Multi-core optical fiber

Abstract

A multi-core optical fiber (1) is provided with a plurality of cores (2), a first cladding (3), and a second cladding (4). The refractive index of the second cladding (4) is lower than the refractive index of the core (2) and higher than the refractive index of the first cladding (3). And the transmission loss at a wavelength of 1550 nm is 0.22 dB / km or less. When the effective cross-sectional area of the core (2) at a wavelength of 1550 nm is Aeff1 [[mu] m], the center spacing of two adjacent cores (2) in a cross-section orthogonal to the optical fiber axis is lambda [[mu] m], and the relative refractive index difference of the core (2) based on the refractive index of the first cladding (3) is [delta] core [%], the following condition 1a is satisfied by using Aeff1 defined by Aeff1 = 18.6-4.63 ln ([delta] core) + 1.24 lambda + 24.1 [ln ([delta] core)] 2-6.05 * 10-3 [lambda] 2-0.858 [lambda] ln ([delta] core). 0.35 [%] < = [delta] core [%] < = 1.00 [%]; 25 [mu] m < = [lambda] [mu] m < = 50 [mu] m; 0.80 Aeff1 < = Aeff1 < = 1.35 Aeff1.

Description

Technical Field

[0001] The present disclosure relates to a multi-core optical fiber. This application claims the benefit of priority based on Japanese Application No. 2023-012598, filed on January 31, 2023, and incorporates herein the entire contents of the aforementioned Japanese Application. Background Art

[0002] Non-patent documents 1 to 3 describe that the smaller the effective cross-sectional area (hereinafter also referred to as Aeff) of each core of a multi-core optical fiber (hereinafter also referred to as MCF), the more it can strengthen the restriction of light entering each core, and can reduce the inter-core power coupling coefficient (hereinafter also referred to as PCC) and inter-core crosstalk (hereinafter also referred to as XT). Non-patent document 1 describes a step-index (hereinafter also referred to as SI) type MCF, non-patent document 2 describes a groove-assisted MCF, and non-patent document 3 describes a hole-assisted MCF. In an uncoupled MCF that uses a single core as an independent optical spatial channel, reducing PCC will reduce the inter-core XT and improve the signal quality of each core.

[0003] Prior art literature

[0004] Non-patent literature

[0005] Non-Patent Literature 1: T. Hayashi et al., “Design strategy of uncoupled multicore fiber enabling high spatial capacity transmission”, SUM2013, MC2.4

[0006] Non-patent document 2: JHChang et al., "Achievable capacity improvement by using multi-level modulation format in trench-assisted multi-core fibersystem", OPTICS EXPRESS14262Vol.21, No.12,17June 2013

[0007] Non-Patent Document 3: F. Ye et al., “Capacity of Space-Division Multiplexing with Heterogeneous Multi-Core Fibers”, 2013 18th OECC / PS FC

[0008] Non-patent document 4: H. Yoshikawa et al., "Analysis of Two-Core Single-ModeOptical Fibers for Subscriber Lines", Electronics and Communications in Japan, Part 2, Vol. 72, No. 8, P. 45-53, 1989

[0009] Non-patent document 5: RJ Black et al., “Developments in the theory of equivalent-step-index fibers”, J. Opt. Soc. Am. A, vol. 1, No. 11, p. 1129-1131, November 1984 Summary of the Invention

[0010] The MCF disclosed in the present invention is a common depressed type MCF used for parallel propagation, comprising: a plurality of fiber cores; a common first cladding surrounding each of the plurality of fiber cores; and a second cladding surrounding the first cladding. The refractive index of the second cladding is lower than that of the fiber core and higher than that of the first cladding. The transmission loss at a wavelength of 1550nm is less than 0.22dB / km. When the effective cross-sectional area of the fiber core at a wavelength of 1550nm is set to Aeff[μm 2 ], assuming that the center distance between two adjacent cores in a cross section perpendicular to the fiber axis is Λ [μm], and the relative refractive index difference of the cores relative to the refractive index of the first cladding is Δcore [%], the formula Aeff1 = 18.6 - 4.63ln (Δcore) + 1.24Λ + 24.1 [ln (Δcore)] is used. 2 -6.05×10 -3 Λ 2 -0.858Λln(Δcore)

[0011] The defined Aeff1 satisfies the following condition 1a:

[0012] 0.35[%]≤Δcore[%]≤1.00[%]

[0013] 25[μm]≤Λ[μm]≤50[μm]

[0014] 0.80Aeff1≤Aeff≤1.35Aeff1. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a graph showing the relationship between PCC and Aeff of each core.

[0016] Figure 2 It is a diagram showing a cross section perpendicular to the optical fiber axis and a refractive index distribution of an MCF according to an embodiment.

[0017] Figure 3 This is a graph showing the relationship between the core radius and the relative refractive index difference of the core with respect to the refractive index of the first cladding.

[0018] Figure 4 It is a graph showing the relationship between PCC and Aeff of each core.

[0019] Figure 5 It is a graph that plots how much h increases when Aeff deviates from Aeff1.

[0020] Figure 6 It is a graph that plots how much h increases when Aeff deviates from Aeff1.

[0021] Figure 7 It is a diagram showing a cross section perpendicular to the optical fiber axis and a refractive index distribution of an MCF according to a modification. DETAILED DESCRIPTION

[0022] [Technical Problems to be Solved by the Present Disclosure]

[0023] The inventors discovered that in a so-called common-depression MCF, if Aeff is too small, the PCC between cores actually increases. A common-depression MCF is an MCF that has multiple cores, a common first cladding surrounding the multiple cores, and a second cladding surrounding the first cladding, where the refractive index of the second cladding is lower than that of the cores but higher than that of the first cladding.

[0024] An object of the present disclosure is to provide a common recessed MCF capable of reducing inter-core XT.

[0025] [Effects of the Present Disclosure]

[0026] According to the present disclosure, a common recessed MCF capable of reducing inter-core XT can be provided.

[0027] [Description of the Implementation Aspects of the Present Disclosure]

[0028] First, implementation aspects of the present disclosure are listed for description.

[0029] (1) The MCF involved in the first aspect of the present disclosure is a common depressed type MCF used for parallel propagation, comprising: a plurality of fiber cores; a common first cladding surrounding each of the plurality of fiber cores; and a second cladding surrounding the first cladding. The refractive index of the second cladding is lower than that of the fiber core and higher than that of the first cladding. The transmission loss at a wavelength of 1550 nm is less than 0.22 dB / km. When the effective cross-sectional area of the fiber core at a wavelength of 1550 nm is set to Aeff [μm 2 ], assuming that the center distance between two adjacent cores in a cross section perpendicular to the fiber axis is Λ [μm], and the relative refractive index difference of the cores relative to the refractive index of the first cladding is Δcore [%], the formula Aeff1 = 18.6 - 4.63ln (Δcore) + 1.24Λ + 24.1 [ln (Δcore)] is used. 2 -6.05×10 -3 Λ 2 -0.858Λln(Δcore)

[0030] Defined Aeff1[μm 2 ], then the following condition 1a is satisfied:

[0031] 0.35[%]≤Δcore[%]≤1.00[%]

[0032] 25[μm]≤Λ[μm]≤50[μm]

[0033] 0.80Aeff1≤Aeff≤1.35Aeff1.

[0034] In this MCF, the first cladding functions as a common notch. This reduces the PCC between two adjacent cores (hereinafter referred to as parallel propagation PCC) when signal light propagates in parallel, thereby reducing the XT between two adjacent cores (hereinafter referred to as parallel propagation XT) when signal light propagates in parallel. The increase in parallel propagation PCC and parallel propagation XT from their minimum values can be suppressed to less than 3 dB.

[0035] (2) In the above (1), condition 1a can also be further specified by the following formula:

[0036] 0.83Aeff1≤Aeff≤1.18Aeff1.

[0037] In this case, the parallel propagation PCC and the parallel propagation XT can be suppressed to an increase of 1 dB or less from the minimum value.

[0038] (3) In the above (1), condition 1a can also be further specified by the following formula:

[0039] 0.90Aeff1≤Aeff≤1.12Aeff1.

[0040] In this case, the parallel propagation PCC and the parallel propagation XT can be suppressed to an increase of 0.5 dB or less from the minimum value.

[0041] (4) In the above (1), condition 1a can also be further specified by the following formula:

[0042] 0.95Aeff1≤Aeff≤1.05Aeff1.

[0043] In this case, the parallel propagation PCC and the parallel propagation XT can be suppressed to an increase of 0.1 dB or less from the minimum value.

[0044] (5) The MCF involved in the second aspect of the present disclosure is a common depressed type MCF used for parallel propagation, comprising: a plurality of fiber cores; a common first cladding surrounding the plurality of fiber cores; and a second cladding surrounding the first cladding. The refractive index of the second cladding is lower than that of the fiber core and higher than that of the first cladding. The transmission loss at a wavelength of 1550 nm is less than 0.22 dB / km. When the effective cross-sectional area of the fiber core at a wavelength of 1550 nm is set to Aeff [μm 2 ], assuming that the center distance between two adjacent cores in a cross section perpendicular to the fiber axis is Λ [μm], and the relative refractive index difference of the cores relative to the refractive index of the first cladding is Δcore [%], the formula Aeff1 = 18.6 - 4.63ln (Δcore) + 1.24Λ + 24.1 [ln (Δcore)] is used. 2 -6.05×10 -3 Λ 2 -0.858Λln(Δcore)

[0045] Defined Aeff1[μm 2 ], then the following condition 2a is satisfied:

[0046] 0.30[%]≤Δcore[%]≤0.35[%]

[0047] 20[μm]≤Λ[μm]≤25[μm]

[0048] 0.88Aeff1≤Aeff≤1.35Aeff1.

[0049] In this MCF, the first cladding layer functions as a common notch. This reduces the parallel propagation PCC, thereby reducing the parallel propagation XT. The increase in the parallel propagation PCC and XT from their minimum values can be suppressed to a value of 3 dB or less.

[0050] (6) In (5) above, condition 2a can also be further specified by the following formula:

[0051] 0.90Aeff1≤Aeff≤1.18Aeff1.

[0052] In this case, the parallel propagation PCC and the parallel propagation XT can be suppressed to an increase of 1 dB or less from the minimum value.

[0053] (7) In (5) above, condition 2a can also be further specified by the following formula:

[0054] 0.92Aeff1≤Aeff≤1.12Aeff1.

[0055] In this case, the parallel propagation PCC and the parallel propagation XT can be suppressed to an increase of 0.5 dB or less from the minimum value.

[0056] (8) In (5) above, condition 2a can also be further specified by the following formula:

[0057] 0.96Aeff1≤Aeff≤1.05Aeff1.

[0058] In this case, the parallel propagation PCC and the parallel propagation XT can be suppressed to an increase of 0.1 dB or less from the minimum value.

[0059] (9) The MCF involved in the third aspect of the present disclosure is a common depressed type MCF used in counter-propagation, comprising: a plurality of cores; a common first cladding surrounding the plurality of cores; and a second cladding surrounding the first cladding. The refractive index of the second cladding is lower than that of the core and higher than that of the first cladding. The transmission loss at a wavelength of 1550 nm is less than 0.22 dB / km. When the effective cross-sectional area of the core at a wavelength of 1550 nm is set to Aeff [μm 2 ], assuming that the center distance between two adjacent cores in a cross section perpendicular to the fiber axis is Λ [μm], and the relative refractive index difference of the cores based on the refractive index of the first cladding is Δcore [%], the formula Aeff2 = 19.2-3.01ln(Δcore)+1.40Λ+25.6[ln(Δcore)] is used. 2 -7.72×10 -3 Λ 2 -0.980Λln(Δcore)

[0060] Defined Aeff2[μm 2 ], then the following condition 1b is met:

[0061] 0.35[%]≤Δcore[%]≤1.00[%]

[0062] 25[μm]≤Λ[μm]≤50[μm]

[0063] 0.75Aeff2≤Aeff≤1.36Aeff2.

[0064] In this MCF, the first cladding functions as a common notch. This reduces the PCC between two adjacent cores when signal light propagates in opposite directions (hereinafter referred to as counterpropagating PCC), thereby reducing the XT between two adjacent cores when signal light propagates in opposite directions (hereinafter referred to as counterpropagating XT). The counterpropagating PCC and counterpropagating XT can be suppressed to an increase of less than 3 dB from their minimum values.

[0065] (10) In (9) above, condition 1b can also be further specified by the following formula:

[0066] 0.85Aeff2≤Aeff≤1.18Aeff2.

[0067] In this case, the counterpropagation PCC and the counterpropagation XT can be suppressed to an increase of 1 dB or less from the minimum value.

[0068] (11) In (9) above, condition 1b can also be further specified by the following formula:

[0069] 0.89Aeff2≤Aeff≤1.12Aeff2.

[0070] In this case, the counterpropagation PCC and the counterpropagation XT can be suppressed to an increase of 0.5 dB or less from the minimum value.

[0071] (12) In (9) above, condition 1b can also be further specified by the following formula:

[0072] 0.95Aeff2≤Aeff≤1.05Aeff2.

[0073] In this case, the counterpropagation PCC and the counterpropagation XT can be suppressed to an increase of 0.1 dB or less from the minimum value.

[0074] (13) The MCF involved in the fourth aspect of the present disclosure is a common depressed type MCF used in counter-propagation, comprising: a plurality of fiber cores; a common first cladding surrounding the plurality of fiber cores; and a second cladding surrounding the first cladding. The refractive index of the second cladding is lower than that of the fiber core and higher than that of the first cladding. The transmission loss at a wavelength of 1550 nm is less than 0.22 dB / km. When the effective cross-sectional area of the fiber core at a wavelength of 1550 nm is set to Aeff [μm 2], assuming that the center distance between two adjacent cores in a cross section perpendicular to the fiber axis is Λ [μm], and the relative refractive index difference of the cores based on the refractive index of the first cladding is Δcore [%], the formula Aeff2 = 19.2-3.01ln(Δcore)+1.40Λ+25.6[ln(Δcore)] is used. 2 -7.72×10 -3 Λ 2 -0.980Λln(Δcore)

[0075] Defined Aeff2[μm 2 ], then the following condition 2b is satisfied:

[0076] 0.30[%]≤Δcore[%]≤0.35[%]

[0077] 20[μm]≤Λ[μm]≤25[μm]

[0078] 0.84Aeff2≤Aeff≤1.36Aeff2.

[0079] In this MCF, the first cladding layer functions as a common notch. This reduces the counterpropagation PCC, and thus the counterpropagation XT. The counterpropagation PCC and counterpropagation XT can be suppressed to an increase of 3 dB or less from their minimum values.

[0080] (14) In (13) above, condition 2b can also be further specified by the following formula:

[0081] 0.88Aeff2≤Aeff≤1.18Aeff2.

[0082] In this case, the counterpropagation PCC and the counterpropagation XT can be suppressed to an increase of 1 dB or less from the minimum value.

[0083] (15) In (13) above, condition 2b can also be further specified by the following formula:

[0084] 0.90Aeff2≤Aeff≤1.12Aeff2.

[0085] In this case, the counterpropagation PCC and the counterpropagation XT can be suppressed to an increase of 0.5 dB or less from the minimum value.

[0086] (16) In (13) above, condition 2b can also be further specified by the following formula:

[0087] 0.95Aeff2≤Aeff≤1.05Aeff2.

[0088] In this case, the counterpropagation PCC and the counterpropagation XT can be suppressed to an increase of 0.1 dB or less from the minimum value.

[0089] (17) In any of the above (1) to (16), it is also possible that when the radius of the fiber core is set to ra [μm], the optical cable cutoff wavelength is set to λcc [nm], and the relative refractive index difference of the fiber core based on the refractive index of the first cladding is set to Δcore [%], condition 3, condition 4, condition 5 or condition 6 is satisfied.

[0090] Condition 3 is defined by the following formula:

[0091] λcc[nm]≤1260[nm]

[0092] ra[nm]≥2.51(Δcore) -0.605 [μm].

[0093] Condition 4 is defined by the following formula:

[0094] λcc[nm]≤1360[nm]

[0095] ra[nm]≥2.71(Δcore) -0.607 [μm].

[0096] Condition 5 is defined by the following formula:

[0097] λcc[nm]≤1460[nm]

[0098] ra[nm]≥2.92(Δcore) -0.609 [μm].

[0099] Condition 6 is defined by the following formula:

[0100] λcc[nm]≤1530[nm]

[0101] ra[nm]≥3.06(Δcore) -0.611 [μm].

[0102] In this case, confinement of signal light in each core can be strengthened.

[0103] (18) In any one of the above (1) to (17), it is also possible that there is also a third cladding surrounded by the first cladding, and the third cladding is arranged on a line segment connecting the centers of the two adjacent fiber cores, separated from each other in a cross section perpendicular to the fiber axis, and the refractive index of the third cladding is lower than the refractive index of the first cladding.

[0104] In this case, the inter-core PCC and inter-core XT can be further reduced.

[0105] (19) In the above (18), the third cladding may have a circular shape in a cross section perpendicular to the optical fiber axis.

[0106] In this case, the MCF can be manufactured more easily.

[0107] (20) In any one of the above (1) to (19), the plurality of cores, the first cladding, and the second cladding may include silica-based glass.

[0108] In this case, low transmission loss can be achieved in the wavelength band used for optical communication between 1260 nm and 1625 nm, and high manufacturability can also be achieved.

[0109] [Details of the embodiments of the present disclosure]

[0110] Specific examples of the MCF involved in this embodiment will be described with reference to the accompanying drawings as needed. It should be noted that the present disclosure is not limited to these examples but is set forth in the claims, which are intended to encompass all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, identical elements are denoted by the same reference numerals, and duplicate descriptions are omitted.

[0111] In an SI-type (more precisely, a matched cladding type) MCF, such as that described in Non-Patent Document 1, the cable cutoff wavelength (hereinafter also referred to as λcc) of each core is set to 1530nm or less, for example, to enable each core to operate in single-mode at a wavelength of 1550nm for optical signal transmission. In this case, to reduce the inter-core PCC and inter-core XT, it is ideal to make λcc as long as possible to strengthen the confinement of light entering the core. By reducing the inter-core PCC and thus the inter-core XT, the core density (the number of cores per unit cross-sectional area) on the MCF cross section can be increased while maintaining the quality of the transmitted signal.

[0112] Figure 1 This is a graph showing the relationship between the PCC between two adjacent cores and the Aeff of each core at a wavelength of 1550 nm, for example, when the λcc of each core is fixed at 1530 nm. Figure 1 The vertical axis represents the PCC between fiber cores [ / km]. Figure 1 The horizontal axis represents Aeff[μm 2 ].exist Figure 1 , the cases where the center interval Λ of the core is 25 μm, 30 μm, 35 μm, 40 μm, 45 μm and 50 μm are shown.

[0113] It can be seen that while the value varies depending on the center-to-center distance between adjacent cores, the smaller Aeff, the more PCC (represented by the symbol h) decreases exponentially. In SI-type and matched-cladding cores, if λcc and Aeff are determined, the relative refractive index difference of the core relative to the refractive index of the cladding and the core radius are uniquely determined. This is because the only degrees of freedom in design lie in the relative refractive index difference of the core and the core radius.

[0114] Non-Patent Document 4 shows that in a graded-index core, if the spot diameter and λcc are of the same magnitude, the PCC is the same as that of an SI core. Therefore, it can be seen that, similar to the SI core, in a graded-index core, as Aeff decreases, the PCC h decreases exponentially.

[0115] Trench-assisted and hole-assisted MCFs offer increased design freedom. Therefore, λcc and Aeff alone do not uniquely determine the refractive index profile and the confinement of light entering the core. When the values from the lowest refractive index of the trenches, holes, etc. to the highest refractive index of the core are fixed, as shown in Non-Patent Documents 2 and 3, a smaller Aeff strengthens the confinement of light entering the core, and PCCh decreases exponentially.

[0116] Figure 2 : is a diagram showing a cross section perpendicular to the optical fiber axis and a refractive index distribution of the MCF involved in the embodiment. Figure 2 As shown, the MCF 1 according to the embodiment includes multiple cores 2, a common first cladding 3, and a common second cladding 4. The cores 2, first cladding 3, and second cladding 4 each comprise silica-based glass. In the MCF 1, the transmission loss at a wavelength of 1550 nm is 0.22 dB / km or less. Silica-based glass is glass primarily composed of silicon dioxide, containing at least 90% silicon dioxide.

[0117] The plurality of cores 2 have the same circular shape as each other in a cross section perpendicular to the optical fiber axis. The radius ra of each core 2 is, for example, greater than 3 μm and less than 7 μm. Each core 2 extends along the optical fiber axis of the MCF1. In the present embodiment, the MCF1 is a dual-core optical fiber and the number of cores 2 is 2, but it is not limited to this. In a cross section perpendicular to the optical fiber axis, the two cores 2 are arranged so as to be opposed to each other across the central axis of the MCF1. In a cross section perpendicular to the optical fiber axis, the two cores 2 are arranged at an equal distance from the central axis. In a cross section perpendicular to the optical fiber axis, the center spacing Λ of two adjacent cores 2 is, for example, greater than 30 μm and less than 50 μm.

[0118] The first cladding 3 is a common cladding surrounding the plurality of cores 2. The first cladding 3 covers the outer circumference of the plurality of cores 2. In a cross section perpendicular to the optical fiber axis, the first cladding 3 has a circular shape. The second cladding 4 is a common cladding surrounding the first cladding 3. The second cladding 4 covers the outer circumference of the first cladding 3. For example, the shortest distance between the interface between the first cladding 3 and the second cladding 4 and the center of the plurality of cores 2 is greater than or equal to 15 μm and less than or equal to 20 μm. In a cross section perpendicular to the optical fiber axis, the second cladding 4 has a circular shape. The radius of the second cladding 4 is, for example, greater than or equal to 50 μm and less than or equal to 100 μm, or greater than or equal to 62 μm and less than or equal to 63 μm.

[0119] The refractive index of the second cladding 4 is lower than that of the core 2 and higher than that of the first cladding 3. The relative refractive index difference of the second cladding 4 with respect to the refractive index of the first cladding 3 is 0.02% or more, the relative refractive index difference of the core 2 is 0.30% or more and 1.00% or less, and the difference between the relative refractive index difference of the core 2 and the relative refractive index difference of the second cladding 4 is 0.20% or more. Figure 2 The refractive index distribution is shown with the vertical axis representing the relative refractive index difference Δ relative to the refractive index of the first cladding 3, and the horizontal axis representing the radial position r. At the center of the fiber, radial position r is 0. MCF1 is a common-depression MCF. The first cladding 3 functions as a common depression.

[0120] In MCF1, the effective cross-sectional area of the core 2 at a wavelength of 1550 nm is set to Aeff [μm 2 ], let the center distance between two adjacent cores 2 in the cross section perpendicular to the optical fiber axis be Λ [μm], let the relative refractive index difference of the core 2 based on the refractive index of the first cladding 3 be Δcore [%], and define Aeff1 [μm] using the following formula 2 ], condition 1a or condition 2a is met.

[0121] Aeff1=18.6-4.63ln(Δcore)+1.24Λ+24.1[ln(Δcore)] 2 -6.05×10 -3 Λ 2 -0.858Λln(Δcore)

[0122] Conditions 1a and 2a are defined by the following equations.

[0123] In the above formula, the unit of Aeff1 is [μm 2 ], the coefficient of the constant is the following unit.

[0124] 18.6[μm 2 ]. -4.63[μm 2 ]. 1.24[μm]. 24.1[μm2 ].-6.05×10 -3 [-(unitless)]. -0.858[μm].

[0125] That is, Aeff means [μm 2 ] is used to represent the value of the effective cross-sectional area, Λ means the value of the center interval in [μm], Δcore means the value of the relative refractive index difference in [%], Aeff1 means the value of the relative refractive index difference in [μm] 2 ] when .

[0126] (Condition 1a)

[0127] 0.35[%]≤Δcore[%]≤1.00[%]

[0128] 25[μm]≤Λ[μm]≤50[μm]

[0129] 0.80Aeff1≤Aeff≤1.35Aeff1

[0130] (Condition 2a)

[0131] 0.30[%]≤Δcore[%]≤0.35[%]

[0132] 20[μm]≤Λ[μm]≤25[μm]

[0133] 0.88Aeff1≤Aeff≤1.35Aeff1

[0134] Condition 1a may be further defined by any of the following formulas.

[0135] 0.83Aeff1≤Aeff≤1.18Aeff1

[0136] 0.90Aeff1≤Aeff≤1.12Aeff1

[0137] 0.95Aeff1≤Aeff≤1.05Aeff1

[0138] Condition 2a may be further defined by any of the following formulas.

[0139] 0.90Aeff1≤Aeff≤1.18Aeff1

[0140] 0.92Aeff1≤Aeff≤1.12Aeff1

[0141] 0.96Aeff1≤Aeff≤1.05Aeff1

[0142] In MCF1, the effective cross-sectional area of the core 2 at a wavelength of 1550 nm is set to Aeff [μm 2 ], let the center distance between two adjacent cores 2 in the cross section perpendicular to the optical fiber axis be Λ [μm], let the relative refractive index difference of the core 2 based on the refractive index of the first cladding 3 be Δcore [%], and define Aeff2 [μm] by the following formula 2 ], condition 1b or condition 2b is met.

[0143] Aeff2=19.2-3.01ln(Δcore)+1.40Λ+25.6[ln(Δcore)] 2 -7.72×10 -3 Λ 2 -0.980Λln(Δcore)

[0144] In the above formula, the unit of Aeff2 is [μm 2 ], the coefficient of the constant is the following unit.

[0145] 19.2[μm 2 ]. -3.01[μm 2 ]. 1.40[μm]. 25.6[μm 2 ]. -7.72×10 -3 [-(unitless)]. -0.980[μm].

[0146] That is, Aeff means [μm 2 ] is used to represent the value of the effective cross-sectional area, Λ means the value of the center interval in [μm], Δcore means the value of the relative refractive index difference in [%], Aeff2 means the value of the relative refractive index difference in [μm] 2 ] when .

[0147] Conditions 1b and 2b are defined by the following equations.

[0148] (Condition 1b)

[0149] 0.35[%]≤Δcore[%]≤1.00[%]

[0150] 25[μm]≤Λ[μm]≤50[μm]

[0151] 0.75Aeff2≤Aeff≤1.36Aeff2

[0152] (Condition 2b)

[0153] 0.30[%]≤Δcore[%]≤0.35[%]

[0154] 20[μm]≤Λ[μm]≤25[μm]

[0155] 0.84Aeff2≤Aeff≤1.36Aeff2

[0156] Condition 1b may be further defined by any of the following formulas.

[0157] 0.85Aeff2≤Aeff≤1.18Aeff2

[0158] 0.89Aeff2≤Aeff≤1.12Aeff2

[0159] 0.95Aeff2≤Aeff≤1.05Aeff2

[0160] Condition 2b may be further defined by any of the following formulas.

[0161] 0.88Aeff2≤Aeff≤1.18Aeff2

[0162] 0.90Aeff2≤Aeff≤1.12Aeff2

[0163] 0.95Aeff2≤Aeff≤1.05Aeff2

[0164] In the common-depression type, λcc, when considering only the waveguide structure portion including the core 2 and the common depression (i.e., the first cladding 3), is larger than the desired λcc. By adjusting the relative refractive index difference of the second cladding 4 relative to the refractive index of the first cladding 3, and adjusting the shortest distance between the center of the core 2 and the second cladding 4 in a cross section perpendicular to the fiber axis, high-order modes leak into the second cladding 4. This allows λcc to be adjusted to the desired value. Consequently, the radius ra of the core 2 and the relative refractive index difference Δcore of the core 2 relative to the common first cladding 3 can freely assume independent values relative to λcc. Furthermore, MCF1 can reduce PCC h when achieving the same λcc compared to SI-type / matched cladding-type and trench-assisted MCFs.

[0165] To strengthen the constraints on each core 2, at least compared to SI and matched cladding types, it is ideal to set λcc to a value greater than the desired upper limit of λcc when considering only the waveguide structure portion consisting of the core 2 and the common recess (i.e., the first cladding 3). To achieve this in a common recess-type MCF, it is ideal to satisfy Condition 3, Condition 4, Condition 5, or Condition 6 defined by the following equations, when the radius of the core 2 is ra [μm], the cable cutoff wavelength is λcc [nm], and the relative refractive index difference of the core 2 relative to the refractive index of the first cladding 3 is Δcore [%].

[0166] (Condition 3)

[0167] λcc[nm]≤1260[nm]

[0168] ra[μm]≥2.51(Δcore) -0.605 [μm]

[0169] (Condition 4)

[0170] λcc[nm]≤1360[nm]

[0171] ra[μm]≥2.71(Δcore) -0.607 [μm]

[0172] (Condition 5)

[0173] λcc[nm]≤1460[nm]

[0174] ra[μm]≥2.92(Δcore) -0.609 [μm]

[0175] (Condition 6)

[0176] λcc[nm]≤1530[nm]

[0177] ra[μm]≥3.06(Δcore) -0.611 [μm]

[0178] MCF1 satisfies conditions 3, 4, 5, and 6. Here, λcc represents the cable cutoff wavelength in nm, ra represents the radius in μm, and Δcore represents the relative refractive index difference in %. The coefficients of the constants on the right side of conditions 3 through 6 are in μm. Specifically, they are 2.51 μm in condition 3, 2.71 μm in condition 4, 2.92 μm in condition 5, and 3.06 μm in condition 6.

[0179] Figure 3 This is a graph showing the relationship between the core radius and the relative refractive index difference of the core with respect to the refractive index of the first cladding. Figure 3 The vertical axis represents the core radius ra [μm]. Figure 3 The horizontal axis represents the relative refractive index difference [%] of the core with respect to the refractive index of the first cladding. Figure 3 In FIG, the cases where λcc is 1.26μm, 1.36μm, 1.46μm and 1.53μm are shown. In other words, Figure 3 The indicated lines correspond to the case where the equality of the two equations is satisfied in each of Conditions 3, 4, 5, and 6. The region above these lines is an ideal region.

[0180] Strictly speaking, the refractive index profile of actual optical fibers is often not SI-type. In such cases, by approximating an equivalent SI-type refractive index profile based on the ESI (Equivalent Step Index) described in Non-Patent Document 5, Conditions 3, 4, 5, and 6 can also be applied to the MCF of a non-SI core. In this case, the ESI-approximated Δcore, Δcore,s, can be expressed using the following equation using the effective normalized frequency Veff described in Non-Patent Document 5, the core radius ρs, wavelength λ, and the core refractive index n1.

[0181]

[0182] Here, Δcore,s is assumed to be in %, and λ and ρs use the same unit. Veff and n1 are dimensionless quantities. ρs and Δcore,s can also be replaced with a and Δcore in this application.

[0183] Next, refer to Figure 4 , the results of studying the relationship between h and Aeff for each Δcore and each Λ are explained. Figure 4 It is a graph showing the relationship between PCC and Aeff of each core. Figure 4 The vertical axis represents the PCC between fiber cores [ / km]. Figure 4 The horizontal axis represents Aeff[μm 2 ].exist Figure 4 , the cases where the center interval Λ of the core is 25 μm, 30 μm, 35 μm, 40 μm, 45 μm and 50 μm are shown. Figure 4 This is the case where Δcore is 0.40%. Although not shown in the figure, the relationship between h and Aeff was also examined in the same manner when Δcore was within the range of 0.30% to 1.00%.

[0184] From this result, it can be seen that h reaches its minimum at different Aeff for each Λ. In parallel propagation between two adjacent cores, the Aeff at which PCC h reaches its minimum hmin is approximated by the aforementioned Aeff1. This approximation is valid at least within the range of Δcore from 0.30% to 1.00% and Λ from 20 μm to 50 μm. Parallel propagation refers to the situation where the signal light propagating in two adjacent cores is oriented in the same direction.

[0185] Figure 5 and Figure 6 It is a graph that plots how much h increases when Aeff deviates from Aeff1. Figure 5 and Figure 6The horizontal axis represents Aeff / Aeff1. Figure 5 and Figure 6 The vertical axis represents 10log 10 (h / hmin). hmin represents the minimum value of h under each combination of Δcore and Λ. Figure 5 In the graph, the values in the range of Δcore being 0.35% or more and 1.00% or less and Λ being 25 μm or more and 50 μm or less are plotted. Figure 6 In FIG. 5 , values in the range where Δcore is 0.30% or more and 1.00% or less and Λ is 20 μm or more and 50 μm or less are plotted.

[0186] Depend on Figure 5 The following can be confirmed.

[0187] 0.80Aeff1≤Aeff≤1.35Aeff1

[0188] When hmin is , the increase in h is suppressed to less than 3dB in dB value, which is ideal.

[0189] 0.83Aeff1≤Aeff≤1.18Aeff1

[0190] When , the increase of h is suppressed to less than 1dB in dB value compared with hmin, which is more ideal.

[0191] 0.90Aeff1≤Aeff≤1.12Aeff1

[0192] When compared with hmin, the increase in h is suppressed to less than 0.5dB in dB value, which is further ideal.

[0193] 0.95Aeff1≤Aeff≤1.05Aeff1

[0194] When hmin is 0.1dB, the increase in h is suppressed to 0.1dB or less, which is the most ideal result. This result corresponds to the above-mentioned condition 1a.

[0195] Depend on Figure 6 It can be confirmed that if the lower limit of Δcore is extended to 0.30% and the lower limit of Λ is extended to 20μm, the lower limit of the ideal Aeff will be particularly narrowed.

[0196] 0.88Aeff1≤Aeff≤1.35Aeff1

[0197] When hmin is , the increase in h is suppressed to less than 3dB in dB value, which is ideal.

[0198] 0.90Aeff1≤Aeff≤1.18Aeff1

[0199] When compared with hmin, the increase of h is suppressed to less than 1dB in dB value, so it is more ideal.

[0200] 0.92Aeff1≤Aeff≤1.12Aeff1

[0201] When compared with hmin, the increase in h is suppressed to less than 0.5dB in dB value, which is further ideal.

[0202] 0.96Aeff1≤Aeff≤1.05Aeff1

[0203] When , the increase in h is suppressed to 0.1 dB or less compared to hmin, which is the most ideal result. This result corresponds to the above-mentioned condition 2a.

[0204] As described in Non-Patent Document 1, by aligning the propagation directions of signal light between adjacent cores, i.e., in two adjacent cores, the PCC and XT between adjacent cores can be reduced. In this case, the Aeff at which the PCC caused by Rayleigh backscattering (PCCbs) reaches its minimum value, or the Aeff at which the XT caused by Rayleigh backscattering (XTbs) reaches its minimum value, i.e., the Aeff at which the PCC h reaches its minimum value hmin during counterpropagation, is approximated by the aforementioned Aeff2. This approximation is valid at least within the range of Δcore from 0.30% to 1.00% and Λ from 20 μm to 50 μm. Having signal light propagating in two adjacent cores in opposite directions is called counterpropagation.

[0205] Based on the above approximation using Aeff2, the following can be confirmed.

[0206] 0.75Aeff2≤Aeff≤1.36Aeff2

[0207] When the increase from the minimum value of PCCbs and the increase from the minimum value of XTbs are suppressed to less than 3dB in dB value, it is ideal.

[0208] 0.85Aeff2≤Aeff≤1.18Aeff2

[0209] When the increase from the minimum value of PCCbs and the increase from the minimum value of XTbs are suppressed to less than 1dB in dB value, it is more ideal.

[0210] 0.89Aeff2≤Aeff≤1.12Aeff2

[0211] When the increase from the minimum value of PCCbs and the increase from the minimum value of XTbs are suppressed to less than 0.5dB in dB value, it is further ideal.

[0212] 0.95Aeff2≤Aeff≤1.05Aeff2

[0213] When , the increase from the minimum value of PCCbs and the increase from the minimum value of XTbs are suppressed to 0.1 dB or less in dB, which is the most ideal result. This result corresponds to the above-mentioned condition 1b.

[0214] It can be confirmed that if the lower limit of Δcore is extended to 0.30% and the lower limit of Λ is extended to 20μm, the lower limit of the ideal Aeff will be particularly narrowed.

[0215] 0.84Aeff2≤Aeff≤1.36Aeff2

[0216] When the increase from the minimum value of PCCbs and the increase from the minimum value of XTbs are suppressed to less than 3dB in dB value, it is ideal.

[0217] 0.88Aeff2≤Aeff≤1.18Aeff2

[0218] When the increase from the minimum value of PCCbs and the increase from the minimum value of XTbs are suppressed to less than 1dB in dB value, it is more ideal.

[0219] 0.90Aeff2≤Aeff≤1.12Aeff2

[0220] When the increase from the minimum value of PCCbs and the increase from the minimum value of XTbs are suppressed to less than 0.5dB in dB value, it is further ideal.

[0221] 0.95Aeff2≤Aeff≤1.05Aeff2

[0222] When , the increase from the minimum value of PCCbs and the increase from the minimum value of XTbs are suppressed to 0.1 dB or less in dB, which is the most ideal. This result corresponds to the above-mentioned condition 2b.

[0223] As described above, in the MCF1 involved in this embodiment, the first cladding 3 functions as a common depression. In MCF1, by satisfying condition 1a or condition 2a, the parallel propagation PCC and the parallel propagation XT can be reduced. By satisfying condition 1b or condition 2b, the counter-propagation PCC and the counter-propagation XT can be reduced. That is, in the MCF1 of the common depression type, the PCC and XT can be reduced more than those of the SI type and matched cladding type MCFs. In addition, by setting the Aeff of each core 2 to an appropriate range, the PCC and XT can be suppressed to values close to the minimum. Moreover, since MCF1 does not have a refractive index structure for strengthening the individual limitations of each core 2 like the trench-assisted type and the hole-assisted type, it is easy to manufacture and can reduce PCC and XT.

[0224] As mentioned above, although embodiment was demonstrated, this disclosure is not necessarily limited to the said embodiment, Various changes are possible within the range which does not deviate from the summary.

[0225] Figure 7 : is a diagram showing a cross section perpendicular to the optical fiber axis and a refractive index distribution of an MCF according to a modified example. Figure 7 As shown, the MCF1A according to the modification further includes a third cladding layer 5. Figure 2 The MCF 1 shown is different. The third cladding 5 is surrounded by the first cladding 3. In a cross section perpendicular to the fiber axis, the third cladding 5 is separated from each of the two adjacent cores 2 and arranged on a line segment L connecting the centers of the two adjacent cores 2. None of the cores 2 is covered by the third cladding 5. This further reduces the PCC h between the two adjacent cores 2.

[0226] The third cladding 5 has a circular shape in a cross section perpendicular to the optical fiber axis. This makes manufacturing easier. In the present embodiment, the radius of the third cladding 5 is the same size as the radius of the core 2, but it may also be different from the radius of the core 2. The third cladding 5 contains silica-based glass. The third cladding 5 has a refractive index lower than that of the first cladding 3 and functions as a low-refractive-index pit. The relative refractive index difference of the third cladding 5 based on the refractive index of the first cladding 3 is, for example, greater than -0.8% and less than -0.1%. In this modified example, in a cross section perpendicular to the optical fiber axis, the third cladding 5 is located at a position equidistant from the centers of the two cores 2, and the center of the third cladding 5 overlaps with the center of MCF1A.

[0227] It should be noted that the characteristic quantities and characteristics of the MCF disclosed in the present invention can be measured by the following methods. The refractive indices of the core, the first cladding, and the second cladding can be measured, for example, by the refractive near-field method and the lateral interference method. The transmission loss can be measured by applying the method described in ITU-T G.650.1 to each core of the MCF. The effective cross-sectional area can be measured, for example, by the method described in Appendix III of ITU-TG.650.2 (08 / 2015). The center spacing between two adjacent cores in a cross section perpendicular to the optical fiber axis can be measured, for example, based on the refractive near-field method, the lateral interference method, and a microscope observation image of the MCF cross section (transmission near-field method).

[0228] The above description uses an uncoupled MCF as an example, but is not limited to this. In a coupled MCF (also called a randomly coupled MCF) that allows inter-core XT to increase the core density, if the coupling between the cores is too strong, the difficulty of numerical calculation to compensate for the inter-core XT also increases. To this end, by adopting a core structure that reduces the inter-core PCC, the core density can be increased (the core spacing can be reduced). Therefore, whether in an uncoupled MCF or in a coupled MCF, the individual optical properties of each core can be maintained at the desired value within the range of an economically feasible refractive index distribution, while reducing the inter-core PCC and inter-core XT.

[0229] Description of Reference Numerals

[0230] 1. 1A MCF

[0231] 2 fiber cores

[0232] 3 First cladding

[0233] 4 Second cladding

[0234] 5 Third cladding

[0235] ra is the radius of the fiber core

[0236] L segment

[0237] Λ Center spacing between adjacent fiber cores

[0238] ΔRelative refractive index difference.

Claims

1. A multi-core optical fiber, being a common depressed type multi-core optical fiber used for parallel propagation, comprising: Multiple fiber cores; a common first cladding layer surrounding the plurality of fiber cores respectively; and a second cladding layer, surrounding said first cladding layer, The refractive index of the second cladding is lower than the refractive index of the core and higher than the refractive index of the first cladding, The transmission loss of the multi-core optical fiber at a wavelength of 1550 nm is less than 0.22 dB / km. When the effective cross-sectional area of the core at a wavelength of 1550 nm is set to Aeff [μm 2 ], the center distance between two adjacent cores in the cross section perpendicular to the optical fiber axis is Λ [μm], and the relative refractive index difference of the cores based on the refractive index of the first cladding is Δcore [%], using Aeff1=18.6-4.63ln(Δcore)+1.24Λ+24.1[ln(Δcore)] 2 -6.05×10 -3 L 2 -0. 858Λln(Δcore) Defined Aeff1[μm 2 ], the multi-core optical fiber satisfies the following condition 1a: 0.35[%]≤Δcore[%]≤1.00[%] 25[μm]≤Λ[μm]≤50[μm] 0.80Aeff1≤Aeff≤1.35Aeff1.

2. The multi-core optical fiber according to claim 1, wherein: The condition 1a is further defined by the following formula: 0.83Aeff1≤Aeff≤1.18Aeff1.

3. The multi-core optical fiber according to claim 1, wherein: The condition 1a is further defined by the following formula: 0.90Aeff1≤Aeff≤1.12Aeff1.

4. The multi-core optical fiber according to claim 1, wherein: The condition 1a is further defined by the following formula: 0.95Aeff1≤Aeff≤1.05Aeff1.

5. A multi-core optical fiber, being a common depressed type multi-core optical fiber used for parallel propagation, comprising: Multiple fiber cores; a common first cladding layer surrounding the plurality of fiber cores respectively; and a second cladding layer, surrounding said first cladding layer, The refractive index of the second cladding is lower than the refractive index of the core and higher than the refractive index of the first cladding, The transmission loss of the multi-core optical fiber at a wavelength of 1550 nm is less than 0.22 dB / km. When the effective cross-sectional area of the core at a wavelength of 1550 nm is set to Aeff [μm 2 ], the center distance between two adjacent cores in the cross section perpendicular to the optical fiber axis is Λ [μm], and the relative refractive index difference of the cores based on the refractive index of the first cladding is Δcore [%], using Aeff1=18.6-4.63ln(Δcore)+1.24Λ+24.1[ln(Δcore)] 2 -6.05×10 -3 L 2 -0. 858Λln(Δcore) Defined Aeff1[μm 2 ], the multi-core optical fiber satisfies the following condition 2a: 0.30[%]≤Δcore[%]≤0.35[%] 20[μm]≤Λ[μm]≤25[μm] 0.88Aeff1≤Aeff≤1.35Aeff1.

6. The multi-core optical fiber according to claim 5, wherein: The condition 2a is further defined by the following formula: 0.90Aeff1≤Aeff≤1.18Aeff1.

7. The multi-core optical fiber according to claim 5, wherein: The condition 2a is further defined by the following formula: 0.92Aeff1≤Aeff≤1.12Aeff1.

8. The multi-core optical fiber according to claim 5, wherein: The condition 2a is further defined by the following formula: 0.96Aeff1≤Aeff≤1.05Aeff1.

9. A multi-core optical fiber, being a common depressed type multi-core optical fiber used in counterpropagation, the multi-core optical fiber comprising: Multiple fiber cores; a common first cladding layer surrounding the plurality of fiber cores respectively; and a second cladding layer, surrounding said first cladding layer, The refractive index of the second cladding is lower than the refractive index of the core and higher than the refractive index of the first cladding, The transmission loss of the multi-core optical fiber at a wavelength of 1550 nm is less than 0.22 dB / km. When the effective cross-sectional area of the core at a wavelength of 1550 nm is set to Aeff [μm 2 ], the center distance between two adjacent cores in the cross section perpendicular to the optical fiber axis is Λ [μm], and the relative refractive index difference of the cores based on the refractive index of the first cladding is Δcore [%], using Aeff2=19.2-3.01ln(Δcore)+1.40Λ+25.6[ln(Δcore)] 2 -7.72×10 -3 L 2 -0. 980Λln(Δcore) Defined Aeff2[μm 2 ], the multi-core optical fiber satisfies the following condition 1b: 0.35[%]≤Δcore[%]≤1.00[%] 25[μm]≤Λ[μm]≤50[μm] 0.75Aeff2≤Aeff≤1.36Aeff2.

10. The multi-core optical fiber according to claim 9, wherein: The condition 1b is further defined by the following formula: 0.85Aeff2≤Aeff≤1.18Aeff2.

11. The multi-core optical fiber according to claim 9, wherein: The condition 1b is further defined by the following formula: 0.89Aeff2≤Aeff≤1.12Aeff2.

12. The multi-core optical fiber according to claim 9, wherein: The condition 1b is further defined by the following formula: 0.95Aeff2≤Aeff≤1.05Aeff2.

13. A multi-core optical fiber, being a common depressed type multi-core optical fiber used in counterpropagation, the multi-core optical fiber comprising: Multiple fiber cores; a common first cladding layer surrounding the plurality of fiber cores respectively; and a second cladding layer, surrounding said first cladding layer, The refractive index of the second cladding is lower than the refractive index of the core and higher than the refractive index of the first cladding, The transmission loss of the multi-core optical fiber at a wavelength of 1550 nm is less than 0.22 dB / km. When the effective cross-sectional area of the core at a wavelength of 1550 nm is set to Aeff [μm 2 ], the center distance between two adjacent cores in the cross section perpendicular to the optical fiber axis is Λ [μm], and the relative refractive index difference of the cores based on the refractive index of the first cladding is Δcore [%], using Aeff2=19.2-3.01ln(Δcore)+1.40Λ+25.6[ln(Δcore)] 2 -7.72×10 -3 L 2 -0. 980Λln(Δcore) Defined Aeff2[μm 2 ], the multi-core optical fiber satisfies the following condition 2b: 0.30[%]≤Δcore[%]≤0.35[%] 20[μm]≤Λ[μm]≤25[μm] 0.84Aeff2≤Aeff≤1.36Aeff2.

14. The multi-core optical fiber according to claim 13, wherein: The condition 2b is further defined by the following formula: 0.88Aeff2≤Aeff≤1.18Aeff2.

15. The multi-core optical fiber according to claim 13, wherein: The condition 2b is further defined by the following formula: 0.90Aeff2≤Aeff≤1.12Aeff2.

16. The multi-core optical fiber according to claim 13, wherein: The condition 2b is further defined by the following formula: 0.95Aeff2≤Aeff≤1.05Aeff2.

17. The multi-core optical fiber according to any one of claims 1 to 16, wherein: When the radius of the core is set to ra [μm], the cable cutoff wavelength is set to λcc [nm], and the relative refractive index difference of the core with respect to the refractive index of the first cladding is set to Δcore [%], the multi-core optical fiber satisfies condition 3, condition 4, condition 5, or condition 6. The condition 3 is defined by the following formula: λcc[nm]≤1260[nm] ra≥2.51(Δcore) -0.605 μm, The condition 4 is defined by the following formula: λcc[nm]≤1360[nm] ra≥2.71(Δcore) -0.607 [μm], The condition 5 is defined by the following formula: λcc[nm]≤1460[nm] ra≥2.92(Δcore) -0.609 [μm], The condition 6 is defined by the following formula: λcc[nm]≤1530[nm] ra≥3.06(Δcore) -0.611 [μm]。 18. The multi-core optical fiber according to any one of claims 1 to 17, wherein The multi-core optical fiber further comprises a third cladding surrounded by the first cladding, The third cladding is arranged on a line segment connecting the centers of the two adjacent cores, spaced apart from each other in a cross section perpendicular to the optical fiber axis. The refractive index of the third cladding layer is lower than the refractive index of the first cladding layer.

19. The multi-core optical fiber according to claim 18, wherein: The third cladding has a circular shape in a cross section orthogonal to the optical fiber axis.

20. The multi-core optical fiber according to any one of claims 1 to 19, wherein The plurality of cores, the first cladding, and the second cladding include silica-based glass.

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