JP6503513B2 - Multicore fiber - Google Patents

Description

The present invention relates to multi-core fibers.
Priority is claimed on Japanese Patent Application No. 2017-051904, filed March 16, 2017, the content of which is incorporated herein by reference.

  In order to cope with the increase in communication traffic in recent years, a further increase in communication (transmission) capacity is required. However, in an optical communication system using a single mode fiber (SMF) used in conventional optical communication, a limit to the increase in capacity is expected. As a technology to overcome the limitation, space-multiplexed research and development are actively conducted.

As an optical fiber that realizes spatial multiplexing, a number mode fiber (FMF) that increases the capacity by propagating a plurality of modes in one core and placing a signal in each mode, and each of a plurality of cores There is a multi-core fiber (MCF) in which the capacity is increased by loading a signal. For example, Non-Patent Document 1 discloses a few mode fiber.
In addition, MCFs are roughly divided into non-bonded MCFs that allow each core to transmit information independently, and (in the mode of) each core combine to form a super mode, and information is sent to each super mode. There are two types: coupled MCF (C-MCF: Coupled Multicore fiber) to be transmitted. C-MCF is one of fibers for Mode Division Multiplexing (MDM) transmission as disclosed in, for example, Non-Patent Documents 2 and 3.

G. Milione et al., “1.2-Tb / s MIMO-less Transmission Over 1 km of Four-Core Elliptical-Core Few-Mode Fiber with 125-μm Diameter Cladding”, Proc. Of OECC / PS2016, PD2-1 ( 2016). C. Xia et al., "Supermodes for optical transmission", Opt. Exp., Vol. 19, no. 17, pp. 16653-16664 (2011). R. Ryf et al., “Long-Distance Transmission over Coupled-Core Multicore Fiber”, Proc. Of ECOC 2016, Th.3.C.3 (2016).

In FMF and C-MCF, a plurality of modes are propagated to one core. For example, in the case of the 2LP mode, the information is transmitted by being put on the 3 modes of LP ₀₁ , LP _11a and LP _11b . Here, degenerate modes such as LP _11a and LP _11b are not easily identified on the receiving side because they are easily mixed due to structural fluctuations in the fiber and disturbances such as twist and bending applied to the fiber. There is a case. In the case of long distance transmission, it is preferable to perform digital signal processing such as Multiple-input and Multiple-output (MIMO) to separate and receive mixed modes. However, in the case of short distance transmission in a data center or the like, it is preferable that such processing can be omitted from the viewpoint of cost and the like. Further, even in the case of long distance transmission, it is required to reduce the load of the MIMO processing by reducing the differential group delay (DGD) or the like.

Non-Patent Document 1 discloses a multi-mode multi-core fiber in which four cores are circularly arranged, and each core can propagate two LP modes of LP ₀₁ and LP ₁₁ . In Non-Patent Document 1, it is possible to suppress coupling between two LP ₁₁ modes and omit MIMO processing by making the shape of the core elliptical to cut off one degenerate LP ₁₁ mode. It is disclosed. In Non-Patent Document 1, since each core is an independent transmission path, the super mode is not formed.
In Non-Patent Document 2, forming a super mode and suppressing DGD in each mode by adjusting the pitch and arrangement of each core using a coupled MCF in which adjacent cores are arranged at equal intervals. Is disclosed.
Non-Patent Document 3 discloses a four-core coupled multi-core fiber (CC-MCF) for long distance transmission. In Non-Patent Document 3, DGDs of a plurality of modes propagating CC-MCF are reduced, and the number of taps of MIMO processing calculated by an 8 × 8 matrix is reduced (with a reduction in processing load). It is disclosed to separate the

  However, in any of the above documents, mode separation can be easily performed by MIMO processing with respect to a configuration in which a core forming a super mode in short distance transmission can omit MIMO processing or in a plurality of super modes existing in long distance transmission. The configuration of the core is not fully disclosed.

  The present invention has been made in view of the above-described circumstances, and it is possible to omit MIMO processing in short distance transmission, or a coupled multi-core fiber in which mode separation can be easily performed by MIMO processing in long distance transmission. provide.

  A first aspect of the present invention is a multi-fiber, comprising a plurality of cores arranged in a matrix of m rows × n columns (m and n are integers of 2 or more) in a cross section in the longitudinal direction of the optical fiber The distance between two cores adjacent to each other in the column direction is wider than the distance between two cores adjacent to each other in the row direction, and light propagating in each core in both the row direction and the column direction is strong It is configured to combine to form a super mode.

  According to a second aspect of the present invention, in the multi-core fiber according to the first aspect, in the case of n 間隔 3, an interval between cores adjacent to each other in the row direction may be substantially constant.

  According to a third aspect of the present invention, in the multi-core fiber according to the first or second aspect, in the case of m 間隔 3, the spacing between cores adjacent to each other in the row direction may be substantially constant.

  According to a fourth aspect of the present invention, in the multi-core fiber according to any one of the first to third aspects, the plurality of cores arranged in a matrix of m rows and n columns have substantially the same structure. May be

  According to a fifth aspect of the present invention, in the multi-core fiber according to any one of the first to fourth aspects, the plurality of cores arranged in a matrix of m rows × n columns in a predetermined transmission wavelength band Each may be configured to operate in a single mode.

  According to a sixth aspect of the present invention, in the multi-core fiber according to any one of the first to fifth aspects, the propagation constant of the highest order supermode which can be transmitted and the refractive index difference of the cladding in a predetermined transmission wavelength band May be configured to be 0.0005 or more.

  A seventh aspect of the present invention is the multi-core fiber according to any one of the first to fifth aspects, wherein a difference in propagation constant between mode groups used for transmission is 0.0005 or more in a predetermined transmission wavelength band. It may be configured as it is.

  According to an eighth aspect of the present invention, in the multi-core fiber according to any one of the first to seventh aspects, the spacing between the cores adjacent to each other in the column direction is the spacing between the cores adjacent to each other in the row direction It may be about √3 times of.

The ninth aspect of the present invention may be n = 2 in the multi-core fiber according to any one of the first, third to eighth aspects.
According to a tenth aspect of the present invention, in the multi-core fiber according to any one of the first, second, and fourth to eighth aspects, m = 2.

  An eleventh aspect of the present invention is the multicore fiber according to any one of the first to tenth aspects, wherein the multi-fiber according to the above aspect is m rows x n columns (m and n are integers of 2 or more) There may be a plurality of regions in which the plurality of cores arranged in a matrix are formed, and the plurality of regions may be arranged such that light propagating in the respective regions is uncoupled.

  According to the above aspect of the present invention, it is possible to realize a coupled multi-core fiber in which MIMO processing can be omitted in short distance transmission. Alternatively, in long distance transmission, it is possible to realize a coupled multi-core fiber in which mode separation can be easily performed by MIMO processing.

FIG. 1 is a schematic view of a multi-core fiber 1 according to a first embodiment of the present invention. FIG. 1 is a cross-sectional view perpendicular to the longitudinal direction of a multi-core fiber 1 according to a first embodiment of the present invention. FIG. 5 is a cross-sectional view perpendicular to the longitudinal direction of a multi-core fiber 1A according to a second embodiment of the present invention. It is sectional drawing perpendicular | vertical to the longitudinal direction of the multi-core fiber 1B which concerns on 2nd Embodiment of this invention. In the first embodiment, it is a graph showing the relationship between the propagation constant n _eff and the inter-core distance lambda ₂ in the column direction Y of the super mode calculated in both (1) and the finite element method (FEM). It is a graph which shows the Fourier-transform result of the spectrum of the recorded image. It is a beam profile in two peaks of FIG. It is a graph which shows an impulse response measurement result. It is a graph which shows the result of having measured XT 3 times with the wavelength in a C + L band. In the first embodiment, (a) shows the refractive index distribution of each core when Λ ₂ = 8.66 μm, (b) shows the FEM electric field distribution of the LP ₀₁ -like mode, and (c) shows the same. a FEM electric field distribution LP _11a -like mode. In the comparative example, (a) shows the refractive index distribution of each core, (b) shows the FEM electric field distribution of the LP ₀₁ -like mode, (c) shows the FEM electric field distribution of the LP _11a- like mode, d) is the FEM electric field distribution of the following mode: In the second embodiment, a graph showing the relationship between the propagation constants n _eff and the inter-core distance lambda ₂ in the column direction Y of the super mode calculated in both (1) and the finite element method (FEM).

  Hereinafter, the present invention will be described based on preferred embodiments with reference to the drawings. Note that the embodiments shown below are described by way of example in order to better understand the spirit of the invention, and the present invention is not limited unless otherwise specified. Further, in the drawings used for the following description, for the sake of easy understanding of the features of the present invention, the main parts may be enlarged for convenience, and the dimensional ratio of each component is the same as the actual Not necessarily. Moreover, in order to make the feature of the present invention intelligible, some parts are omitted for convenience.

First Embodiment The configuration of a multi-core fiber 1 according to a first embodiment of the present invention will be described using FIGS. 1A and 1B. FIG. 1A is a schematic view showing the configuration of a multi-core fiber 1 according to a first embodiment of the present invention, and FIG. 1B is a cross-sectional view perpendicular to the longitudinal direction of the multi-core fiber 1.

As shown in FIGS. 1A and 1B, the multi-core fiber 1 according to the present embodiment is composed of four cores 2 and a clad 3.
The multi-core fiber 1 is a coupled multi-core fiber (C-MCF), and is configured such that light propagating through the cores 2 is strongly coupled to form a super mode.

The four cores 2 are arranged in 2 rows × 2 columns in a cross section in the longitudinal direction of the multi-core fiber 1. As shown in FIG. 1B, the radius of the core 2 is a. The core-to-core distance in the row direction X (core-to-center distance) is Λ ₁ , the core-to-core distance in the column direction Y is Λ ₂ , and in the present embodiment, each core is arranged such that Λ ₁ <Λ ₂ . That is, the distance between two cores adjacent to each other in the column direction Y is wider than the distance between two cores adjacent to each other in the row direction X. Then, light propagating through each core 2 in both the row direction X and the column direction Y is strongly coupled to form a super mode.
By arranging the cores in this manner, it is possible to realize a coupled multi-core fiber in which MIMO processing can be omitted in short distance transmission (for example, transmission of 100 m to 10 km). In addition, in long distance transmission (for example, transmission of several tens of kilometers or more), it is possible to realize a coupled multi-core fiber in which mode separation can be easily performed by MIMO processing.
Alternatively, by forming the core 2 in fours of 2 rows × 2 columns, each core can be smaller than the two-core type coupled MCF. Therefore, when the glass rod of the same rod diameter is manufactured as a base material, the core can be formed long, and the manufacturing efficiency can be improved.

Clad 3 A common clad covering the periphery of all cores 2. Assuming that the refractive index of the core 2 is n _core and the refractive index of the cladding 3 is n _clad , the core 2 is configured such that n _core > n _clad .

  In this embodiment, preferably, all cores 2 are capable of single mode transmission in the transmission band. The cores 2 are preferably all of substantially the same structure (composed of the same kind of core). Here, substantially the same structure means that the size, the shape, the refractive index, and the like are the same to the extent that the characteristics of the light wave propagating through the core 2 are not affected. By forming the cores 2 in this manner, light propagating through the cores is easily coupled, and the formation of the super mode is facilitated.

  As a medium which comprises the core 2 and the clad 3 of the multi-core fiber 1, quartz system glass (silica glass), multicomponent glass, a plastics etc. are mentioned. The quartz-based glass includes pure quartz glass containing no additive and quartz-based glass containing an additive. As an additive, 1 type (s) or 2 or more types, such as Ge, Al, P, B, F, Cl, an alkali metal, are mentioned, A refractive index can be adjusted by adding these to quartz-type glass.

The wavelength band used for transmission in the multicore fiber 1 according to the present embodiment is not particularly limited, and examples thereof include C band (1530-1565 nm), L band (1565-1625 nm), and the like. As a condition for single mode operation in the used wavelength band, it is preferable to satisfy the single mode operation condition of v ≦ 2.405 as normalized frequency v = 2πa (n _core ² −n _clad ² ) ^1/2 / λ. The upper limit of the core radius at which the relative refractive index difference Δ = (n _core ² −n _clad ² ) / (2 n _core ² ) is 0.05% or more and v ≦ 2.405 holds in the C + L band is approximately 13 μm. The value of Δ for which v ≦ 2.405 holds at each core radius can be determined automatically. Here, λ is the wavelength, and 2π / λ is the wavenumber k ₀ .
In addition, in a or Δ such that v ≧ 2.405, the transmission loss of the high-order mode higher than the LP ₁₁ mode may be equal to or more than α _Loss . At this time, α _Loss > 0 dB / m, and for example, 0.1 dB / m, 0.5 dB / m, 1.0 dB / m, 2.0 dB / m and the like can be mentioned. Examples of the cable cutoff wavelength λ _cc of the fiber include 1530 nm or less, 1260 nm or less, and 1000 nm or less.

The number and arrangement of the cores 2 are not limited to four in 2 rows × 2 columns, and m rows × n columns (m and n are 2 or more according to the number of modes of light propagating in the multicore fiber 1 etc. It may be arranged in a matrix of integers). In this case, the distance between two cores adjacent to each other in the column direction Y is wider than the distance between two cores adjacent to each other in the row direction X, and all light propagating through the cores 2 is strongly coupled It may be configured to form a mode.
When m 場合 3, it is preferable that the distance (こ と が₂ ) between cores adjacent to each other in the row direction X be substantially constant. When n 3 3, it is preferable that the distance (好 ま し い₂ ) between the cores adjacent to each other in the column direction Y be substantially constant. Here, that the distance between the cores is substantially constant means that they are the same to such an extent that the characteristics of the light waves propagating through the cores are not affected. By making the spacing in the row direction X and the column direction Y substantially constant, the distribution of super modes spreads almost equally in each core, which is preferable when connecting multi-core fibers or the like.

Further, it is preferable that the distance (Λ ₂ ) between the cores 2 adjacent to each other in the column direction Y is approximately √3 times the distance (Λ ₁ ) between the cores 2 adjacent to each other in the row direction X. In this case, as a base material, a glass rod of a plurality of same diameter by drawing in close-packed arrangement, lambda ₂ can be prepared lambda ₁ of √3 times the multi-core fiber, the glass rod of a plurality of types of diameter There is no need to prepare. Also, if Λ ₂ is √ 3 times 結合₁ , as described later, a coupled multi-core fiber capable of omitting MIMO processing in short distance transmission, and a coupled type capable of mode separation easily in MIMO processing in long distance transmission Both multicore fibers can be realized.

Further, it is preferable to set the relationship between Λ ₁ and Λ ₂ so that the transmission difference of the highest order supermode that can be transmitted and the refractive index difference of the cladding 3 is 0.0005 or more in a predetermined transmission wavelength band. .
By setting the interval between cores in this manner, only the unnecessary mode can be cut off with certainty, and MIMO processing can be more reliably omitted.

Further, in a predetermined transmission wavelength band, it is preferable to set the relationship between よ う₁ and 伝 搬₂ so that the difference in propagation constant between mode groups used for transmission is 0.0005 or more.
By setting the interval between cores in this manner, MIMO processing can be performed separately for a small number of modes included in each mode group, and the load of MIMO processing can be further reduced. Furthermore, in the case where each mode group is defined to include only one mode, setting the above relationship eliminates the need for MIMO processing.

Second Embodiment FIGS. 2A and 2B show cross-sectional views perpendicular to the longitudinal direction of multi-core fibers 1A and 1B, respectively, according to a second embodiment. FIG. 2A shows the case where there are two bonded core regions 4 in which the core 2 of 2 rows × 2 columns is formed, and FIG. 2B shows that there are four bonded core regions 4 in which the cores 2 of 2 rows × 2 columns are formed. That's the case.
In the multicore fibers 1A and 1B, the number of coupled core regions 4 in which the cores 2 of 2 rows × 2 columns are formed with respect to the multicore fiber 1 is different. Therefore, in the following description, the same reference numerals are given to components common to those described above, and redundant description will be omitted.

In FIG. 2A, the multi-core fiber 1A has two coupled core regions 4 in which 2 rows × 2 columns of cores 2 are formed in the cladding 3. The two coupled core regions 4 are arranged side by side in the row direction X with a distance such that the lights propagating in the respective regions become uncoupled.
Similarly, in FIG. 2B, the multi-core fiber 1A has four coupled core regions 4 in which 2 rows × 2 columns of cores 2 are formed in the cladding 3. The four coupled core regions 4 are arranged in a matrix of 2 rows × 2 columns at a distance, respectively, so that the lights propagating in the respective regions become uncoupled to each other. In addition, it is possible to provide a region having a refractive index lower than that of the cladding 3 between the respective coupling core regions 4 so as to suppress the coupling between the coupling core regions 4.

In the multi-core fibers 1A and 1B, in the coupling core region 4, the distance between two cores adjacent to each other in the column direction Y is wider than the distance between two cores adjacent to each other in the row direction X. Then, light propagating through each core 2 in both the row direction X and the column direction Y is strongly coupled to form a super mode. The respective coupling core regions 4 are arranged at a distance from each other so that the lights propagating in the respective regions become non-coupled to each other.
By having such a configuration, it is possible to increase the mode multiplicity for one multi-core fiber. For example, when the mode capable of propagating through each coupled core region 4 is LP ₀₁ -like mode and LP _11a -like mode, the mode multiplicity can be set to 4 in the multicore fiber 1A, and the mode multiplicity in the multicore fiber 1B Can be eight.

The number and arrangement of the cores 2 constituting the coupling core region 4 are not limited to the same as the number and arrangement of the cores 2 in the first embodiment.
Further, the number and the arrangement of the coupled core regions 4 are not limited as long as the lights propagating in the respective regions are arranged so as not to be coupled.

Although the present invention has been described above based on the preferred embodiments, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the present invention.
The MCF of the present invention can be used as a part or all of an optical fiber used for an optical transmission line, an optical waveguide, an optical cable or the like. The optical cable preferably has at least a part of the MCF of the present invention.

  Hereinafter, the present invention will be specifically described using examples. The present invention is not limited to only this embodiment.

In the example, the configuration of the multi-core fiber 1 which is the four-core C-MCF described in the first embodiment described above is used. When all cores 2 are configured to operate in the same kind and in single mode, the propagation constants of the four super mode LP ₀₁ -like mode, LP _11a -like mode, LP _11b -like mode, and LP ₂₁ -like mode are as follows: It is given by the formula of

Here, β ₀ is a non-coupling propagation constant, κ ₁ is a mode coupling coefficient between cores 2 in the row direction X, and κ ₂ is a mode coupling coefficient between cores 2 in the column direction Y, κ ₃ Is a mode coupling coefficient between diagonal cores 2.

First Example From the relationship between the propagation constant n _{eff of the} super mode and the inter-core distance Λ _{2 in the} column direction Y, a configuration in which the LP _11b -like mode and the LP ₂₁ -like mode are cut off was investigated.
Core radius a = 2.5 μm, relative refractive index difference Δ = 0.35%, n _clad = 1.45, Λ ₁ = 5.0 μm, the wavelength of propagating light is 1550 nm, and the above equation (1) and finite elements The relationship between the supermode propagation constant n _eff calculated by both methods (FEM) and the inter-core distance Λ _{2 in the} column direction Y is shown in FIG.

As shown in FIG. 3, the result using the above equation (1) and the result using the finite element method (FEM) roughly coincided. In addition, as a reason which does not correspond completely, the thing which did not consider at the time of strong coupling, and having said that Formula (1) used calculation of the coupling coefficient of a simple 2 core model is mentioned.
As can be seen from FIG. 3, as the difference between Λ ₁ and Λ ₂ increases, the difference between the propagation constants n _eff of the LP ₀₁ -like mode and the LP _{11 a} -like mode decreases. On the other hand, the larger the difference between Λ ₁ and Λ ₂ , the larger the difference between the propagation constants n _eff of the LP _11a -like mode and the LP _11b -like mode. The propagation constant of the _LP 11b -like mode and _LP 21 -like mode lambda ₂ is approximately 8 [mu] m (lambda ₁ of 1.6 times) or more is smaller than the propagation constants of the cladding. At this time, the difference between the propagation constant of the LP _11a -like mode, which is the highest order supermode that can be transmitted, and the refractive index of the cladding is sufficiently large, 0.0005 or more, and only the LP _11b -like mode and the LP ₂₁ -like mode are selected. It turned out that it is possible to cut off. By More particularly, lambda if ₂ is less than (2 × lambda ₁₎ degree _10μm, LP 01 -like mode and _LP 11a -like mode MIMO process if the difference is also sufficiently large short distance transmission propagation constant between such Mode separation is also considered unnecessary. In particular, in the case of 8.6 ₂ = 8.66 μm (√3 times Λ ₁ ), the LP _{11 b} -like mode and the LP ₂₁ -like mode can be cut off, and MIMO processing can be omitted. It was found that if Λ ₂ = √3 作 製_1, it can be produced by close-packing arraying and drawing of seven glass rods of the same diameter, which is preferable because it has merits in manufacturing.

Based on the structure of FIG. 3 (Λ ₂ = 8.66 μm), a 4-core C-MCF was prepared, and S ² measurement was performed to observe its propagation mode. Here, a 4-core C-MCF of 22 m in length was measured. A single mode fiber (SMF) was spliced to excite 4-core C-MCF. Calculated at the wavelength 1.55 _.mu.m, LP 01 effective area of -like mode and _LP 11a -like mode _{(A eff),} respectively 177 .mu.m ^2, since it is 165 .mu.m ^2, greater SMF of _{A eff} than normal SMF (Wavelength 1.55 μm) was used.
The near-field image is near-infrared (NIR) during wavelength sweep (1.547 μm to 1.553 μm at a wavelength interval of 1.953 GHz) using a tunable laser source (TLS). Recorded using a camera. The computer controlled TLS and the camera simultaneously.

The Fourier-transform result of the spectrum of the image recorded on FIG. 4 is shown. Two peaks of differential group delay (DGD) at 0 ps and 86 ps (3.91 ns / km) are clearly observed. As shown in FIG. 5, the beam profiles at the two peaks indicate that the LP ₀₁ -like mode and the LP _11a -like mode were both propagated. Since the high order LP ₀₁ -like mode and the high order LP _11a -like mode are not observed, they are considered to be below the cutoff wavelength. Obtained by calculating the Fourier _transform, the multi-path interference of LP 11a -like mode (MPI: multi-pass interference) was -26.5DB.
Furthermore, as shown in FIG. 6, crosstalk (XT) between two propagation modes was measured by performing impulse response (IR) measurement.

The IR of a 1 km long 4-core C-MCF was measured using a vector network analyzer equipped with a light modulator and a photodetector (PD). The TLS and light modulators are connected to the PMF. To excite the 4-core C-MCF, using the same SMF as used for ^{S 2} measurements. FIG. 6 shows the results of IR measurement at a wavelength of 1.55 μm.
The LP ₀₁ -like mode appears when the value of the normalized DGD is zero. In addition, XT between the LP ₀₁ -like mode and the LP _11a -like mode appears as a “step” like when the value of the normalized DGD is between 0 and 1. The IR measurement shows that the DGD between the two modes is 3.93 ns / km, which is consistent with the results obtained by the S ² measurement.

The dashed line in FIG. 6 represents the fitting curve calculated by the theoretical IR model. From FIG. 6, the power coupling coefficient was estimated to be about 4.1 × 10 ⁻⁶ / m. Therefore, XT is estimated to be about -24 dB / km.
FIG. 7 shows the result of measuring XT three times in the wavelength band of C + L band. The measured XT was less than -23 dB / km throughout the C + L band. It can be seen that XT is sufficiently low to realize short distance transmission where MIMO processing is unnecessary.
From the above results, it can be understood that short distance transmission which does not require MIMO processing is possible according to this embodiment.

FIG. 8 shows (a) refractive index distribution of each core, (b) FEM electric field distribution of LP ₀₁ -like mode, and (c) LP _11a -like mode when 実 施₂ = 8.66 μm in this example. The FEM electric field distribution of
From the FEM electric field distribution shown in FIGS. 8 (b) and 8 (c), each core can be strongly coupled to form the LP ₀₁ -like mode and the LP _11a -like mode super mode in the configuration of this embodiment. I understood it. Here, Table 1 below shows the results of comparing the characteristics at a wavelength of 1550 nm using C-MCF of a two-core type (2 rows × 1 column) having an effective refractive index difference similar to that of this example as a comparative example. Show.

From Table 1, it can be seen that the effective cross sections of the LP ₀₁ -like mode and the LP _11a -like mode are substantially the same in the example and the comparative example. However, in the comparative example, the following mode occurs, and it is understood that the difference between the effective refractive index and the cladding is 0.0005 or more.

FIG. 9 shows (a) refractive index distribution of each core in a comparative example, (b) FEM electric field distribution of LP ₀₁ -like mode, (c) FEM electric field distribution of LP _11a- like mode, and (d) next mode The FEM electric field distribution is shown.
As shown in FIG. 9D, the next mode is generated by combining the LP ₁₁ modes of the respective cores. As described above, the super modes generated by coupling the higher order modes of the cores are compared to the super modes generated by coupling the respective cores in only the fundamental mode because the nodes of the electromagnetic field distribution exist at the centers of the cores. It becomes difficult to excite and receive modes using input / output devices such as fan-in / fan-out devices. In addition, even when the communication system is configured not to use such high-order modes, such high-order modes are excited by a slight axis deviation of the input and output parts, etc. It may be noise. Therefore, by using the configuration of this embodiment, more stable mode excitation and light reception can be performed. Furthermore, since the total area of the core portion is smaller in this embodiment than in the comparative example, the core can be formed long when a glass rod of the same rod diameter is manufactured as a base material, and the manufacturing efficiency can be improved. it can.

Second Embodiment From the relationship between the propagation constant n _{eff of the} super mode and the inter-core distance Λ _{2 in the} column direction Y, a configuration in which four modes can be handled as transmission of two mode groups was investigated.
Core radius a = 4.0 μm, relative refractive index difference Δ = 0.35%, n _clad = 1.45, Λ ₁ = 8.0 μm, the wavelength of the propagating light is 1550 nm, and the above equation (1) and finite elements The relationship between the supermode propagation constant n _eff calculated by both methods (FEM) and the inter-core distance Λ _{2 in the} column direction Y is shown in FIG.

As shown in FIG. 10, the result using the above equation (1) and the result using the finite element method (FEM) were almost the same in this example as well.
As can be seen from FIG. 10, the larger the difference between Λ ₁ and Λ ₂ , the smaller the difference between the propagation constants n _eff of the LP ₀₁ -like mode and the LP _11a- like mode. Further, as the difference between Λ ₁ and Λ ₂ increases, the difference between the propagation constants n _eff of the LP _{11 b} -like mode and the LP ₂₁ -like mode also decreases. Then, at lambda ₂ is approximately 13 .mu.m (lambda ₁ of 1.6 times) or _more, the propagation constant of the LP 01 -like mode and _LP 11a -like mode becomes approximately the _same, LP 11b -like mode and _LP 21 -like The propagation constant with the mode was approximately the same. Further, in lambda ₂ is approximately 13 .mu.m (lambda 1.6 times the _{1) above,} the difference in propagation constant between the LP 11a -like mode and _LP 11b -like mode becomes 0.0005 sufficiently large. Therefore, if the lambda ₂ is lambda ₁ of 1.6 times or _more, and composed of mode group in the LP 01 -like mode and _LP 11a -like _mode, the LP 11b -like mode and _LP 21 -like mode It can be considered that it can be handled as transmission of two mode groups with the configured mode group. Therefore, MIMO processing can be performed separately for the two modes included in each mode group, and the matrix size of the MIMO processing can be further reduced, which leads to a reduction in load. In particular, in the case of 3.8 ₂ = 13.86 μm (Λ3 times え₁ ), it can be treated as a transmission of two mode groups, and as described above, a close-packed arrangement and drawing of seven equal diameter glass rods It was found that it is possible to produce it and that it is preferable because it has manufacturing merits.

  As mentioned above, although the multi-core fiber of this invention was demonstrated, this invention is not limited to said example, It can change suitably in the range which does not deviate from the meaning of invention.

  1 ... multi-core fiber, 2 ... core, 3 ... cladding, 4 ... coupled core region

Claims (11)

And a plurality of cores arranged in a matrix of m rows and n columns (m and n are integers of 2 or more) in a cross section in the longitudinal direction of the optical fiber,
The distance between two cores adjacent to each other in the column direction is wider than the distance between two cores adjacent to each other in the row direction,
A multi-core fiber configured such that light propagating in each core in both the row direction and the column direction is strongly coupled to form a super mode.   The multi-core fiber according to claim 1, wherein when n 3 3, the spacing between cores adjacent to each other in the row direction is substantially constant.   The multi-core fiber according to claim 1 or 2, wherein when m 3 3, the distance between cores adjacent to each other in the row direction is substantially constant.   The multi-core fiber according to any one of claims 1 to 3, wherein the plurality of cores arranged in a matrix of m rows x n columns have substantially the same structure.   The plurality of cores arranged in a matrix of m rows by n columns in a predetermined transmission wavelength band are configured to operate in a single mode. Multicore fiber.   The method according to any one of claims 1 to 5, wherein in the predetermined transmission wavelength band, the difference between the propagation constant of the highest order supermode that can be transmitted and the refractive index of the cladding is 0.0005 or more. Multicore fiber described.   The multi-core fiber according to any one of claims 1 to 5, wherein in the predetermined transmission wavelength band, the difference in propagation constant between mode groups used for transmission is 0.0005 or more.   The multi-core fiber according to any one of claims 1 to 7, wherein an interval between cores adjacent to each other in the column direction is approximately √3 times an interval between cores adjacent to each other in the row direction.   The multi-core fiber according to any one of claims 1 and 3 to 8, wherein n = 2.   The multi-core fiber according to any one of claims 1, 2, and 4-8, wherein m = 2. a plurality of regions in which the plurality of cores are arranged in a matrix of m rows and n columns (m and n are integers of 2 or more);
The multi-core fiber according to any one of claims 1 to 10, wherein the plurality of regions are arranged such that lights propagating in the respective regions are uncoupled from each other.