JP7320788B2 - Coupled multi-core optical fiber - Google Patents

Description

The present disclosure relates to coupled multicore optical fibers.

In optical fiber communication systems, the transmission capacity is limited by nonlinear effects and fiber fuses that occur in optical fibers. Spatial transmission such as parallel transmission using multi-core fiber that has multiple cores in one optical fiber and mode-multiplexed transmission using multi-mode fiber in which multiple propagation modes exist in the core to relax these restrictions. Multiple techniques are being considered.

H. Takara et al. , "1.01-Pb/s (12 SDM/222 WDM/456 Gb/s) Crosstalk-managed Transmission with 91.4-b/s/Hz Aggregate Spectral Efficiency," in ECOC2012, paper Th. 3. C. 1 (2012) T. Sakamoto et al. , "Differential Mode Delay Managed Transmission Line for WDM-MIMO System Using Multi-Step Index Fiber," J. Am. Lightwave Technol. vol. 30, pp. 2783-2787 (2012). Y. Sasaki et al. , “Large-effective-area uncoupled few-mode multi-core fiber,” ECOC2012, paper Tu. 1. F. 3 (2012). T. Ohara et al. , "Over-1000-Channel Ultradense WDM Transmission With Supercontinuum Multicarrier Source," IEEE J. Phys. Lightw. Technol. , vol. 24, pp. 2311-2317 (2006) T. Sakamoto, T.; Mori, M. Wada, T. Yamamoto, F.; Yamamoto, and K. Nakajima, "Fiber Twisting- and Bending-Induced Adiabatic/Nonadiabatic Super-Mode Transition in Coupled Multicore Fiber," J. Am. Lightwave Technol. 34, 1228-1237 (2016). T. Fujisawa et al. , "Group delay spread analysis of coupled-multicore fibers: A comparison between weak and tight bending conditions," Opt. Commun. , vol. 393, no. 9, pp. 232-237, 2017. Y. Kokubun, and M.; Koshiba "Novel multi-core fibers for mode division multiplexing: Proposal and design principle." IEICE Electronics Express, vol. 6, pp. 522-528 (2009) R. Ryf, N. K. Fontaine, B.; Guan, R. -J. Essiambre, S.; Randel, A.; H. Gnauck, S.; Chandrasekhar, A.; Adamiecki, G.; Raybon, B, Ercan, R.; P. Scott, S. J. Ben Yoo, T.; Hayashi, T.; Nagashima, and T. Sasaki, "1705-km transmission over coupled-core fiber supporting 6 spatial modes," ECOC, paper PD. 3.2 (2014). Written by Okamoto Fundamentals of Optical Waveguides, Corona Publishing ISBN 4-339-00602-5

In transmission using a multi-core fiber, if crosstalk occurs between cores, the signal quality deteriorates. Therefore, the cores must be separated by a certain distance or more in order to suppress the crosstalk. In general, in order to ensure sufficient transmission quality in an optical communication system, it is desirable to keep the power penalty at 1 dB or less. must.

On the other hand, when using MIMO technology, it is possible to compensate for crosstalk at the receiving end, reduce the distance between cores, and reduce the power penalty to less than 1dB by signal processing even if the crosstalk is -26dB or more. It is possible to improve space utilization efficiency. However, when applying the MIMO technology, if the group delay spread (GDS) caused by the differential group delay (DMD) between a plurality of signal lights occurring in the transmission line is large, the impulse response width of the transmission line becomes large. Incurs an increase in signal processing.

In a single-mode multi-core fiber in which each core structure propagates a single mode, the core structure and core spacing are adjusted to induce random coupling between modes, as described in Non-Patent Document 5. Coupled single-mode MCFs have been considered.

In general, even with a homogenous core single-mode MCF, the structure of each core is slightly different due to manufacturing errors, and the group velocities of the modes propagating through each core are different. However, by inducing random coupling between modes, the GDS becomes larger in proportion to the square root of the distance. It is possible to

However, as described in Non-Patent Document 5, if the core spacing is too small, the effective refractive index difference between the propagation modes increases, the amount of coupling between modes in the fiber decreases, and random coupling cannot be obtained. Because of the resulting increase in GDS, there is a preferred core spacing range for obtaining random binding.

On the other hand, in order to perform high-capacity communication with a limited fiber cross-sectional area, it is desirable to increase the arrangement density of the cores (that is, reduce the core spacing). It is desirable to reduce the spacing.

That is, an object of the present invention is to provide a coupled multi-core optical fiber capable of reducing the core spacing while realizing random inter-mode coupling.

In order to achieve the above object, a coupled multi-core optical fiber according to the present invention has cores arranged in a specific arrangement.

Specifically, the coupled multi-core optical fiber according to the present invention is a coupled multi-core optical fiber having three or more cores,
In cross section, the cores are rotationally asymmetrical with respect to the center of the clad and are arranged at a minimum core spacing such that inter-core crosstalk is -10 dB/km or more.

For example, in the coupled multi-core optical fiber according to the present invention, in cross section, at least one of the cores is located outside the regular polygon from the position of the core, assuming that all the cores are arranged in the regular polygon. It is characterized by being in a position shifted to . At this time, it is preferable that an angle formed by a side of the regular polygon and a line segment connecting the center of the core at the shifted position and the center of the core adjacent to the core is 10° or more.

Unlike the conventional multi-core optical fiber in which all adjacent core spacings are equal, the coupled multi-core optical fiber of this embodiment increases the spacing between some cores to increase the maximum effective refractive index difference of the propagation mode. can be reduced and the GDS can be reduced. Therefore, the present invention can provide a coupled multi-core optical fiber capable of reducing core spacing while realizing random inter-mode coupling.

Further, in the coupled multi-core optical fiber according to the present invention, the cores may be spirally arranged in a cross section.

Furthermore, a coupled multi-core optical fiber according to the present invention is a coupled multi-core optical fiber having p or more (p is an integer of 4 or more) cores,
In the cross section, the core is arranged in an n-sided polygon with n vertices out of p so as to have m-fold rotational symmetry with respect to the center of the clad (m is an integer of 2 or more and less than n, n is 4 or more). p), and are arranged at a minimum core spacing such that inter-core crosstalk is -10 dB/km or more.

GDS can be reduced by arranging cores so as to have m-fold rotational symmetry with respect to the center of the clad, even if all adjacent cores have the same spacing as in a conventional multi-core optical fiber. Therefore, the present invention can provide a coupled multi-core optical fiber capable of reducing core spacing while realizing random inter-mode coupling.

INDUSTRIAL APPLICABILITY As described above, the present invention can provide a coupled multi-core optical fiber capable of reducing core spacing while realizing random inter-mode coupling.

It is a figure explaining the cross section of the coupling type multi-core optical fiber relevant to this invention. FIG. 3 is a diagram explaining GDS with respect to core spacing of a coupled multi-core optical fiber related to the present invention; FIG. 3 is a diagram explaining GDS with respect to core spacing of a coupled multi-core optical fiber related to the present invention; FIG. 3 is a diagram illustrating the effective refractive index with respect to the core spacing of a coupled multi-core optical fiber related to the present invention; It is a figure explaining the cross section of the coupled multi-core optical fiber which concerns on this invention. FIG. 3 is a diagram for explaining effective refractive index with respect to core spacing of a coupled multi-core optical fiber according to the present invention; FIG. 3 is a diagram explaining GDS with respect to the core spacing of the coupled multi-core optical fiber according to the present invention; It is a figure explaining the core number dependence of GDS. It is a figure explaining the cross section of the coupled multi-core optical fiber which concerns on this invention. It is a figure explaining the cross section of the coupled multi-core optical fiber which concerns on this invention. FIG. 5 is a diagram for explaining an impulse response shape when inter-core crosstalk is changed; FIG. 4 is a diagram for explaining the relationship between the amount of coupling (crosstalk) and the correlation coefficient when the impulse response shape is fitted with a Gaussian waveform; FIG. 4 is a diagram illustrating the relationship between the amount of coupling (crosstalk) and the standard deviation obtained by Gaussian fitting of impulse response waveforms;

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In addition, in this specification and the drawings, constituent elements having the same reference numerals are the same as each other.

(related forms)
FIG. 1 is a diagram explaining the cross-sectional structure of a coupled multi-core optical fiber related to the present invention. FIGS. 1A, 1B, and 1C are array examples with 3, 4, and 6 cores, respectively. The cores have a radius of a and are arranged in an annular shape (the cores are arranged at the vertices of a polygon) with a core interval (center-to-center distance between cores) of Λ. In such a structure, the distance between adjacent cores is Λ for all cores.

Assuming that the refractive index of the core is n1 and the refractive index of the clad is n2, the condition for guiding light through the core is n1>n2. For example, the clad is made of pure silica glass, and the core is made of silica glass doped with germanium (Ge), aluminum (Al), phosphorus (P), or other impurities that increase the refractive index. Alternatively, the clad may be made of silica glass doped with fluorine (F), boron (B), or other impurities that reduce the refractive index, and the core may be made of pure silica glass.

FIG. 2 is a diagram for explaining the result of calculating the relationship between the core spacing and the group delay spread (GDS) in the three-core structure arranged in an annular shape as shown in FIG. 1(A).

Here, the refractive index distribution of the core was set to a step type, the core radius was set to 4.5 μm, the relative refractive index difference was set to 0.35%, and the GDS was calculated when the core spacing Λ was changed. Note that the analysis method and parameters described in Non-Patent Document 6 are used to calculate the GDS. Specifically, the bending radius of the optical fiber is 140 mm, the twist speed is 0.5 πrad/m, the standard deviation _σγ of the twist speed is 0.1 rad/m, and the wavelength is 1550 nm. Also, the propagation distance is 1 km. By the way, as described in Non-Patent Document 6, it is not realistic to assume that the refractive index distribution of each core of the multi-core fiber is ideally the same in consideration of the errors involved in the manufacture of the optical fiber. The deviation σ _Δ of the relative refractive index difference between is set to 0.001%.

From the obtained figure, the coupling state between modes can be mainly classified into three. In a region where the core spacing is large, the structure is classified as a so-called non-coupled multi-core fiber, and the GDS spreads in proportion to the distance due to inter-core skew caused by structural deviation of each core. This is defined as a "weak coupling region", and the GDS spreads in proportion to the group velocity difference of each propagation mode when each core is regarded as an independent structure.

On the other hand, in the region where the core spacing is 25-30 μm, random coupling occurs between modes as designed in Non-Patent Documents 5 and 6, and GDS exhibits a characteristic that increases in proportion to the square root of the distance. Such regions are defined as "random coupling regions" and the GDS waveform exhibits a Gaussian waveform.

When the core spacing is further reduced, a mode called a supermode propagates across multiple cores, and when the core spacing is reduced, the effective refractive index difference between them increases, suppressing coupling between supermodes. , the GDS increases in proportion to the group delay difference between modes in the same way as in few-mode fibers. This is defined as a "supermode waveguide region".

As can be seen from FIG. 2, the GDS takes a minimum value when the core spacing is in the range of 20-25 μm, and if the core spacing is reduced beyond that range, the GDS increases. In the core spacing range of 20-25 μm, the GDS waveform exhibits a Gaussian waveform, indicating random coupling between modes. On the other hand, in the region of 25 μm or less, an increase in the propagation mode effective refractive index difference reduces the inter-mode coupling and increases the GDS.

The tendency described above shows the same tendency regardless of the number of cores. FIG. 3 shows the calculation of GDS when the core spacing is 18 or 25 μm and the number of cores is varied from 2 to 7. FIG. Calculation conditions are the same as those described in FIG. Also, the arrangement of the cores is an annular arrangement.

From FIG. 3, the GDS shows a Gaussian waveform at a core spacing of 25 μm for any number of cores, and random coupling is obtained between modes, whereas at a core spacing of 18 μm, the amount of coupling between modes decreases. GDS is increasing as a result.

As mentioned above, the increase in GDS with decreasing core spacing is due to an increase in the intermodal effective refractive index difference. To show this, the calculated core spacing and the effective refractive index of the propagation mode for 3, 4, and 6 core numbers are shown in FIGS. It can be seen that the effective refractive index difference between modes increases as the core spacing decreases for any number of cores.

FIG. 4D shows the maximum effective refractive index difference Δneff _max between modes at each core spacing. For any number of cores, the maximum effective refractive index difference with respect to the core spacing shows the same value, which explains why the GDS characteristics independent of the number of cores as shown in FIG. 3 were obtained.

(Embodiment 1)
FIG. 5 is a diagram for explaining the arrangement of cores in the coupled multi-core optical fiber of this embodiment. FIGS. 5A, 5B, and 5C illustrate 3 cores, 4 cores, and 6 cores, respectively. The coupled multi-core optical fiber of this embodiment has three or more cores 11, and in cross section, the cores are rotationally asymmetric with respect to the center of the clad, and the inter-core crosstalk is -10 dB/km or more. are arranged with a minimum core spacing of

In particular, this embodiment is characterized in that the symmetry of the core arrangement with respect to the clad center is lost by changing the positions of specific cores in comparison with the conventional core arrangement. In other words, in the present embodiment, at least one of the cores 11 in the cross section is positioned outside the regular polygon from the position of the core, assuming that all the cores are arranged in the regular polygon. characterized by

Specifically, cores 11-1 and 11-2 are offset from conventional core positions 11-1 _d and 11-2 _d . For example, in the case of 6 cores in FIG. 5(C), a dotted line connecting the centers of cores adjacent to each other with the minimum core spacing .LAMBDA. On the other hand, by increasing the distance between the cores 11-1 and 11-2 as in the present embodiment, rotational symmetry is lost. In this specification, the absence of rotational symmetry is referred to as "rotational asymmetry".

Also, the multi-core optical fiber with the rotationally asymmetric core arrangement as shown in FIG. 5 can be regarded as the multi-core optical fiber shown in Non-Patent Document 7 in which the cores are arranged pseudo-one-dimensionally on a straight line. Therefore, in a multi-core optical fiber with a rotationally asymmetric core arrangement as shown in FIG. 5, there are propagation modes having N different effective refractive indices for N cores.

On the other hand, in the core arrangement having rotational symmetry as shown in FIG. 1, a plurality of rotationally symmetric propagation modes are generated in the propagating modes. Although the electric field distributions are different, being rotationally symmetric can effectively be regarded as the same electric field, ie the modes are degenerate. In other words, a multi-core optical fiber with a rotationally symmetrical core arrangement as shown in FIG. 1 cannot form propagation mode groups having N different effective refractive indices for N cores.

In addition, with the core arrangement of Non-Patent Document 7, the cores can only be arranged on a straight line, and the cores cannot be arranged so that the core arrangement density is high with respect to the circular clad cross section, which is not preferable. .

Here, as shown in FIG. 5, let θ be the difference in angle from the position where two cores with different core spacings in the annular arrangement are arranged in the conventional structure and the adjacent cores. In other words, a line segment (chain line) connecting the center of the core 11-1 and the center of the core 11-3 and a line segment connecting the center of the conventional core position _11-1d and the center of the core 11-3 ( Dotted line) is the angle θ. It is preferable that θ≧10°.

FIG. 6 is a diagram for explaining the relationship between the core spacing and the effective refractive index of the propagation mode in the coupled multicore optical fiber of this embodiment. FIGS. 6A, 6B, and 6C are calculation results for 3, 4, and 6 cores, respectively. Note that θ is 10°. Comparing with FIG. 4, which is the calculation result of the multi-core optical fiber with the conventional structure, it can be seen that the coupled multi-core optical fiber of this embodiment propagates modes with different effective refractive indices corresponding to the number of cores.

FIG. 6D shows the maximum effective refractive index difference Δneff _max between modes at each core spacing. Comparing with FIG. 4, which shows the result of the multi-core optical fiber with the conventional structure, if the core spacing is the same, the maximum effective refractive index difference of the coupled multi-core optical fiber of this embodiment is equal to that of the multi-core optical fiber with the conventional structure. is about half the value of

FIG. 7 is a diagram for explaining the result of calculating the relationship between the core spacing and the GDS in a multi-core optical fiber having a three-core structure in which the cores are arranged as shown in FIG. 5(A). Here, θ=10°. When compared with FIG. 2, which shows the result of the multi-core optical fiber with the conventional structure, it can be seen that the multi-core optical fiber of the present embodiment provides random coupling and expands the area of the core spacing Λ where the GDS is reduced. Specifically, the region is expanded to 17.5 to 30 μm, particularly in the direction in which the core spacing Λ is narrowed. This means that the multi-core optical fiber of this embodiment can arrange the cores at a higher density than the conventional multi-core optical fiber.

The relationship between the core spacing and the GDS was also calculated for the multi-core optical fiber with 4 cores in FIG. 5B and the multi-core optical fiber with 6 cores in FIG. It has been confirmed that random bonding can be obtained even if the length is shortened.

FIG. 8 is a diagram for explaining the results of calculating the core number dependency of GDS for each value of θ. Calculations are made with the core spacing Λ=18 μm. θ=0° corresponds to a multi-core optical fiber having a conventional structure. From FIG. 8, it can be seen that GDS can be reduced in any number of cores by making θ greater than 0°. Further, from FIG. 8, it can be said that θ should be 10° or more in order to obtain a significant GDS reduction effect.

Although FIG. 5 shows an example in which the positions of two cores are displaced from the conventional core arrangement, the same effect can be obtained by displacing only one core.

In this embodiment, calculations and effects are described for a core capable of propagating a single mode at 1260 nm or more. However, even when the core has two or more propagation modes, the effect of reducing the GDS at the maximum effective refractive index difference can be obtained as in the above description. Therefore, the present invention is not limited to multi-core optical fibers with single-mode cores, but may be multi-core optical fibers with cores in which several modes propagate.

ここで、ａは係数である。図９はａ＝０．１の例である。コア１１をこのように配置すると、任意のコア数においてクラッド中心における対称性を無くすことができ、ＧＤＳを低減できる。なお、コアを渦巻状に配置することは、任意のコア数でコア配置を一意に決めることができ、コア数の拡張性があるという特徴もある。 (Embodiment 2)
FIG. 9 is a diagram illustrating an example of a multi-core optical fiber with more than seven cores. This example is characterized in that the core 11 is spirally arranged in the cross section. A solid line connecting the centers of the spirally arranged cores 11 is called an involute curve represented by Equation (1).

where a is a coefficient. FIG. 9 is an example of a=0.1. By arranging the cores 11 in this way, the symmetry at the clad center can be eliminated for an arbitrary number of cores, and the GDS can be reduced. Arranging the cores spirally also has the characteristic that the core arrangement can be uniquely determined with an arbitrary number of cores, and that the number of cores can be expanded.

(Embodiment 3)
In Embodiments 1 and 2, structural examples of core arrangement having no symmetry with respect to the clad center have been described. In this embodiment, it will be explained that similar effects can be obtained by reducing the symmetry with respect to the clad center even with a conventional core arrangement.

An example of FIG. 10 in which the cores 11 are arranged in a square lattice in the clad 12 will be described. In the core arrangement structure shown in FIG. 10(A), the number of symmetry is 2, and the structure (4-fold rotational symmetry) in which the core 11 is arranged in a square shape in the clad 12 as shown in FIG. become smaller. Also, as shown in FIG. 10(B), the core interval Λ1 between the four cores 11 in the upper stage and the four cores 11 in the lower stage may be made larger than Λ. The core arrangement structure of FIG. 10(B) can reduce the maximum value of the effective refractive index difference more than the core arrangement structure of FIG. 10(A), and can further reduce the GDS.

(Embodiment 4)
In the present embodiment, it will be quantitatively described how much binding amount random binding occurs and GDS can be reduced.
Considering that the repeater section sandwiched between optical amplifiers is generally 40 km or longer, the case of a transmission distance of 40 km will be described. FIG. 11 is a diagram for explaining calculation results of impulse response shapes when inter-core crosstalk is changed. In the calculation, the differential group delay DMD between modes is assumed to be 1 ns/km and the number of propagation modes is two.

Here, the input pulse width is sufficiently small with respect to the impulse response (the peak waveforms present at both ends in the case of -20 dB/km are the same as the input pulse width), and the output waveforms are , which is obtained by adding the output light of .

At −20 dB/km, there are pulses with high intensity at both ends, and the width is 40 ns, which is the same value as the cumulative DMD (1 ns/km×40 km). In the case of -10 dB/km, the impulse response width is the same as the cumulative DMD, although the pulse intensity at both ends is reduced.

On the other hand, at a coupling amount of −5 dB/km or more, the impulse response shape becomes a Gaussian shape. It is well known that when the core-to-core coupling is strong, the impulse response shape becomes Gaussian. In the case of −3 dB/km, the shape is similarly Gaussian, but it can be seen that the width (for example, the half-value width) is even smaller.

According to Non-Patent Document 8, when the impulse response shape is Gaussian, the impulse response width is proportional to the square root of the distance. Compared to uncoupled fibers whose impulse response width is proportional to distance, it is known that coupled optical fibers can reduce the impulse response width especially in long-distance transmission (see, for example, Non-Patent Document 8). Therefore, the relationship between the amount of coupling (crosstalk) and the Gaussian shape of the impulse response width was confirmed.

FIG. 12 is a diagram for explaining the relationship between the amount of coupling (crosstalk) and the correlation coefficient when the impulse response shape is fitted with a Gaussian waveform. As shown in FIG. 12, the impulse response becomes an ideal Gaussian waveform with a correlation coefficient of 95% or more at a coupling amount of -10 dB/km or more.

FIG. 13 is a diagram for explaining the relationship between the amount of coupling and the standard deviation obtained by Gaussian fitting of the impulse response waveform. It can be seen that the greater the amount of coupling, the smaller the impulse response width.

κは隣接コア間結合係数、
βはモードの伝搬定数、
Ｒは曲げ半径、
である。 From these results, it can be seen that when coupling of -10 dB/km or more is obtained between the cores, the half-value width can be greatly reduced when the impulse response width is Gauss-fitted. Therefore, the distance Λ between adjacent cores should be designed so as to obtain coupling of -10 dB/km or more. Here, the inter-core crosstalk amount XT (/km) can be calculated by the following equation.

κ is the coupling coefficient between adjacent cores,
β is the propagation constant of the mode,
R is the bending radius,
is.

That is, the condition for the distance Λ between adjacent cores for obtaining crosstalk between cores of -10 dB/km or more is given by the following equation.

By substituting XT=0.1 (equivalent to −10 dB/km) into the number C1, the desired distance Λ between adjacent cores can be obtained. As for the bending radius, since the bending radius is set to 140 mm in the measurement of the cut-off wavelength of the optical fiber, if the bending radius is similarly set to 140 mm, the value of the crosstalk under the worst conditions can be calculated. Incidentally, this radius of 140 mm is used as an effective value of the bending radius given to the optical fiber in the cable. However, since the adjacent cores do not contact with respect to the core diameter 2a, at least 2a≦Λ
Must. As described in Non-Patent Document 5, there is a phenomenon that the impulse response width increases when the cores are close to each other, so empirically, 2a≦2Λ
It is desirable to set it to a degree.

Here, κ can be calculated from the effective refractive index and electric field distribution of each mode, as described in Non-Patent Document 9. It is a parameter generally used for calculating the amount of crosstalk between cores in a multi-core optical fiber.

[Appendix]
The following is a description of the multi-core optical fiber of this embodiment.
This multi-core optical fiber has three or more cores, each core is arranged so as not to have rotational symmetry with respect to the clad center, and the core spacing is -10 dB/km or more. adjusted to be

i.e.
(1):
This multi-core optical fiber is a coupled multi-core optical fiber having three or more cores, in which the arrangement of the cores in the cross section is rotationally asymmetric with respect to the center of the clad, and the shortest inter-core distance Λ is the number C1. characterized by being

(2):
In the multi-core optical fiber described in (1) above, in the cross section, at least one of the cores extends outside the regular polygon from a position of the core on the assumption that all the cores are arranged in a regular polygon. It is characterized by being in a shifted position.

(3):
In the multi-core optical fiber described in (2) above, an angle between a line segment connecting the center of the core located at a displaced position and the center of the core adjacent to the core and a side of the regular polygon is 10°. It is characterized by the above.

(4):
The multi-core optical fiber described in (1) above is characterized in that the cores are arranged spirally in a cross section.

(5):
This multi-core optical fiber is a coupled multi-core optical fiber having three or more cores, and in a cross section, an n-gon (n is an integer of 4 or more) formed by the arrangement of the cores is m with respect to the center of the clad. It is characterized by rotational symmetry (m is an integer of 2 or more and less than n), and the shortest inter-core distance Λ is the number C1.

Since the optical fiber of the present invention can arrange a large number of cores in a smaller area, the multiplexing degree of the cores is improved and the transmission capacity is increased. In addition, since the optical fiber of the present invention has a small group delay spread after signal propagation, it has the effect of reducing the calculation load in MIMO processing for compensating inter-mode crosstalk at the receiving end.

INDUSTRIAL APPLICABILITY The present invention can be used as a transmission medium in an optical transmission system.

11: core 12: clad

Claims (2)

A coupled multi-core optical fiber having three or more cores,
In the cross section, the cores are rotationally asymmetric with respect to the center of the clad and are arranged at a minimum core spacing at which inter-core crosstalk is −10 dB/km or more , and at least one of the cores is arranged with all the cores Being located outside the regular polygon from the position of the core assumed to be arranged in the regular polygon
A coupled multi-core optical fiber characterized by: a line segment connecting the center of the core at the shifted position and the center of the core adjacent to the core;
2. The coupled multi-core optical fiber according to claim 1 , wherein the angles formed with the sides of the regular polygon are 10[deg.] or more.

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