative refractive index difference Δ, a number of combinations of Aeff and λcc were created, and Dmin for each combination was calculated.

[0030] The common cladding 3 surrounds the four cores 2. The common cladding 3 is provided in contact with the outer peripheral surfaces of the four cores 2. No depressed cladding is provided between the cores 2 and the common cladding 3. As such, the MCF 1 does not use a complex refractive index structure, which improves manufacturability.

[0031] The common cladding 3 is made of glass containing silica as its main component. The common cladding 3 has a refractive index lower than that of each of the four cores 2. To create a refractive index difference between the cores 2 and the common cladding 3, the cores 2 may be doped with germanium (Ge). Alternatively, the common cladding 3 may be doped with fluorine (F). Doping the cores 2 and the common cladding 3 with a small amount of F allows a depressed profile to be achieved with good manufacturability.

[0032] The relative refractive index difference Δc of each core 2 is 0.50% or less, based on the refractive index of the cladding in contact with the core 2. In this embodiment, since the common cladding 3 is in contact with each core 2, the cladding that serves as the reference for the refractive index is the common cladding 3. The relative refractive index difference Δc of each core 2 based on the refractive index of the common cladding 3 is indicated by Δ1. In other words, the relative refractive index difference Δ1 of each of the four cores 2 based on the refractive index of the common cladding 3 is 0.50% or less. Since a deep depressed cladding or refractive index trench is not used, the manufacturability of the MCF 1 can be improved. By preventing an excessive refractive index difference between the cores 2 and the common cladding 3, transmission loss can be reduced.

[0033] The diameter (cladding diameter) of the common cladding 3 is 124.5 μm or more and 125.5 μm or less. The common cladding 3 has the same diameter as the cladding diameter of a widely used general-purpose single-mode optical fiber. Diameter Therefore, it is possible to realize the same ease of handling and mechanical reliability.

[0034] The coating resin 4 surrounds the common clad 3. The coating resin 4 is provided in contact with the outer peripheral surface of the common clad 3. The coating resin 4 is made of, for example, an ultraviolet curable resin.

[0035] The four cores 2 are arranged so that, in a cross section perpendicular to the central axis AX, the OCT (outer cladding thickness) [μm] is the shortest distance between the interface between the common cladding 3 and the coating resin 4 and the center of the first core, the OCT, Aeff, and λcc of the first core satisfy equation (5). JPEG2024116792000033.jpg14149 This makes it possible to reduce the leakage loss at a wavelength of 1625 nm to 0.001 dB / km or less.

[0036] The four cores 2 may be arranged so that the OCT, Aeff, and λcc of the first core satisfy equation (9). JPEG2024116792000034.jpg14149 This makes it possible to reduce the leakage loss at a wavelength of 1625 nm to 0.0005 dB / km or less.

[0037] Figure 3 is a graph plotting the relationship between the lower limit of OCT (OCTmin) at which the leakage loss is 0.001 dB / km or less at a wavelength of 1625 nm and Aeff for multiple λcc. The multiple λcc are 1.26 μm, 1.36 μm, 1.46 μm, and 1.53 μm. The horizontal axis of Figure 3 is Aeff [μm 2 ], and the vertical axis represents OCTmin [μm]. For example, when λcc = 1.53 μm, the relationship between OCTmin and Aeff must be in the range above the bottom curve in Figure 3.

[0038] Equation (5) summarizes the relationship shown in Figure 3. Figure 3 was obtained by creating multiple combinations of Aeff, λcc, and OCTmin. Specifically, multiple combinations of Aeff and λcc were created by varying the radius ra of the core 2 and the relative refractive index difference Δ1 of the core 2 based on the refractive index of the common cladding 3, and OCTmin was calculated for each combination.

[0039] Figure 4 is a graph plotting the relationship between the lower limit of OCT (OCTmin) at which the leakage loss is 0.0005 dB / km or less at a wavelength of 1625 nm and Aeff for multiple λcc. The multiple λcc are 1.26 μm, 1.36 μm, 1.46 μm, and 1.53 μm. The horizontal axis of Figure 4 is Aeff [μm 2 ], and the vertical axis represents OCTmin [μm]. For example, when λcc = 1.53 μm, the relationship between OCTmin and Aeff must be in the range above the bottom curve in Figure 4.

[0040] The relationship shown in Figure 4 is summarized in Equation (9). Figure 4 was obtained by creating multiple combinations of Aeff, λcc, and OCTmin. Specifically, the radius ra of core 2 and the core 2 By varying the relative refractive index difference Δ, multiple combinations of Aeff and λcc were created, and the OCTmin for each combination was calculated.

[0041] The diameter of the common cladding 3 is 124.5 μm or more and 125.5 below Therefore, the leakage loss is greatest when the diameter of the common cladding 3 is 124.5 μm. Even if the diameter of the common cladding 3 is 124.5 μm and the core pitch (Dc) varies by ±1 μm from the design center value, the opposite XT is 10 -4 / (100km) 2 below The relationship between Aeff and λcc where there is a design center value of Dc where the leakage loss is 0.001 dB / km or less is expressed by equation (1). That is, if the relationship between Aeff and λcc satisfies equation (1), when the diameter of the common cladding 3 is 124.5 μm or more and 125.5 μm or less, even if the core pitch (Dc) varies by ±1 μm from the design center value, the opposing XT at a wavelength of 1625 nm will be 10 -4 / (100km) 2 below There is a central design value for Dc that satisfies the above and results in a leakage loss of 0.001 dB / km or less.

[0042] Figure 5 shows that for multiple cladding diameters, even if the core pitch varies by ±1 μm from the design center value, the opposing XT at a wavelength of 1625 nm remains within 10 -4 / (100km) 2 5 is a graph showing the relationship between the upper limit of Aeff and λcc, which satisfies the following conditions and results in a leakage loss of 0.001 dB / km or less. The cladding diameters were set in the range of 124.5 μm or more and 125.5 μm or less. The cladding diameters were 124.5 μm, 124.75 μm, 125 μm, 125.25 μm, and 125.5 μm. The horizontal axis of FIG. 5 represents λcc [μm], and the vertical axis represents the upper limit of Aeff. In FIG. 5, the innermost part of the approximately triangular region represented by the curved line and the straight line corresponds to the range that satisfies Equation (1), Equation (2), and Equation (3), respectively.

[0043] Figure 6 shows that for multiple cladding diameters in the range of 124.5 μm to 125.5 μm, even if the core pitch varies by ±1 μm from the design center value, the opposing XT remains 10 at a wavelength of 1625 nm. -4 / (100km) 2 below 6 is a graph showing the relationship between λcc and the upper limit of Aeff, where the leakage loss is 0.0005 dB / km or less and the value satisfies the above equation. The horizontal axis of FIG. 6 represents λcc [μm], and the vertical axis represents the upper limit of Aeff. In FIG. 6, the innermost part of the approximately triangular region shown by the curved line and the straight line corresponds to the range that satisfies Equation (1), Equation (6), and Equation (7), respectively.

[0044] When formulas (1), (2), and (3) are satisfied, and formula (5) is also satisfied, the bending loss when bending with a radius of 25 mm or more is significantly less than 0.1 dB per 100 turns. Therefore, even if the excess length of MCF1 is wound with a radius of 25 mm or more and stored in a repeater or station building, an increase in loss can be suppressed.

[0045] As described above, in the MCF1 according to this embodiment, XT during parallel propagation between adjacent cores at a wavelength of 1625 nm is 10 -4 / km or less. Furthermore, the leakage loss at a wavelength of 1625 nm is 0.001 dB / km or less. Therefore, degradation of signal transmission quality at the wavelength of 1625 nm can be suppressed. With the MCF1, degradation of signal transmission quality can be suppressed in long-distance transmission by counter propagation at least in the wavelength range of 1530 nm to 1625 nm.

[0046] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

[0047] FIG. 7 is a cross-sectional view perpendicular to the central axis of the MCF according to the first modification. Figure 7 As shown in Fig. 1, in the MCF1A according to the first modification, in a cross section perpendicular to the central axis AX, the centers of the four cores 2 are arranged at the four vertices of an isosceles trapezoid with three sides of length Dc and one side longer than Dc. In the MCF1A, the cores 2 can be identified without providing markers.

[0048] FIG. 8 is a cross-sectional view perpendicular to the central axis of the MCF according to the second modification. Figure 8 As shown in Fig. 1, the MCF 1B according to the second modification further includes four individual claddings 5. The four individual claddings 5 surround the four cores 2, respectively, inside the common cladding 3. When the relative refractive index difference of each of the four individual claddings 5 with respect to the refractive index of the common cladding 3 is Δic [%], Δic satisfies formula (10). JPEG2024116792000035.jpg8149 As described above, the relative refractive index difference Δc of each core 2 is 0.50% or less, with the refractive index of the cladding in contact with the core 2 as the reference. In this modification, the cladding that serves as the reference for the refractive index is the corresponding individual cladding 5, i.e., the individual cladding 5 that surrounds the core 2 from the inside. MCF1B also does not use a deep depressed cladding or a refractive index trench, which improves manufacturability. By preventing an excessive refractive index difference between the core 2 and the corresponding individual cladding 5, transmission loss can be reduced.

[0049] FIG. 9 is a diagram showing a refractive index profile around a core that can be applied to the MCF of the present disclosure. Regarding the core structure of the MCF of the present disclosure, an appropriate structure can be selected for the core refractive index profile and the associated optical characteristics depending on the application. For example, the refractive index profiles of patterns (A) to (J) shown in FIG. 9 are applicable. In FIG. 9, Δ is the relative refractive index difference based on the refractive index of the common cladding, and r is the radius from the center of each core, shown in a local coordinate system with the origin O being the center of each core, Δ=0%. The structures of the cores may be the same or different.

[0050] 9, pattern (A) is a step-type refractive index profile, pattern (B) is a ring-type refractive index profile, pattern (C) is a double-step-type refractive index profile, pattern (D) is a graded-type refractive index profile, and pattern (E) is a tapered-type refractive index profile, and these are applicable to the core structure of the MCF of the present disclosure. Furthermore, patterns (F) and (H) in which a depressed-type refractive index profile is provided around the core, patterns (G), (I) and (J) in which a raised-type refractive index profile is provided around the core, and pattern (E) in which a matched-type refractive index profile is provided around the core are also applicable to the core structure.

[0051] Patterns (A), (B), (C), and (D) correspond to the MCF1 according to the embodiment and the MCF1A according to the first modified example. Patterns (F) and (H) that satisfy formula (10) correspond to the MCF1B according to the second modified example.

[0052] For refractive index profiles other than the step-type refractive index profile of pattern (A), the ESI (equivalent-step-index) approximation can be used to calculate the core radius ra and relative refractive index difference Δ1 when approximated by a step type (Non-Patent Document 4). Non-Patent Document 4 can be easily applied when the boundary between the core and the cladding is clear. For the refractive index profiles of patterns (E), (H), (I), and (J), where the boundary between the core and the common cladding is unclear, the ESI approximation can be performed by regarding r, where dΔ / dr of the refractive index profile before ESI approximation is smallest (the slope downward to the right is steepest), as the core radius of the core before approximation. The relative refractive index difference (Δic) of the individual cladding can be the average value of the relative refractive index difference of the core before approximation in the individual cladding portion. In other words, the average value from ra to rb on the horizontal axis (r axis) can be used: (rb - ra) / 3 + ra ≦ r ≦ 2(rb - ra) / 3 + ra. 2rb is the diameter of the individual cladding.

[0053] FIG. 10 is a diagram showing a refractive index profile around a core applicable to the MCF of the present disclosure. In FIG. 9, Δ is the relative refractive index difference based on the refractive index of the common cladding, whereas in FIG. 10, Δ is the relative refractive index difference based on the refractive index of the cladding in contact with the core. Therefore, in FIG. 10, the relative refractive index difference of the core is indicated by Δc. In patterns (A) to (E) shown in FIG. 10, the cladding that serves as the reference for the refractive index is the common cladding, so the refractive index profiles are substantially the same as patterns (A) to (E) shown in FIG. 9. In patterns (F) to (J) shown in FIG. 10, a cladding other than the common cladding is provided in contact with the core, so the refractive index profiles are different from patterns (F) to (J) shown in FIG. 9.

[0054] The refractive index profile around the core is not limited to the refractive index profiles of patterns (A) to (J) shown in FIGS. 9 and 10, respectively.

[0055] The features and characteristics of the MCF disclosed herein can be measured by the following methods. The refractive indices of the core, common cladding, and individual cladding can be measured, for example, by the refraction near-field method or the lateral interferometry. The diameter of the common cladding can be measured, for example, by the refraction near-field method or the lateral interferometry, or from a microscope image (transmission near-field method) of the MCF cross section. The effective area Aeff can be measured, for example, by the Appendix of ITU-T G.650.2 (08 / 2015). III The cable cutoff wavelength λcc can be measured, for example, by the method described in Section 6.3 of ITU-T G.650.1 (10 / 2020). The center-to-center spacing Dc between the first core and the second core located closest to the first core can be measured, for example, by the refraction near-field method, lateral interferometry, or a microscopic image of the MCF cross section (transmission near-field method). The shortest distance OCT between the interface between the common cladding and the coating resin and the center of the first core can be measured, for example, by the refraction near-field method, lateral interferometry, or a microscopic image of the MCF cross section (transmission near-field method). XT during parallel propagation can be measured by the method described in Non-Patent Document 5. XT during counter-propagation can be predicted from XT during parallel propagation based on the formula described in Non-Patent Document 3. Leakage loss can be measured by the method described in Patent Document 3. The components constituting a multicore fiber can be measured by X-ray fluorescence analysis.

[0056] Although units are omitted in the above formulas, the units of the constants and coefficients in formulas (1) to (10) are as follows, where [-] indicates a dimensionless quantity. Formula (1):-11.919[-],153.67[μm],-105.98[μm] 2 ] Formula (2): 70[μm 2 ],101.2[μm 2 ] Formula (3): 1.270 [μm], 1.530 [μm] Equation (4): 62.67[μm],-44.75[-],0.2217[μm] -1 ],9.911[μm -1 ],-8.461×10 -4 [μm -3 ], 3.981×10 -2 [μm -2 ] Equation (5): 76.53[μm],-70.55[-],0.3821[μm] -1 ],19.56[μm -1 ],-6.480×10 -4 [μm -3 ],7.279×10 -2 [μm -2 ] Formula (6): 70[μm 2 ],93.0[μm 2 ] Equation (7): 1.270 [μm], 1.460 [μm] Formula (8): 1.360[μm] Equation (9): 78.90[μm],-72.75[-],0.3936[μm] -1 ],20.14[μm -1 ], -6.704×10 -4 [μm -3 ],7.480×10 -2 [μm -2 ] Formula (10):-0.20[%],0[%]

[0057] Furthermore, the symbols for the physical quantities mentioned above are considered not to include units, and the symbols are considered to represent numerical values in the units listed next to the symbols. Even if equations (1) to (10) are considered dimensionless, the equations still hold. In this case, the numerical value of λcc used in the equations is a numerical value in [μm]. In the above, the numerical value of λcc is sometimes listed in [nm] for comparison with the wavelength of the optical signal. [Explanation of symbols]

[0058] 1,1A,1B...MCF 2...Core 3...Common clad 4...Coating resin 5...Individual cladding AX…Central axis Dc: Center-to-center spacing of adjacent cores OCT: Outer cladding thickness Δ…relative refractive index difference Δc: Cladding in contact with the core Refractive index of The relative refractive index difference of the core based on Δ1: Relative refractive index difference of the core based on the refractive index of the common cladding Δic: relative refractive index difference of individual cladding ra...core radius rb… Individual Cladding Radius

Claims

1. Four cores extending along the central axis of a multicore optical fiber, A common cladding surrounds the four cores and has a refractive index lower than that of each of the four cores, The coating resin surrounding the aforementioned common cladding, Equipped with, The diameter of the common cladding is 124.5 μm or more and 125.5 μm or less. In each of the four cores, the effective cross-section at a wavelength of 1550 nm is Aef [μm]. ２ Let ] and the cable cutoff wavelength be λcc [μm], then Aef satisfies equations (1) and (2), and λcc satisfies equations (1) and (3), The four cores are arranged such that, in a cross-section perpendicular to the central axis, the distance between the centers of one of the four cores, the first core, and the second core located closest to the first core is Dc [μm], and the shortest distance between the interface between the common cladding and the coating resin and the center of the first core is OCT [μm], such that Dc satisfies equation (4) for both the first core and the second core's Aef and λcc, and the OCT, Aef, and λcc of the first core satisfy equation (5). Multicore optical fiber. [Math 1] [Math 2] [Math 3] [Math 4] [Math 5]

2. In each of the four cores, Aeff satisfies equation (6) and λcc satisfies equation (7). A multicore optical fiber according to claim 1. [Math 6] [Number 7]

3. In each of the four cores, λcc satisfies equation (8), A multicore optical fiber according to claim 1. [Number 8]

4. The four cores are arranged such that the OCT, Aef, and λcc of the first core satisfy equation (9). A multicore optical fiber according to claim 1. [Number 9]

5. In the cross-section, the centers of the four cores are located at each of the four vertices of a square with side length Dc. A multicore optical fiber according to any one of claims 1 to 4.

6. In the cross-section, the centers of the four cores are located one at each of the four vertices of an isosceles trapezoid with three sides of length Dc and one side longer than Dc. A multicore optical fiber according to any one of claims 1 to 4.

7. The common cladding is provided in contact with the outer circumferential surfaces of the four cores. A multicore optical fiber according to any one of claims 1 to 4.

8. The difference in relative refractive index of each of the four cores, relative to the refractive index of the common cladding, is 0.50% or less. The multicore optical fiber according to claim 7.

9. Inside the common cladding, there are further four individual claddings surrounding each of the four cores, If Δic [%] is the difference in relative refractive index of each of the four individual claddings relative to the refractive index of the common cladding, then Δic satisfies equation (10). A multicore optical fiber according to any one of claims 1 to 4. [Number 10]

10. Inside the common cladding, there are further four individual claddings surrounding each of the four cores, The difference in the relative refractive index of the core inside each individual cladding, relative to the refractive index of each individual cladding, is 0.50% or less. A multicore optical fiber according to any one of claims 1 to 4.

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