WO2018168266A1 - Multi-core fiber - Google Patents

Multi-core fiber Download PDF

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Publication number
WO2018168266A1
WO2018168266A1 PCT/JP2018/004228 JP2018004228W WO2018168266A1 WO 2018168266 A1 WO2018168266 A1 WO 2018168266A1 JP 2018004228 W JP2018004228 W JP 2018004228W WO 2018168266 A1 WO2018168266 A1 WO 2018168266A1
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mode
core
cores
core fiber
interval
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PCT/JP2018/004228
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French (fr)
Japanese (ja)
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翔太 斉藤
竹永　勝宏
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株式会社フジクラ
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Priority to JP2018512438A priority Critical patent/JP6503513B2/en
Publication of WO2018168266A1 publication Critical patent/WO2018168266A1/en

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating

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  • the present invention relates to a multi-core fiber.
  • This application claims priority based on Japanese Patent Application No. 2017-051904 filed in Japan on March 16, 2017, the contents of which are incorporated herein by reference.
  • a plurality of modes are propagated in one core, and a signal is placed in each mode.
  • MCF multi-core fiber
  • the number mode fiber is disclosed in Non-Patent Document 1, for example.
  • the MCF is broadly divided into a non-coupled MCF in which each core independently transmits information, and each core (mode) forms a super mode, and information is stored in each super mode.
  • C-MCF Coupled Multicore fiber
  • the C-MCF is one of mode division multiplexing (MDM) transmission fibers as disclosed in Non-Patent Documents 2 and 3, for example.
  • FMF and C-MCF a plurality of modes are propagated to one core.
  • the 2LP mode information is transmitted in three modes of LP 01 , LP 11a and LP 11b .
  • the degenerate modes such as LP 11a and LP 11b are easily mixed due to structural fluctuations in the fiber and disturbances such as torsion and bending applied to the fiber, and thus are not properly identified on the receiving side.
  • digital signal processing such as MIMO (Multiple-input and Multiple-output) is preferably performed, and mixed modes are separated and received.
  • Non-Patent Document 1 discloses a number-mode multi-core fiber in which four cores are arranged in a circle and each core can propagate two LP modes, LP 01 and LP 11 .
  • Non-Patent Document 1 it is possible to suppress the coupling between two LP 11 modes by omitting one LP 11 mode which is degenerated by making the core shape an ellipse and omitting the MIMO processing. It is disclosed.
  • the super mode is not formed.
  • Non-Patent Document 2 by using a coupled MCF in which adjacent cores are arranged at equal intervals, a super mode is formed by adjusting the pitch and arrangement of each core, and DGD of each mode is suppressed.
  • Non-Patent Document 3 discloses a four-core long-distance transmission multi-core fiber (CC-MCF).
  • each mode reduces DGD of a plurality of modes propagating in CC-MCF, reduces the number of taps of MIMO processing calculated with an 8 ⁇ 8 matrix (reducing processing load) Is disclosed.
  • the core that forms the super mode in short-distance transmission can omit the MIMO processing, and mode separation is easy with MIMO processing for a plurality of super modes in long-distance transmission.
  • the structure 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 a MIMO process in short-distance transmission, or a coupled multicore fiber that can be easily mode-separated by MIMO process 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 through the cores in both the row direction and the column direction is stronger. It is configured to combine to form a super mode.
  • the interval between cores adjacent to each other in the row direction may be substantially constant.
  • the interval between cores adjacent to each other in the column direction may be substantially constant.
  • the plurality of cores arranged in a matrix of m rows ⁇ n columns have substantially the same structure. May be.
  • 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.
  • a multicore fiber according to any one of the first to fifth aspects, wherein the propagation constant of the highest supermode that 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 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 to be.
  • an interval between cores adjacent to each other in the column direction is equal to an interval between cores adjacent to each other in the row direction. May be approximately ⁇ 3 times.
  • n 2 in the multicore fiber according to any one of the first, third to eighth aspects.
  • An eleventh aspect of the present invention is the multicore fiber according to any one of the first to tenth aspects, wherein the multifiber according to the aspect has m rows ⁇ 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 through the respective regions is uncoupled.
  • a coupled multi-core fiber that can omit the MIMO processing in short-distance transmission. Or, in long-distance transmission, a coupled multi-core fiber that can be easily mode-separated by MIMO processing can be realized.
  • FIG. 1 is a schematic diagram of a multi-core fiber 1 according to a first embodiment of the present invention. It is sectional drawing perpendicular
  • FEM finite element method
  • FIG. 1A is a schematic diagram illustrating the configuration of the multicore fiber 1 according to the first embodiment of the present invention
  • FIG. 1B is a cross-sectional view perpendicular to the longitudinal direction of the multicore fiber 1.
  • the multi-core fiber 1 includes 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 each core 2 is strongly coupled to form a super mode.
  • C-MCF coupled multi-core fiber
  • the four cores 2 are arranged in 2 rows ⁇ 2 columns in the cross section in the longitudinal direction of the multicore fiber 1.
  • the radius of the core 2 is a.
  • the inter-core distance in the row direction X core-center distance
  • the inter-core distance in the column direction Y is ⁇ 2.
  • the cores are arranged so that ⁇ 1 ⁇ 2 . . That is, the interval between two cores adjacent to each other in the column direction Y is wider than the interval between two cores adjacent to each other in the row direction X.
  • the light propagating through the cores 2 in both the row direction X and the column direction Y is strongly coupled to form a super mode.
  • each core can be made smaller than the two-core coupled MCF by forming the cores 2 by four of 2 rows ⁇ 2 columns. Therefore, when producing a glass rod having the same rod diameter as a base material, the core can be formed long, and the production efficiency can be improved.
  • the clad 3 is a common clad covering the periphery of all the cores 2.
  • the core 2 is configured so that n core > n clad .
  • all the cores 2 are capable of single mode transmission in the transmission band. Moreover, it is preferable that all the cores 2 are substantially the same structure (it is comprised from the core of the same kind).
  • substantially the same structure means that the size, shape, refractive index, and the like are the same so as not to affect the characteristics of the light wave propagating through the core 2.
  • Examples of the medium constituting the core 2 and the clad 3 of the multi-core fiber 1 include quartz glass (silica glass), multicomponent glass, and plastic.
  • quartz glass there are pure quartz glass containing no additive and quartz glass containing the additive.
  • the additive include one or more of Ge, Al, P, B, F, Cl, alkali metal, and the like, and the refractive index can be adjusted by adding them to quartz glass.
  • the wavelength band used for transmission in the multi-core fiber 1 according to the present embodiment is not particularly limited, and examples thereof include C band (1530 to 1565 nm), L band (1565 to 1625 nm) and the like.
  • the normalized frequency v 2 ⁇ a (n core 2 ⁇ n clad 2 ) 1/2 / ⁇ satisfies the single mode operation condition of v ⁇ 2.405.
  • ⁇ is a wavelength
  • 2 ⁇ / ⁇ is a wave number k 0 .
  • the transmission loss of the higher-order mode of the LP 11 mode or higher may be ⁇ Loss or higher.
  • ⁇ Loss > 0 dB / m, for example, 0.1 dB / m, 0.5 dB / m, 1.0 dB / m, 2.0 dB / m, and the like.
  • the fiber cutoff wavelength ⁇ cc of the fiber include 1530 nm or less, 1260 nm or less, 1000 nm or less.
  • the number and arrangement of the cores 2 are not limited to four of 2 rows ⁇ 2 columns, and m rows ⁇ n columns (m and n are 2 or more depending on the number of modes of light propagating in the multicore fiber 1). It suffices if they are arranged in an integer) matrix. In this case, the distance between the two cores adjacent to each other in the column direction Y is wider than the distance between the two cores adjacent to each other in the row direction X, and all the light propagating through the cores 2 is strongly coupled. What is necessary is just to be comprised so that a mode may be formed. When m ⁇ 3, it is preferable that the interval ( ⁇ 2 ) between adjacent cores in the row direction X is substantially constant.
  • the interval ( ⁇ 2 ) between adjacent cores in the column direction Y is substantially constant.
  • that the interval between the cores is substantially constant means that they are the same so as not to affect the characteristics of the light wave propagating through each core.
  • the interval ( ⁇ 2 ) between the cores 2 adjacent to each other in the column direction Y is preferably approximately ⁇ 3 times the interval ( ⁇ 1 ) between the cores 2 adjacent to each other in the row direction X.
  • a base material a plurality of glass rods of the same diameter are arranged in a close-packed arrangement and drawn, whereby a multi-core fiber having ⁇ 2 of ⁇ 3 times ⁇ 1 can be produced. There is no need to prepare. If ⁇ 2 is ⁇ 3 times ⁇ 1 , as will be described later, a coupled multicore fiber that can omit MIMO processing in short-distance transmission, and a coupled type that can be easily mode-separated by MIMO processing in long-distance transmission. Both multi-core fibers can be realized.
  • ⁇ 1 and ⁇ 2 it is preferable to set the relationship between ⁇ 1 and ⁇ 2 so that the transmission constant of the highest super mode that can be transmitted and the refractive index difference between the cladding 3 is 0.0005 or more in a predetermined transmission wavelength band.
  • ⁇ 1 and ⁇ 2 it is preferable to set the relationship between ⁇ 1 and ⁇ 2 so that a difference in propagation constant between mode groups used for transmission is 0.0005 or more in a predetermined transmission wavelength band.
  • FIGS. 2A and 2B are cross-sectional views perpendicular to the longitudinal direction of multi-core fibers 1A and 1B, respectively, according to a second embodiment.
  • FIG. 2A shows a case where there are two coupled core regions 4 where the cores 2 of 2 rows ⁇ 2 columns are formed
  • FIG. 2B shows four coupled core regions 4 where the cores 2 of 2 rows ⁇ 2 columns are formed. Is the case.
  • the number of coupled core regions 4 where the 2 rows ⁇ 2 columns of cores 2 are formed differs from the multi-core fiber 1. Therefore, in the following description, about the structure which is common in what was already demonstrated, the same code
  • the multi-core fiber 1 ⁇ / b> A has two coupled core regions 4 in which 2 rows ⁇ 2 columns of cores 2 are formed in the clad 3.
  • the two coupled core regions 4 are arranged side by side in the row direction X at a distance so that the lights propagating through the respective regions are not coupled to each other.
  • the multi-core fiber 1 ⁇ / b> A has four coupled core regions 4 in which the cores 2 in 2 rows ⁇ 2 columns 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 from each other so that lights propagating through the respective regions are not coupled to each other.
  • a structure in which a region having a refractive index lower than that of the cladding 3 is provided between the coupling core regions 4 to suppress the coupling between the coupling core regions 4 may be employed.
  • the interval between two cores adjacent to each other in the column direction Y is wider than the interval between two cores adjacent to each other in the row direction X.
  • the light propagating through the cores 2 in both the row direction X and the column direction Y is strongly coupled to form a super mode.
  • region 4 is arrange
  • the multiplicity of the multicore fiber 1A can be set to 4
  • the multiplicity of the multicore fiber 1B can be set to mode multiplicity. Can be set to 8.
  • region 4 it is not limited similarly to the number and arrangement
  • the number and arrangement of the coupled core regions 4 are not limited as long as the light propagating through the respective regions is disposed so as not to be coupled.
  • 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.
  • the configuration of the multi-core fiber 1 that is the 4-core C-MCF described in the first embodiment is used.
  • the propagation constants of the four super modes LP 01 -like mode, LP 11a -like mode, LP 11b -like mode, and LP 21 -like mode are as follows: Is given by
  • ⁇ 0 is a propagation constant at the time of non-coupling
  • ⁇ 1 is a mode coupling coefficient between the cores 2 in the row direction X
  • ⁇ 2 is a mode coupling coefficient between the cores 2 in the column direction Y
  • ⁇ 3 Is a mode coupling coefficient between the diagonal cores 2.
  • the propagation constants of the LP 11b -like mode and the LP 21 -like mode are smaller than the propagation constant of the cladding.
  • the propagation constant of the LP 11a -like mode, which is the highest super mode that can be transmitted, and the refractive index difference of the cladding are sufficiently large, 0.0005 or more, and only the LP 11b -like mode and LP 21 -like mode are used. It was found that cut-off was possible.
  • ⁇ 2 is smaller than about 10 ⁇ m (twice ⁇ 1 )
  • the difference in propagation constant between the LP 01 -like mode and the LP 11a -like mode is also sufficiently large. Mode separation is also considered unnecessary.
  • ⁇ 2 8.66 ⁇ m ( ⁇ 3 times ⁇ 1 )
  • the LP 11b -like mode and the LP 21 -like mode can be cut off and the MIMO process can be omitted.
  • a 4-core C-MCF was fabricated, and S 2 measurement was performed to observe its propagation mode.
  • a 4-core C-MCF having a length of 22 m was measured.
  • Single mode fiber (SMF) was spliced for 4 core C-MCF excitation. 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 ⁇ m) was used.
  • NIR near-infrared
  • TLS tunable laser source
  • FIG. 4 shows a Fourier transform result of the spectrum of the recorded image.
  • Two peaks of differential group delay (DGD) at 0 ps and 86 ps (3.91 ns / km) are clearly observed.
  • the beam profiles at the two peaks indicate that both the LP 01 -like mode and the LP 11a -like mode have 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.
  • the multipath interference (MPI) of LP 11a -like mode obtained by calculating the Fourier transform was ⁇ 26.5 dB.
  • the 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 an optical modulator and a photodetector (PD). The TLS and the optical modulator are connected to the PMF.
  • FIG. 6 shows IR measurement results at a wavelength of 1.55 ⁇ m.
  • the LP 01 -like mode appears.
  • the XT between the LP 01 -like mode and the LP 11a -like mode appears in a “step” state when the value of the standardized DGD is between 0 and 1.
  • DGD between the two modes is 3.93ns / km, it is found to be consistent with results obtained by S 2 measurements.
  • FIG. 6 The broken line in FIG. 6 represents a fitting curve calculated by a 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 the C + L band. The measured XT was less than -23 dB / km throughout the C + L band. It can be seen that the XT is sufficiently low to realize short-distance transmission that does not require MIMO processing. From the above results, it can be seen that short distance transmission that does not require MIMO processing is possible according to the present embodiment.
  • the FEM electric field distribution is shown. From the FEM electric field distribution shown in FIG. 8B and FIG. 8C, the cores are strongly coupled in the configuration of this embodiment, and a super mode of LP 01 -like mode and LP 11a -like mode can be formed. I understood it.
  • Table 1 below shows the results of comparing the characteristics at a wavelength of 1550 nm using a 2-core type (2 rows ⁇ 1 column) C-MCF having an effective refractive index difference of the same level as that of this example as a comparative example. Show.
  • FIG. 9 shows (a) the refractive index distribution of each core in the comparative example, (b) the FEM electric field distribution in the LP 01 -like mode, (c) the FEM electric field distribution in the LP 11a -like mode, and (d) the next mode.
  • the FEM electric field distribution is shown.
  • the next mode is generated by combining the LP 11 modes of the respective cores.
  • the super mode generated by combining higher-order modes of each core is compared with the super mode generated by combining each core only in the fundamental mode because the node of the electromagnetic field distribution exists at the center of each core. It becomes difficult to excite and receive the mode using an input / output device such as a fan-in / fan-out device.
  • the total area of the core portion is smaller than that of the comparative example, so that when a glass rod having the same rod diameter is produced as a base material, the core can be formed longer, and the production efficiency can be improved. it can.
  • FIG. 11 shows the relationship between the supermode propagation constant n eff calculated by both the method (FEM) and the inter-core distance ⁇ 2 in the column direction Y.
  • ⁇ 2 is 1.6 times or more of ⁇ 1
  • a mode group composed of LP 01 -like mode and LP 11a -like mode, LP 11b -like mode, and LP 21 -like mode are used. It is considered that it can be handled as transmission of two mode groups with the configured mode group. Therefore, the MIMO processing can be performed separately for the two modes included in each mode group, the matrix size of the MIMO processing can be further reduced, and the load is reduced.
  • ⁇ 2 13.86 ⁇ m ( ⁇ 3 times ⁇ 1 )

Abstract

This multi-core fiber is provided with a plurality of cores arranged in the form of a matrix of m rows and n columns (where, m and n are integers of two or more) in a longitudinal section of an optical fiber, wherein the distance between two neighboring cores in a column direction is greater than that between two neighboring cores in a row direction. The multi-core fiber is configured so that light beams, which propagate along each of the cores in both the row and column directions, strongly couple to each other to form a super mode.

Description

マルチコアファイバMulti-core fiber
　本発明は、マルチコアファイバに関する。
　本願は、２０１７年３月１６日に、日本に出願された特願２０１７－０５１９０４号に基づき優先権を主張し、これらの内容をここに援用する。 The present invention relates to a multi-core fiber.
This application claims priority based on Japanese Patent Application No. 2017-051904 filed in Japan on March 16, 2017, the contents of which are incorporated herein by reference.
　近年の通信トラフィックの増大に対処するため、更なる通信（伝送）容量の増大が求められている。しかし、従来の光通信に用いられるシングルモードファイバ（ＳＭＦ）を用いた光通信システムでは、容量の増大に対する限界が予想されている。その限界を超えるための技術として、空間多重の研究開発が盛んに行われている。 In order to cope with the recent increase in communication traffic, further increase in communication (transmission) capacity is required. However, in an optical communication system using a single mode fiber (SMF) used for conventional optical communication, a limit to an increase in capacity is expected. Spatial multiplexing research and development has been actively conducted as a technology to overcome this limitation.
　空間多重を実現する光ファイバとして、１つのコア内に複数のモードを伝搬させ、それぞれのモードに信号を載せることで容量の増大を図った数モードファイバ（ＦＭＦ）と、複数のコアのそれぞれに信号を載せることで容量の増大を図ったマルチコアファイバ（ＭＣＦ）とがある。数モードファイバについては例えば非特許文献１に開示される。
　また、ＭＣＦには、大きく分けて、各コアが独立して情報を伝送させる非結合型ＭＣＦと、各コア（のモード）が結合することによってスーパーモードを形成し、それぞれのスーパーモードに情報を伝送させる結合型ＭＣＦ（Ｃ－ＭＣＦ：Coupled Multicore fiber）との２種類がある。Ｃ－ＭＣＦは、例えば非特許文献２及び３に開示されるようなモード多重伝送（ＭＤＭ：Mode Division Multiplexing）伝送用ファイバの一つである。 As an optical fiber that realizes spatial multiplexing, a plurality of modes are propagated in one core, and a signal is placed in each mode. There is a multi-core fiber (MCF) whose capacity is increased by placing a signal. The number mode fiber is disclosed in Non-Patent Document 1, for example.
In addition, the MCF is broadly divided into a non-coupled MCF in which each core independently transmits information, and each core (mode) forms a super mode, and information is stored in each super mode. There are two types of coupled MCF (C-MCF: Coupled Multicore fiber) to be transmitted. The C-MCF is one of mode division multiplexing (MDM) transmission fibers as disclosed in Non-Patent Documents 2 and 3, for example.
　ＦＭＦやＣ－ＭＣＦでは、１つのコアに複数のモードを伝搬する。例えば、２ＬＰモードの場合，ＬＰ０１，ＬＰ１１ａ，ＬＰ１１ｂの３モードに情報を乗せて発信する。ここで、ＬＰ１１ａ，ＬＰ１１ｂのように縮退しているモードは、ファイバ中の構造揺らぎや、ファイバに加わるねじれや曲げなどの外乱によって、容易に混ざってしまうため、受信側で適切に識別されない場合がある。長距離伝送の場合は、ＭＩＭＯ（Multiple-input and Multiple-output）等のディジタル信号処理を行い、混ざったモードを分離して受信することが好ましい。しかしながら、データセンター等での短距離伝送の場合では、コスト等の観点からこのような処理は省略できることが好ましい。また、長距離伝送の場合でも、群遅延時間差（ＤＧＤ：Differential Group Delay）の低減などによってＭＩＭＯ処理の負荷を削減することが求められる。 In FMF and C-MCF, a plurality of modes are propagated to one core. For example, in the case of the 2LP mode, information is transmitted in three modes of LP 01 , LP 11a and LP 11b . Here, the degenerate modes such as LP 11a and LP 11b are easily mixed due to structural fluctuations in the fiber and disturbances such as torsion and bending applied to the fiber, and thus are not properly identified on the receiving side. There is a case. In the case of long-distance transmission, digital signal processing such as MIMO (Multiple-input and Multiple-output) is preferably performed, and mixed modes are separated and received. 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. Even in the case of long-distance transmission, it is required to reduce the load of MIMO processing by reducing a group delay time difference (DGD: Differential Group Delay).
　非特許文献１では、４つのコアが円形配列され、それぞれのコアがＬＰ０１とＬＰ１１との２つのＬＰモードを伝搬可能である数モードマルチコアファイバが開示されている。非特許文献１では、コアの形状を楕円形にして、縮退している片方のＬＰ１１モードをカットオフさせることで、２つのＬＰ１１モード間の結合を抑制し、ＭＩＭＯ処理を省略することが開示されている。なお、非特許文献１では、各コアが独立した伝送路となるため、スーパーモードは形成されない。
　非特許文献２では、隣接するコアが等間隔で配置される結合型ＭＣＦを用いて、各コアのピッチと配置を調整することで、スーパーモードを形成し、また各モードのＤＧＤを抑制することが開示されている。
　非特許文献３では、４コア型の長距離伝送用の結合型マルチコアファイバ（ＣＣ－ＭＣＦ）が開示されている。非特許文献３においては、ＣＣ－ＭＣＦを伝搬する複数のモードのＤＧＤを低減し、８×８の行列で演算するＭＩＭＯ処理のタップ数を削減して（処理の負荷を低減して）各モードを分離することが開示されている。 Non-Patent Document 1 discloses a number-mode multi-core fiber in which four cores are arranged in a circle and each core can propagate two LP modes, LP 01 and LP 11 . In Non-Patent Document 1, it is possible to suppress the coupling between two LP 11 modes by omitting one LP 11 mode which is degenerated by making the core shape an ellipse and omitting the MIMO processing. 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, by using a coupled MCF in which adjacent cores are arranged at equal intervals, a super mode is formed by adjusting the pitch and arrangement of each core, and DGD of each mode is suppressed. Is disclosed.
Non-Patent Document 3 discloses a four-core long-distance transmission multi-core fiber (CC-MCF). In Non-Patent Document 3, each mode reduces DGD of a plurality of modes propagating in CC-MCF, reduces the number of taps of MIMO processing calculated with an 8 × 8 matrix (reducing processing load) Is disclosed.
　しかしながら、上記のいずれの文献においても、短距離伝送においてスーパーモードを形成するコアがＭＩＭＯ処理を省略可能な構成や、長距離伝送において複数存在するスーパーモードに対してＭＩＭＯ処理でモード分離が容易なコアの構成については十分に開示されていない。 However, in any of the above documents, the core that forms the super mode in short-distance transmission can omit the MIMO processing, and mode separation is easy with MIMO processing for a plurality of super modes in long-distance transmission. The structure of the core is not fully disclosed.
　本発明は、上述した事情に鑑みてなされたものであって、短距離伝送においてＭＩＭＯ処理を省略可能であり、もしくは、長距離伝送においてはＭＩＭＯ処理で容易にモード分離可能な結合型マルチコアファイバを提供する。 The present invention has been made in view of the above-described circumstances, and it is possible to omit a MIMO process in short-distance transmission, or a coupled multicore fiber that can be easily mode-separated by MIMO process in long-distance transmission. provide.
　本発明の第１態様は、マルチファイバであって、光ファイバの長手方向における断面において、ｍ行×ｎ列（ｍ，ｎは２以上の整数）のマトリックス状に配置された複数のコアを備え、行方向において互いに隣り合う２つのコア同士の間隔より列方向において互いに隣り合う２つのコア同士の間隔の方が広く、前記行方向及び前記列方向の両方向において各コアを伝搬する光同士が強結合してスーパーモードを形成するように構成される。 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 through the cores in both the row direction and the column direction is stronger. It is configured to combine to form a super mode.
　本発明の第２態様は、上記第１態様に係るマルチコアファイバにおいて、ｎ≧３の場合に、前記行方向において互いに隣り合うコア同士の間隔が実質的に一定であってもよい。 In the second aspect of the present invention, in the multi-core fiber according to the first aspect, when n ≧ 3, the interval between cores adjacent to each other in the row direction may be substantially constant.
　本発明の第３態様は、上記第１又は第２態様に係るマルチコアファイバにおいて、ｍ≧３の場合に、前記列方向において互いに隣り合うコア同士の間隔が実質的に一定であってもよい。 In the third aspect of the present invention, in the multi-core fiber according to the first or second aspect, when m ≧ 3, the interval between cores adjacent to each other in the column direction may be substantially constant.
　本発明の第４態様は、上記第１～第３態様のいずれか一態様に係るマルチコアファイバにおいて、ｍ行×ｎ列のマトリックス状に配置された前記複数のコアが実質的に同一構造であってもよい。 According to a fourth aspect of the present invention, in the multicore fiber according to any one of the first to third aspects, the plurality of cores arranged in a matrix of m rows × n columns have substantially the same structure. May be.
　本発明の第５態様は、上記第１～第４態様のいずれか一態様に係るマルチコアファイバにおいて、所定の伝送波長帯域において、ｍ行×ｎ列のマトリックス状に配置された前記複数のコアがそれぞれシングルモードで動作するように構成されてもよい。 According to a fifth aspect of the present invention, in the multicore 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.
　本発明の第６態様は、上記第１～第５態様のいずれか一態様に係るマルチコアファイバにおいて、所定の伝送波長帯域において、伝送可能な最高次のスーパーモードの伝搬定数とクラッドの屈折率差が０．０００５以上であるように構成されてもよい。 According to a sixth aspect of the present invention, there is provided a multicore fiber according to any one of the first to fifth aspects, wherein the propagation constant of the highest supermode that 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.
　本発明の第７態様は、上記第１～第５態様のいずれか一態様に係るマルチコアファイバにおいて、所定の伝送波長帯域において、伝送に用いるモード群間の伝搬定数の差が０．０００５以上であるように構成されてもよい。 According to a seventh aspect of the present invention, in the multicore fiber according to any one of the first to fifth aspects, 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 to be.
　本発明の第８態様は、上記第１～第７態様のいずれか一態様に係るマルチコアファイバにおいて、前記列方向において互いに隣り合うコア同士の間隔が、前記行方向において互いに隣り合うコア同士の間隔のおよそ√３倍であってもよい。 According to an eighth aspect of the present invention, in the multicore fiber according to any one of the first to seventh aspects, an interval between cores adjacent to each other in the column direction is equal to an interval between cores adjacent to each other in the row direction. May be approximately √3 times.
　本発明の第９態様は、上記第１、第３～第８態様のいずれか一態様に係るマルチコアファイバにおいて、ｎ＝２であってもよい。
　本発明の第１０態様は、上記第１、第２、第４～第８態様のいずれか一態様に係るマルチコアファイバにおいて、ｍ＝２であってもよい。 According to a ninth aspect of the present invention, n = 2 in the multicore fiber according to any one of the first, third to eighth aspects.
A tenth aspect of the present invention may be m = 2 in the multicore fiber according to any one of the first, second, and fourth to eighth aspects.
　本発明の第１１態様は、上記第１～第１０態様のいずれか一態様に係るマルチコアファイバにおいて、上記態様に係るマルチファイバが、ｍ行×ｎ列（ｍ，ｎは２以上の整数）のマトリックス状に配置された前記複数のコアが形成される領域を複数有し、前記複数の領域は、それぞれの領域を伝搬する光同士がそれぞれ非結合となるように配置されてもよい。 An eleventh aspect of the present invention is the multicore fiber according to any one of the first to tenth aspects, wherein the multifiber according to the aspect has m rows × 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 through the respective regions is uncoupled.
　本発明の上記態様によれば、短距離伝送においてはＭＩＭＯ処理を省略可能な結合型マルチコアファイバを実現できる。もしくは、長距離伝送においてはＭＩＭＯ処理で容易にモード分離可能な結合型マルチコアファイバを実現できる。 According to the above aspect of the present invention, it is possible to realize a coupled multi-core fiber that can omit the MIMO processing in short-distance transmission. Or, in long-distance transmission, a coupled multi-core fiber that can be easily mode-separated by MIMO processing can be realized.
本発明の第１実施形態に係るマルチコアファイバ１の模式図である。1 is a schematic diagram of a multi-core fiber 1 according to a first embodiment of the present invention. 本発明の第１実施形態に係るマルチコアファイバ１の長手方向に垂直な断面図である。It is sectional drawing perpendicular | vertical to the longitudinal direction of the multi-core fiber 1 which concerns on 1st Embodiment of this invention. 本発明の第２実施形態に係るマルチコアファイバ１Ａの長手方向に垂直な断面図である。It is sectional drawing perpendicular | vertical to the longitudinal direction of 1 A of multi-core fibers which concern on 2nd Embodiment of this invention. 本発明の第２実施形態に係るマルチコアファイバ１Ｂの長手方向に垂直な断面図である。It is sectional drawing perpendicular | vertical to the longitudinal direction of the multi-core fiber 1B which concerns on 2nd Embodiment of this invention. 第１実施例における、（１）式及び有限要素法（ＦＥＭ）の両方で算出したスーパーモードの伝搬定数ｎｅｆｆと列方向Ｙのコア間距離Λ２との関係を示すグラフである。It is a graph which shows the relationship between the propagation constant neff of the super mode calculated by both (1) Formula and the finite element method (FEM) and the inter-core distance Λ 2 in the column direction Y in the first embodiment. 記録された画像のスペクトルのフーリエ変換結果を示すグラフである。It is a graph which shows the Fourier-transform result of the spectrum of the recorded image. 図４の２つのピークにおけるビームプロファイルである。It is a beam profile in two peaks of FIG. インパルス応答測定結果を示すグラフである。It is a graph which shows an impulse response measurement result. Ｃ＋Ｌバンド内の波長で、ＸＴを３回測定した結果を示すグラフである。It is a graph which shows the result of having measured XT 3 times at the wavelength in a C + L band. 第１実施例において、Λ２＝８．６６μｍとした際の、（ａ）は各コアの屈折率分布を示し、（ｂ）はＬＰ０１－ｌｉｋｅモードのＦＥＭ電界分布を示し、（ｃ）はＬＰ１１ａ－ｌｉｋｅモードのＦＥＭ電界分布である。In the first example, when Λ 2 = 8.66 μm, (a) shows the refractive index distribution of each core, (b) shows the F 01 electric field distribution of LP 01 -like mode, and (c) shows It is a FEM electric field distribution of LP 11a -like mode. 比較例において、（ａ）は各コアの屈折率分布を示し、（ｂ）はＬＰ０１－ｌｉｋｅモードのＦＥＭ電界分布を示し、（ｃ）はＬＰ１１ａ－ｌｉｋｅモードのＦＥＭ電界分布を示し、（ｄ）は次のモードのＦＥＭ電界分布である。In the comparative example, (a) shows the refractive index distribution of each core, (b) shows the FEM electric field distribution of LP 01 -like mode, (c) shows the FEM electric field distribution of LP 11a -like mode, d) is the FEM electric field distribution in the next mode. 第２実施例における、（１）式及び有限要素法（ＦＥＭ）の両方で算出したスーパーモードの伝搬定数ｎｅｆｆと列方向Ｙのコア間距離Λ２との関係を示すグラフである。It is a graph which shows the relationship between the propagation constant neff of the super mode calculated by both (1) Formula and the finite element method (FEM) and the inter-core distance Λ 2 in the column direction Y in the second embodiment.
　以下、好適な実施形態に基づき、図面を参照して本発明を説明する。なお、以下に示す実施形態は、発明の趣旨をより良く理解させるために、例を挙げて説明するものであり、特に指定のない限り、本発明を限定しない。また、以下の説明に用いる図面は、本発明の特徴を分かりやすくするために、便宜上、要部となる部分を拡大している場合があり、各構成要素の寸法比率などが実際と同じであるとは限らない。また、本発明の特徴を分かりやすくするために、便宜上、省略した部分がある。 Hereinafter, based on a preferred embodiment, the present invention will be described with reference to the drawings. The following embodiments are described by way of example in order to better understand the gist of the invention, and the present invention is not limited unless otherwise specified. In addition, in the drawings used in the following description, in order to make the features of the present invention easier to understand, the main part may be enlarged for convenience, and the dimensional ratios of the respective components are the same as the actual ones. Not necessarily. In addition, in order to make the features of the present invention easier to understand, some parts are omitted for convenience.
第１実施形態
　本発明の第１実施形態に係るマルチコアファイバ１の構成について、図１Ａ、図１Ｂを用いて説明する。図１Ａは、本発明の第１実施形態に係るマルチコアファイバ１の構成を示す模式図であり、図１Ｂはマルチコアファイバ１の長手方向に垂直な断面図である。 1st Embodiment The structure of the multi-core fiber 1 which concerns on 1st Embodiment of this invention is demonstrated using FIG. 1A and FIG. 1B. FIG. 1A is a schematic diagram illustrating the configuration of the multicore fiber 1 according to the first embodiment of the present invention, and FIG. 1B is a cross-sectional view perpendicular to the longitudinal direction of the multicore fiber 1.
　図１Ａ、図１Ｂに示すように、本実施形態に係るマルチコアファイバ１は、４つのコア２と、クラッド３とで構成される。
　マルチコアファイバ１は、結合型マルチコアファイバ（Ｃ－ＭＣＦ）であり、各コア２を伝搬する光同士が強結合してスーパーモードを形成するように構成される。 As shown in FIGS. 1A and 1B, the multi-core fiber 1 according to this embodiment includes 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 each core 2 is strongly coupled to form a super mode.
　４つのコア２は、マルチコアファイバ１の長手方向における断面において２行×２列に配列される。図１Ｂに示すように、コア２の半径はａである。行方向Ｘのコア間距離（コア中心間距離）はΛ１、列方向Ｙのコア間距離はΛ２であり、本実施形態ではΛ１＜Λ２となるように各コアが配列されている。つまり、行方向Ｘにおいて互いに隣り合う２つのコア同士の間隔より列方向Ｙにおいて互いに隣り合う２つのコア同士の間隔の方が広い。そして、行方向Ｘ及び列方向Ｙの両方向において各コア２を伝搬する光同士が強結合してスーパーモードを形成するように構成される。
　このようにコアを配置することで、短距離伝送（例えば、１００ｍ以上１０ｋｍ以下の伝送）においてはＭＩＭＯ処理を省略可能な結合型マルチコアファイバを実現できる。また長距離伝送（例えば、数１０ｋｍ以上の伝送）においてはＭＩＭＯ処理で容易にモード分離可能な結合型マルチコアファイバを実現できる。
　もしくは、コア２を２行×２列の４つで形成することで、２コア型結合ＭＣＦよりも各コアを小さくできる。そのため、同じロッド径のガラスロッドを母材として作製する場合はコアを長く形成することができ、作製効率を向上することができる。 The four cores 2 are arranged in 2 rows × 2 columns in the cross section in the longitudinal direction of the multicore fiber 1. As shown in FIG. 1B, the radius of the core 2 is a. The inter-core distance in the row direction X (core-center distance) is Λ 1 , and the inter-core distance in the column direction Y is Λ 2. In this embodiment, the cores are arranged so that Λ 1 <Λ 2 . . That is, the interval between two cores adjacent to each other in the column direction Y is wider than the interval between two cores adjacent to each other in the row direction X. The light propagating through the cores 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 way, it is possible to realize a coupled multi-core fiber that can omit the MIMO process in short-distance transmission (for example, transmission of 100 to 10 km). In long-distance transmission (for example, transmission of several tens of kilometers or more), a coupled multi-core fiber that can be easily mode-separated by MIMO processing can be realized.
Alternatively, each core can be made smaller than the two-core coupled MCF by forming the cores 2 by four of 2 rows × 2 columns. Therefore, when producing a glass rod having the same rod diameter as a base material, the core can be formed long, and the production efficiency can be improved.
　クラッド３すべてのコア２の周囲を覆う共通のクラッドである。コア２の屈折率をｎｃｏｒｅとし、クラッド３の屈折率をｎｃｌａｄとすると、コア２のｎｃｏｒｅ＞ｎｃｌａｄとなるように構成される。 The clad 3 is a common clad covering the periphery of all the cores 2. When the refractive index of the core 2 is n core and the refractive index of the clad 3 is n clad , the core 2 is configured so that n core > n clad .
　本実施形態において、すべてのコア２が、伝送帯域においてシングルモード伝送が可能であることが好ましい。またコア２は、すべて実質的に同一構造である（同種のコアから構成されている）ことが好ましい。ここで、実質的に同一構造とは、コア２を伝搬する光波の特性に影響がない程度に大きさ、形状、屈折率などが同一であることをいう。このようにコア２を形成することで各コアを伝搬する光が結合しやすくなり、スーパーモードの形成が容易になる。 In this embodiment, it is preferable that all the cores 2 are capable of single mode transmission in the transmission band. Moreover, it is preferable that all the cores 2 are substantially the same structure (it is comprised from the core of the same kind). Here, substantially the same structure means that the size, shape, refractive index, and the like are the same so as not to affect the characteristics of the light wave propagating through the core 2. By forming the core 2 in this manner, light propagating through the cores can be easily coupled, and the super mode can be easily formed.
　マルチコアファイバ１のコア２及びクラッド３を構成する媒質としては、石英系ガラス（シリカガラス）、多成分ガラス、プラスチック等が挙げられる。石英系ガラスとしては、添加物を含まない純石英ガラスと、添加物を含む石英系ガラスがある。添加物としては、Ｇｅ，Ａｌ，Ｐ，Ｂ，Ｆ，Ｃｌ，アルカリ金属等の１種又は２種以上が挙げられ、これらを石英系ガラスに添加することにより屈折率を調整することができる。 Examples of the medium constituting the core 2 and the clad 3 of the multi-core fiber 1 include quartz glass (silica glass), multicomponent glass, and plastic. As the quartz glass, there are pure quartz glass containing no additive and quartz glass containing the additive. Examples of the additive include one or more of Ge, Al, P, B, F, Cl, alkali metal, and the like, and the refractive index can be adjusted by adding them to quartz glass.
　本実施形態に係るマルチコアファイバ１で伝送に使用される波長帯域は、特に限定されないが、Ｃバンド（１５３０～１５６５ｎｍ）、Ｌバンド（１５６５～１６２５ｎｍ）等が挙げられる。使用波長帯域でシングルモード動作する条件としては、正規化周波数ｖ＝２πａ（ｎｃｏｒｅ ２－ｎｃｌａｄ ２）１／２／λとして、ｖ≦２．４０５のシングルモード動作条件を満たすことが好ましい。比屈折率差Δ＝（ｎｃｏｒｅ ２－ｎｃｌａｄ ２）／（２ｎｃｏｒｅ ２）が０．０５％以上で、Ｃ＋Ｌバンドでｖ≦２．４０５が成り立つコア半径の上限はおおよそ１３μｍである。それぞれのコア半径でｖ≦２．４０５が成り立つΔの値は自動的に決めることができる。なお、λは波長であり、２π／λは波数ｋ０である。
　また、ｖ≧２．４０５となるようなａあるいはΔにおいては、ＬＰ１１モード以上の高次モードの伝送損失がαＬｏｓｓ以上であってもよい。このとき、αＬｏｓｓ＞０ｄＢ／ｍであり、例えば０．１ｄＢ／ｍ、０．５ｄＢ／ｍ、１．０ｄＢ／ｍ、２．０ｄＢ／ｍ等が挙げられる。ファイバのケーブルカットオフ波長λｃｃとしては、例えば１５３０ｎｍ以下、１２６０ｎｍ以下、１０００ｎｍ以下等が挙げられる。 The wavelength band used for transmission in the multi-core fiber 1 according to the present embodiment is not particularly limited, and examples thereof include C band (1530 to 1565 nm), L band (1565 to 1625 nm) and the like. As a condition for the single mode operation in the used wavelength band, it is preferable that the normalized frequency v = 2πa (n core 2 −n clad 2 ) 1/2 / λ satisfies the single mode operation condition of v ≦ 2.405. The upper limit of the core radius at which the relative refractive index difference Δ = (n core 2 −n clad 2 ) / (2n core 2 ) is 0.05% or more and v ≦ 2.405 is established in the C + L band is approximately 13 μm. The value of Δ for which v ≦ 2.405 is established for each core radius can be automatically determined. Note that λ is a wavelength, and 2π / λ is a wave number k 0 .
In addition, in a or Δ such that v ≧ 2.405, the transmission loss of the higher-order mode of the LP 11 mode or higher may be α Loss or higher. At this time, α Loss > 0 dB / m, for example, 0.1 dB / m, 0.5 dB / m, 1.0 dB / m, 2.0 dB / m, and the like. Examples of the fiber cutoff wavelength λ cc of the fiber include 1530 nm or less, 1260 nm or less, 1000 nm or less.
　なお、コア２の数及び配置は２行×２列の４つに限定されず、マルチコアファイバ１中を伝搬する光のモード数等に応じてｍ行×ｎ列（ｍ及びｎは２以上の整数）のマトリックス状に配置されていればよい。この場合、行方向Ｘにおいて互いに隣り合う２つのコア同士の間隔より列方向Ｙにおいて互いに隣り合う２つのコア同士の間隔の方が広く、各コア２を伝搬する光同士がすべて強結合してスーパーモードを形成するように構成されていればよい。
　またｍ≧３である場合、行方向Ｘにおいて互いに隣り合うコア同士の間隔（Λ２）が実質的に一定であることが好ましい。また、ｎ≧３である場合、列方向Ｙにおいて互いに隣り合うコア同士の間隔（Λ２）が実質的に一定であることが好ましい。ここで、コア同士の間隔が実質的に一定とは、各コアを伝搬する光波の特性に影響がない程度に同一であることをいう。行方向Ｘや列方向Ｙの間隔を実質的に一定にすることで、スーパーモードの分布が各コアにほぼ均等に広がるため、マルチコアファイバ同士の接続などの際に好ましい。 The number and arrangement of the cores 2 are not limited to four of 2 rows × 2 columns, and m rows × n columns (m and n are 2 or more depending on the number of modes of light propagating in the multicore fiber 1). It suffices if they are arranged in an integer) matrix. In this case, the distance between the two cores adjacent to each other in the column direction Y is wider than the distance between the two cores adjacent to each other in the row direction X, and all the light propagating through the cores 2 is strongly coupled. What is necessary is just to be comprised so that a mode may be formed.
When m ≧ 3, it is preferable that the interval (Λ 2 ) between adjacent cores in the row direction X is substantially constant. When n ≧ 3, it is preferable that the interval (Λ 2 ) between adjacent cores in the column direction Y is substantially constant. Here, that the interval between the cores is substantially constant means that they are the same so as not to affect the characteristics of the light wave propagating through each core. By making the interval in the row direction X and the column direction Y substantially constant, the distribution of the super mode spreads almost uniformly in each core, which is preferable when connecting multi-core fibers.
　また列方向Ｙにおいて互いに隣り合うコア２同士の間隔（Λ２）が、行方向Ｘにおいて互いに隣り合うコア２同士の間隔（Λ１）のおよそ√３倍であることが好ましい。この場合、母材として、複数の同径のガラスロッドを最密充填配列して線引きすることで、Λ２がΛ１の√３倍のマルチコアファイバを作製でき、複数種の径のガラスロッドを用意する必要がない。また、Λ２がΛ１の√３倍であれば、後述するように短距離伝送においてＭＩＭＯ処理を省略可能な結合型マルチコアファイバ、及び長距離伝送においてＭＩＭＯ処理で容易にモード分離可能な結合型マルチコアファイバのどちらも実現できる。 In addition, the interval (Λ 2 ) between the cores 2 adjacent to each other in the column direction Y is preferably approximately √3 times the interval (Λ 1 ) between the cores 2 adjacent to each other in the row direction X. In this case, as a base material, a plurality of glass rods of the same diameter are arranged in a close-packed arrangement and drawn, whereby a multi-core fiber having Λ 2 of √3 times Λ 1 can be produced. There is no need to prepare. If Λ 2 is √3 times Λ 1 , as will be described later, a coupled multicore fiber that can omit MIMO processing in short-distance transmission, and a coupled type that can be easily mode-separated by MIMO processing in long-distance transmission. Both multi-core fibers can be realized.
　また、所定の伝送波長帯域において、伝送可能な最高次のスーパーモードの伝搬定数とクラッド３の屈折率差が０．０００５以上となるようにΛ１とΛ２との関係を設定することが好ましい。
　このようにコア同士の間隔を設定することで、必要ないモードのみを確実にカットオフすることができ、より確実にＭＩＭＯ処理を省略することができる。 In addition, it is preferable to set the relationship between Λ 1 and Λ 2 so that the transmission constant of the highest super mode that can be transmitted and the refractive index difference between the cladding 3 is 0.0005 or more in a predetermined transmission wavelength band. .
By setting the interval between the cores in this way, only unnecessary modes can be cut off reliably, and the MIMO processing can be omitted more reliably.
　また、所定の伝送波長帯域において、伝送に用いるモード群間の伝搬定数の差が０．０００５以上となるようにΛ１とΛ２との関係を設定することが好ましい。
　このようにコア同士の間隔を設定することで、各モード群に含まれる少数のモードに対して別々にＭＩＭＯ処理を行うことができ、ＭＩＭＯ処理の負荷をより削減することができる。さらに、各モード群がそれぞれ１つのモードのみを含むように定義する場合には、上記の関係を設定することでＭＩＭＯ処理が不要となる。 In addition, it is preferable to set the relationship between Λ 1 and Λ 2 so that a difference in propagation constant between mode groups used for transmission is 0.0005 or more in a predetermined transmission wavelength band.
By setting the interval between the cores in this way, it is possible to separately perform MIMO processing for a small number of modes included in each mode group, and it is possible to further reduce the load of the MIMO processing. Furthermore, when each mode group is defined so as to include only one mode, MIMO processing is not required by setting the above relationship.
第２実施形態
　図２Ａ、図２Ｂは第２実施形態に係る、それぞれマルチコアファイバ１Ａ、１Ｂの長手方向に垂直な断面図を示す。図２Ａは２行×２列のコア２が形成される結合コア領域４が２つある場合であり、図２Ｂは２行×２列のコア２が形成される結合コア領域４が４つある場合である。
　なお、マルチコアファイバ１Ａ、１Ｂでは、マルチコアファイバ１に対して２行×２列のコア２が形成される結合コア領域４の数が異なる。そのため、以降の説明において、すでに説明したものと共通する構成については、同一の符号を付して重複する説明を省略する。 Second Embodiment FIGS. 2A and 2B are cross-sectional views perpendicular to the longitudinal direction of multi-core fibers 1A and 1B, respectively, according to a second embodiment. FIG. 2A shows a case where there are two coupled core regions 4 where the cores 2 of 2 rows × 2 columns are formed, and FIG. 2B shows four coupled core regions 4 where the cores 2 of 2 rows × 2 columns are formed. Is the case.
In the multi-core fibers 1 </ b> A and 1 </ b> B, the number of coupled core regions 4 where the 2 rows × 2 columns of cores 2 are formed differs from the multi-core fiber 1. Therefore, in the following description, about the structure which is common in what was already demonstrated, the same code | symbol is attached | subjected and the overlapping description is abbreviate | omitted.
　図２Ａでは、マルチコアファイバ１Ａが、クラッド３中に２行×２列のコア２が形成される結合コア領域４を２つ有する。２つの結合コア領域４は、それぞれの領域を伝搬する光同士がそれぞれ非結合となるように距離を空けて行方向Ｘに並べて配置される。
　また、同様に図２Ｂでは、マルチコアファイバ１Ａが、クラッド３中に２行×２列のコア２が形成される結合コア領域４を４つ有する。４つの結合コア領域４は、それぞれの領域を伝搬する光同士がそれぞれ非結合となるようにそれぞれ距離を空けて２行×２列のマトリックス状に配置される。また、各結合コア領域４の間にクラッド３よりも屈折率の低い領域を設けて結合コア領域４間の結合を抑えるような構造としても良い。 In FIG. 2A, the multi-core fiber 1 </ b> A has two coupled core regions 4 in which 2 rows × 2 columns of cores 2 are formed in the clad 3. The two coupled core regions 4 are arranged side by side in the row direction X at a distance so that the lights propagating through the respective regions are not coupled to each other.
Similarly, in FIG. 2B, the multi-core fiber 1 </ b> A has four coupled core regions 4 in which the cores 2 in 2 rows × 2 columns 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 from each other so that lights propagating through the respective regions are not coupled to each other. Further, a structure in which a region having a refractive index lower than that of the cladding 3 is provided between the coupling core regions 4 to suppress the coupling between the coupling core regions 4 may be employed.
　マルチコアファイバ１Ａ、１Ｂにおいては、結合コア領域４では、行方向Ｘにおいて互いに隣り合う２つのコア同士の間隔より列方向Ｙにおいて互いに隣り合う２つのコア同士の間隔の方が広い。そして、行方向Ｘ及び列方向Ｙの両方向において各コア２を伝搬する光同士が強結合してスーパーモードを形成するように構成される。そして、それぞれの結合コア領域４は、それぞれの領域を伝搬する光同士がそれぞれ非結合となるように互いに距離を空けて配置される。
　このような構成を有することにより、１本のマルチコアファイバに対するモード多重度を増加させることができる。例えば、各結合コア領域４を伝搬可能なモードがＬＰ０１－ｌｉｋｅモード，ＬＰ１１ａ－ｌｉｋｅモードである場合、マルチコアファイバ１Ａではモード多重度を４にすることができ、マルチコアファイバ１Ｂではモード多重度を８にすることができる。 In the multicore fibers 1A and 1B, in the coupled core region 4, the interval between two cores adjacent to each other in the column direction Y is wider than the interval between two cores adjacent to each other in the row direction X. The light propagating through the cores 2 in both the row direction X and the column direction Y is strongly coupled to form a super mode. And each coupling | bonding core area | region 4 is arrange | positioned mutually spaced apart so that the light which propagates each area | region may each become non-coupling | bonding.
By having such a configuration, the mode multiplicity for one multicore fiber can be increased. For example, when the modes capable of propagating through each coupled core region 4 are the LP 01 -like mode and the LP 11a -like mode, the multiplicity of the multicore fiber 1A can be set to 4, and the multiplicity of the multicore fiber 1B can be set to mode multiplicity. Can be set to 8.
　なお、結合コア領域４を構成するコア２の数及び配置については第１実施形態のコア２の数及び配置と同様に限定されない。
　また、結合コア領域４の数及び配置は、それぞれの領域を伝搬する光同士が非結合となるように配置されていれば限定されない。 In addition, about the number and arrangement | positioning of the core 2 which comprise the joint core area | region 4, it is not limited similarly to the number and arrangement | positioning of the core 2 of 1st Embodiment.
Further, the number and arrangement of the coupled core regions 4 are not limited as long as the light propagating through the respective regions is disposed so as not to be coupled.
　以上、本発明を好適な実施形態に基づいて説明してきたが、本発明は上述の実施形態に限定されるものではなく、本発明の要旨を逸脱しない範囲で種々の改変が可能である。
　本発明のＭＣＦは、光伝送路、光導波路、光ケーブル等に使用される光ファイバの一部又は全部として用いることができる。光ケーブルは、本発明のＭＣＦを少なくとも一部に有することが好ましい。 As mentioned above, although this invention has been demonstrated based on suitable embodiment, this invention is not limited to the above-mentioned embodiment, A various change is possible in the range which does not deviate from the summary of this 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.
　以下、実施例を用いて本発明を具体的に説明する。なお、本発明は、本実施例のみに限定されるものではない。 Hereinafter, the present invention will be specifically described with reference to examples. In addition, this invention is not limited only to a present Example.
　実施例では、上述の第１実施形態に記載される４コアＣ－ＭＣＦであるマルチコアファイバ１の構成を用いた。すべてのコア２が同種でかつシングルモード動作するように構成した場合、４つのスーパーモードＬＰ０１－ｌｉｋｅモード、ＬＰ１１ａ－ｌｉｋｅモード、ＬＰ１１ｂ－ｌｉｋｅモード、ＬＰ２１－ｌｉｋｅモードの伝搬定数は以下の式で与えられる。 In the examples, the configuration of the multi-core fiber 1 that is the 4-core C-MCF described in the first embodiment is used. When all cores 2 are configured to be of the same type and operate in a single mode, the propagation constants of the four super modes LP 01 -like mode, LP 11a -like mode, LP 11b -like mode, and LP 21 -like mode are as follows: Is given by
　ここで、β０は非結合時の伝搬定数であり、κ１は行方向Ｘのコア２間のモード結合係数であり、κ２は列方向Ｙのコア２間のモード結合係数であり、κ３は対角のコア２間のモード結合係数である。 Here, β 0 is a propagation constant at the time of non-coupling, κ 1 is a mode coupling coefficient between the cores 2 in the row direction X, κ 2 is a mode coupling coefficient between the cores 2 in the column direction Y, and κ 3 Is a mode coupling coefficient between the diagonal cores 2.
第１実施例
　スーパーモードの伝搬定数ｎｅｆｆと列方向Ｙのコア間距離Λ２との関係からＬＰ１１ｂ－ｌｉｋｅモード、ＬＰ２１－ｌｉｋｅモードがカットオフとなる構成を調査した。
　コア半径ａ＝２．５μｍ、比屈折率差Δ＝０．３５％、ｎｃｌａｄ＝１．４５、Λ１＝５．０μｍ、伝搬する光の波長を１５５０ｎｍとして、上記（１）式及び有限要素法（ＦＥＭ）の両方で算出したスーパーモードの伝搬定数ｎｅｆｆと列方向Ｙのコア間距離Λ２との関係を図３に示す。 First Example The configuration in which the LP 11b -like mode and the LP 21 -like mode are cut off 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 was investigated.
When the core radius a = 2.5 μm, the relative refractive index difference Δ = 0.35%, n clad = 1.45, Λ 1 = 5.0 μm, the wavelength of propagating light is 1550 nm, the above formula (1) and the finite element FIG. 3 shows the relationship between the supermode propagation constant n eff calculated by both the method (FEM) and the inter-core distance Λ 2 in the column direction Y.
　図３に示すように、上記（１）式を用いた結果と有限要素法（ＦＥＭ）を用いた結果はおおよそ一致した。なお、完全に一致しない理由としては、強結合時を考慮していないことや上記（１）式が単純な２コアモデルの結合係数の計算を用いたことが挙げられる。
　図３からわかるように、Λ１とΛ２との差が広くなるほど、ＬＰ０１－ｌｉｋｅモードとＬＰ１１ａ－ｌｉｋｅモードとの伝搬定数ｎｅｆｆの差は小さくなった。一方、Λ１とΛ２との差が広くなるほど、ＬＰ１１ａ－ｌｉｋｅモードとＬＰ１１ｂ－ｌｉｋｅモードとの伝搬定数ｎｅｆｆの差は大きくなった。そして、Λ２がおよそ８μｍ（Λ１の１．６倍）以上でＬＰ１１ｂ－ｌｉｋｅモード及びＬＰ２１－ｌｉｋｅモードの伝搬定数がクラッドの伝搬定数より小さくなった。またこのとき、伝送可能な最高次のスーパーモードであるＬＰ１１ａ－ｌｉｋｅモードの伝搬定数とクラッドの屈折率差が０．０００５以上と十分大きく、ＬＰ１１ｂ－ｌｉｋｅモード及びＬＰ２１－ｌｉｋｅモードのみをカットオフ可能であることがわかった。さらに特に、Λ２が１０μｍ（Λ１の２倍）程度より小さければ、ＬＰ０１－ｌｉｋｅモードとＬＰ１１ａ－ｌｉｋｅモードとの伝搬定数の差も十分に大きく短距離伝送であればＭＩＭＯ処理等によるモードの分離も不要であると考えられる。特に、Λ２＝８．６６μｍ（Λ１の√３倍）の場合、ＬＰ１１ｂ－ｌｉｋｅモード及びＬＰ２１－ｌｉｋｅモードについてカットオフできかつＭＩＭＯ処理も省略可能である。Λ２＝√３Λ１であれば、７つの同径のガラスロッドを最密充填配列して線引きすることで作製可能であり、製造上のメリットもあり好ましいことがわかった。 As shown in FIG. 3, the result using the above equation (1) and the result using the finite element method (FEM) are almost the same. Note that the reasons for the inconsistency are that the strong coupling is not taken into account and the calculation of the coupling coefficient of the two-core model in which the above equation (1) is simple.
As can be seen from FIG. 3, the wider the difference between Λ 1 and Λ 2 , the smaller the difference in the propagation constant n eff between the LP 01 -like mode and the LP 11a -like mode. On the other hand, as the difference between Λ 1 and Λ 2 becomes wider, the difference in the propagation constant n eff between the LP 11a -like mode and the LP 11b -like mode increases. When Λ 2 is about 8 μm (1.6 times Λ 1 ) or more, the propagation constants of the LP 11b -like mode and the LP 21 -like mode are smaller than the propagation constant of the cladding. At this time, the propagation constant of the LP 11a -like mode, which is the highest super mode that can be transmitted, and the refractive index difference of the cladding are sufficiently large, 0.0005 or more, and only the LP 11b -like mode and LP 21 -like mode are used. It was found that cut-off was possible. More particularly, if Λ 2 is smaller than about 10 μm (twice Λ 1 ), the difference in propagation constant between the LP 01 -like mode and the LP 11a -like mode is also sufficiently large. Mode separation is also considered unnecessary. In particular, when Λ 2 = 8.66 μm (√3 times Λ 1 ), the LP 11b -like mode and the LP 21 -like mode can be cut off and the MIMO process can be omitted. If Λ 2 = √3Λ 1, it can be produced by drawing the glass rods having the same diameter in a close-packed arrangement with close packing, and it has been found to be preferable because of manufacturing advantages.
　図３の構造（Λ２＝８．６６μｍ）に基づいて、４コアＣ－ＭＣＦを作製し、その、伝播モードを観察するために、Ｓ２測定を行った。ここでは、２２ｍの長さの４コアＣ－ＭＣＦを測定した。４コアＣ－ＭＣＦの励振のために、シングルモードファイバ（ＳＭＦ）を接合した。波長１．５５μｍにおいて計算した、ＬＰ０１－ｌｉｋｅモードとＬＰ１１ａ－ｌｉｋｅモードの実効断面積（Ａｅｆｆ）は、それぞれ１７７μｍ２、１６５μｍ２であることから、通常のＳＭＦよりもＡｅｆｆの大きいＳＭＦ（波長１．５５μｍ）を用いた。
　近視野画像は、波長可変レーザー（ＴＬＳ：tunable laser source）を用いた波長掃引（１．９５３ＧＨｚの波長間隔で、１．５４７μｍ～１．５５３μｍ）中に、近赤外（ＮＩＲ：near-infrared）カメラを用いて記録した。ＴＬＳとカメラは、コンピュータで同時に制御した。 Based on the structure of FIG. 3 (Λ 2 = 8.66 μm), a 4-core C-MCF was fabricated, and S 2 measurement was performed to observe its propagation mode. Here, a 4-core C-MCF having a length of 22 m was measured. Single mode fiber (SMF) was spliced for 4 core C-MCF excitation. 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.
Near-field image is near-infrared (NIR) during wavelength sweep using a tunable laser source (TLS: 1.547 μm to 1.553 μm at a wavelength interval of 1.953 GHz). Recorded using camera. The TLS and camera were controlled simultaneously by a computer.
　図４に記録された画像のスペクトルのフーリエ変換結果を示す。０ｐｓ及び８６ｐｓ（３．９１ｎｓ／ｋｍ）における群遅延時間差（ＤＧＤ：differential group delay）の２つのピークが明瞭に観察される。図５に示すように、２つのピークにおけるビームプロファイルは、ＬＰ０１－ｌｉｋｅモードとＬＰ１１ａ－ｌｉｋｅモードが共に伝播したことを示している。高次ＬＰ０１－ｌｉｋｅモードと高次ＬＰ１１ａ－ｌｉｋｅモードは観察されないため、それらはカットオフ波長を下回っていると考えられる。フーリエ変換を計算することで得られた、ＬＰ１１ａ－ｌｉｋｅモードのマルチパス干渉（ＭＰＩ:multi-pass interference）は、－２６．５ｄＢであった。
　さらに、図６に示すように、２つの伝播モード間のクロストーク（ＸＴ）を、インパルス応答（ＩＲ）測定を行うことで測定した。 FIG. 4 shows a Fourier transform result of the spectrum of the recorded image. 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 both the LP 01 -like mode and the LP 11a -like mode have 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. The multipath interference (MPI) of LP 11a -like mode obtained by calculating the Fourier transform was −26.5 dB.
Furthermore, as shown in FIG. 6, the crosstalk (XT) between two propagation modes was measured by performing impulse response (IR) measurement.
　光変調器及びフォトディテクタ（ＰＤ）を備えるベクトルネットワークアナライザを用いて、１ｋｍ長の４コアＣ－ＭＣＦのＩＲを測定した。ＴＬＳ及び光変調器はＰＭＦに接続されている。４コアＣ－ＭＣＦを励起するために、Ｓ２測定に用いたものと同じＳＭＦを用いた。図６は、波長１．５５μｍにおけるＩＲ測定結果を示す。
　規格化ＤＧＤの値が０のところでは、ＬＰ０１－ｌｉｋｅモードが現れている。また、ＬＰ０１－ｌｉｋｅモードとＬＰ１１ａ－ｌｉｋｅモードの間のＸＴは、規格化ＤＧＤの値が０～１の間で「ステップ」状に現れている。ＩＲ測定によって、２つのモード間のＤＧＤは、３．９３ｎｓ／ｋｍであり、Ｓ２測定によって得られた結果と一致することがわかる。 The IR of a 1 km long 4-core C-MCF was measured using a vector network analyzer equipped with an optical modulator and a photodetector (PD). The TLS and the optical modulator 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 IR measurement results at a wavelength of 1.55 μm.
When the value of the standardized DGD is 0, the LP 01 -like mode appears. Further, the XT between the LP 01 -like mode and the LP 11a -like mode appears in a “step” state when the value of the standardized DGD is between 0 and 1. By IR measurement, DGD between the two modes is 3.93ns / km, it is found to be consistent with results obtained by S 2 measurements.
　図６の破線は、理論的なＩＲのモデルによって計算されたフィッティングカーブを表している。図６から、パワー結合係数は、約４．１×１０－６／ｍであると推定された。従って、ＸＴは約－２４ｄＢ／ｋｍと見積もられる。
　図７は、Ｃ＋Ｌバンドの波長帯域で、ＸＴを３回測定した結果を示す。測定されたＸＴは、Ｃ＋Ｌバンド内全体で、－２３ｄＢ／ｋｍ未満であった。ＭＩＭＯ処理が不要な短距離伝送を実現する上で十分低いＸＴであることがわかる。
　以上の結果により、本実施例によって、ＭＩＭＯ処理が不要な短距離伝送が可能であることがわかる。 The broken line in FIG. 6 represents a fitting curve calculated by a 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 the C + L band. The measured XT was less than -23 dB / km throughout the C + L band. It can be seen that the XT is sufficiently low to realize short-distance transmission that does not require MIMO processing.
From the above results, it can be seen that short distance transmission that does not require MIMO processing is possible according to the present embodiment.
　図８は、本実施例においてΛ２＝８．６６μｍとした際の、（ａ）各コアの屈折率分布、（ｂ）ＬＰ０１－ｌｉｋｅモードのＦＥＭ電界分布、（ｃ）ＬＰ１１ａ－ｌｉｋｅモードのＦＥＭ電界分布を示す。
　図８（ｂ）及び図８（ｃ）に示されるＦＥＭ電界分布から、本実施例の構成で各コアが強結合しＬＰ０１－ｌｉｋｅモード及びＬＰ１１ａ－ｌｉｋｅモードのスーパーモードを形成可能であることがわかった。ここで、本実施例と同程度の実効屈折率差を有する２コア型（２行×１列）のＣ－ＭＣＦを比較例として、波長１５５０ｎｍにおける各特性を比較した結果を以下の表１に示す。 FIG. 8 shows (a) refractive index distribution of each core, (b) LP 01 -like mode FEM electric field distribution, and (c) LP 11a -like mode when Λ 2 = 8.66 μm in this example. The FEM electric field distribution is shown.
From the FEM electric field distribution shown in FIG. 8B and FIG. 8C, the cores are strongly coupled in the configuration of this embodiment, and a super mode of LP 01 -like mode and LP 11a -like mode can be formed. I understood it. Table 1 below shows the results of comparing the characteristics at a wavelength of 1550 nm using a 2-core type (2 rows × 1 column) C-MCF having an effective refractive index difference of the same level as that of this example as a comparative example. Show.
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　表１より、ＬＰ０１－ｌｉｋｅモードならびにＬＰ１１ａ－ｌｉｋｅモードの実効断面積は実施例と比較例とでほぼ同等であることが分かる。しかしながら、比較例では次のモードが発生し、その実効屈折率のクラッドとの差は０．０００５以上であることが分かる。 From Table 1, it can be seen that the effective cross-sectional areas of the LP 01 -like mode and the LP 11a -like mode are almost the same in the example and the comparative example. However, in the comparative example, the following mode occurs, and it can be seen that the difference between the effective refractive index and the cladding is 0.0005 or more.
　図９は、比較例における（ａ）各コアの屈折率分布、（ｂ）ＬＰ０１－ｌｉｋｅモードのＦＥＭ電界分布、（ｃ）ＬＰ１１ａ－ｌｉｋｅモードのＦＥＭ電界分布、（ｄ）次のモードのＦＥＭ電界分布を示す。
　図９（ｄ）に示す通り、次のモードは各コアのＬＰ１１モードが結合して生じる。このように各コアの高次モードが結合して生じるスーパーモードは、電磁界分布の節が各コアの中心に存在するために、各コアが基本モードのみで結合して生じるスーパーモードに比べて、ファンイン／ファンアウトデバイスのような入出力デバイスを用いてモードを励振ならびに受光することが難しくなる。加えて、このような高次モードを使用しないように通信システムを構成した場合であっても、入出力部のわずかな軸ずれなどによってこのような高次モードが励振されてしまい、受光側でノイズとなる場合がある。従って、本実施例の構成を用いることで、より安定したモードの励振ならびに受光が行える。さらに、本実施例では比較例に比べてコア部分の総面積が少ないため、同じロッド径のガラスロッドを母材として作製する場合はコアを長く形成することができ、作製効率を向上することができる。 FIG. 9 shows (a) the refractive index distribution of each core in the comparative example, (b) the FEM electric field distribution in the LP 01 -like mode, (c) the FEM electric field distribution in the LP 11a -like mode, and (d) the 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. In this way, the super mode generated by combining higher-order modes of each core is compared with the super mode generated by combining each core only in the fundamental mode because the node of the electromagnetic field distribution exists at the center of each core. It becomes difficult to excite and receive the mode using an input / output device such as a fan-in / fan-out device. In addition, even when the communication system is configured not to use such a higher order mode, such a higher order mode is excited by a slight axis misalignment of the input / output unit, etc. There may be noise. Therefore, by using the configuration of this embodiment, more stable mode excitation and light reception can be performed. Furthermore, in this embodiment, the total area of the core portion is smaller than that of the comparative example, so that when a glass rod having the same rod diameter is produced as a base material, the core can be formed longer, and the production efficiency can be improved. it can.
第２実施例
　スーパーモードの伝搬定数ｎｅｆｆと列方向Ｙのコア間距離Λ２との関係から４モードを２つのモード群の伝送として扱える構成を調査した。
　コア半径ａ＝４．０μｍ、比屈折率差Δ＝０．３５％，ｎｃｌａｄ＝１．４５、Λ１＝８．０μｍ、伝搬する光の波長を１５５０ｎｍとして、上記（１）式及び有限要素法（ＦＥＭ）の両方で算出したスーパーモードの伝搬定数ｎｅｆｆと列方向Ｙのコア間距離Λ２との関係を図１１に示す。 Second Example From the relationship between the supermode propagation constant n eff and the inter-core distance Λ 2 in the column direction Y, a configuration that can handle four modes as transmission of two mode groups was investigated.
When the core radius a = 4.0 μm, the relative refractive index difference Δ = 0.35%, n clad = 1.45, Λ 1 = 8.0 μm, the wavelength of propagating light is 1550 nm, the above formula (1) and the finite element FIG. 11 shows the relationship between the supermode propagation constant n eff calculated by both the method (FEM) and the inter-core distance Λ 2 in the column direction Y.
　図１０に示すように、本実施例でも上記（１）式を用いた結果と有限要素法（ＦＥＭ）を用いた結果はおおよそ一致した。
　図１０からわかるように、Λ１とΛ２との差が広くなるほど、ＬＰ０１－ｌｉｋｅモードとＬＰ１１ａ－ｌｉｋｅモードとの伝搬定数ｎｅｆｆの差は小さくなった。また、Λ１とΛ２との差が広くなるほど、ＬＰ１１ｂ－ｌｉｋｅモードとＬＰ２１－ｌｉｋｅモードとの伝搬定数ｎｅｆｆの差も小さくなった。そして、Λ２がおよそ１３μｍ（Λ１の１．６倍）以上で、ＬＰ０１－ｌｉｋｅモードとＬＰ１１ａ－ｌｉｋｅモードとの伝搬定数がおおよそ同じになり、ＬＰ１１ｂ－ｌｉｋｅモードとＬＰ２１－ｌｉｋｅモードとの伝搬定数ともおおよそ同じになった。また、Λ２がおよそ１３μｍ（Λ１の１．６倍）以上で、ＬＰ１１ａ－ｌｉｋｅモードとＬＰ１１ｂ－ｌｉｋｅモードとの伝搬定数の差が０．０００５以上と十分大きくなった。従って、Λ２がΛ１の１．６倍以上であれば、ＬＰ０１－ｌｉｋｅモードとＬＰ１１ａ－ｌｉｋｅモードとで構成されるモード群と、ＬＰ１１ｂ－ｌｉｋｅモードとＬＰ２１－ｌｉｋｅモードとで構成されるモード群との２つのモード群の伝送として扱えると考えられる。従って、各モード群に含まれる２つのモードに対して別々にＭＩＭＯ処理を行うことができ、ＭＩＭＯ処理の行列サイズをより削減することができ、負荷の削減につながる。特に、Λ２＝１３．８６μｍ（Λ１の√３倍）の場合、２つのモード群の伝送として扱え、かつ上述のように７つの同径のガラスロッドを最密充填配列して線引きすることで作製可能であり、製造上のメリットもあり好ましいことがわかった。 As shown in FIG. 10, in this example, the result using the above equation (1) and the result using the finite element method (FEM) were almost the same.
As can be seen from FIG. 10, the wider the difference between Λ 1 and Λ 2 , the smaller the difference in the propagation constant n eff between the LP 01 -like mode and the LP 11a -like mode. Further, as the difference between Λ 1 and Λ 2 becomes wider, the difference in the propagation constant n eff between the LP 11b -like mode and the LP 21 -like mode also becomes smaller. When Λ 2 is approximately 13 μm (1.6 times Λ 1 ) or more, the propagation constants of the LP 01 -like mode and the LP 11a -like mode are approximately the same, and the LP 11b -like mode and the LP 21 -like The propagation constant with the mode is almost the same. Further, when Λ 2 is approximately 13 μm (1.6 times Λ 1 ) or more, the difference in propagation constant between the LP 11a -like mode and the LP 11b -like mode is sufficiently large to be 0.0005 or more. Therefore, if Λ 2 is 1.6 times or more of Λ 1, a mode group composed of LP 01 -like mode and LP 11a -like mode, LP 11b -like mode, and LP 21 -like mode are used. It is considered that it can be handled as transmission of two mode groups with the configured mode group. Therefore, the MIMO processing can be performed separately for the two modes included in each mode group, the matrix size of the MIMO processing can be further reduced, and the load is reduced. In particular, when Λ 2 = 13.86 μm (√3 times Λ 1 ), it can be treated as transmission of two mode groups, and as described above, seven glass rods having the same diameter are arranged in a close-packed arrangement and drawn. It was found that it was preferable because of the manufacturing advantages.
　以上、本発明のマルチコアファイバについて説明してきたが、本発明は上記の例に限定されるものではなく、発明の趣旨を逸脱しない範囲で適宜変更可能である。 The multicore fiber of the present invention has been described above. However, the present invention is not limited to the above example, and can be appropriately changed without departing from the spirit of the invention.
　１…マルチコアファイバ、２…コア、３…クラッド、４…結合コア領域 1 ... multi-core fiber, 2 ... core, 3 ... cladding, 4 ... coupled core region

Claims (11)

  1. 　光ファイバの長手方向における断面において、ｍ行×ｎ列（ｍ，ｎは２以上の整数）のマトリックス状に配置された複数のコアを備え、
    　行方向において互いに隣り合う２つのコア同士の間隔より列方向において互いに隣り合う２つのコア同士の間隔の方が広く、
    　前記行方向及び前記列方向の両方向において各コアを伝搬する光同士が強結合してスーパーモードを形成するように構成される、マルチコアファイバ。     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 interval between two cores adjacent to each other in the column direction is wider than the interval between two cores adjacent to each other in the row direction,
    A multi-core fiber configured such that light propagating through each core in both the row direction and the column direction is strongly coupled to form a super mode.
  2. 　ｎ≧３の場合に、前記行方向において互いに隣り合うコア同士の間隔が実質的に一定である、請求項１に記載のマルチコアファイバ。 2. The multi-core fiber according to claim 1, wherein an interval between cores adjacent to each other in the row direction is substantially constant when n ≧ 3.
  3. 　ｍ≧３の場合に、前記列方向において互いに隣り合うコア同士の間隔が実質的に一定である、請求項１又は２に記載のマルチコアファイバ。 3. The multi-core fiber according to claim 1, wherein when m ≧ 3, an interval between cores adjacent to each other in the column direction is substantially constant.
  4. 　ｍ行×ｎ列のマトリックス状に配置された前記複数のコアが実質的に同一構造である、請求項１～３のうちのいずれか一項に記載のマルチコアファイバ。 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.
  5. 　所定の伝送波長帯域において、ｍ行×ｎ列のマトリックス状に配置された前記複数のコアがそれぞれシングルモードで動作するように構成される、請求項１～４のうちのいずれか一項に記載のマルチコアファイバ。 5. The plurality of cores arranged in a matrix of m rows × n columns in a predetermined transmission wavelength band are each configured to operate in a single mode. Multi-core fiber.
  6. 　所定の伝送波長帯域において、伝送可能な最高次のスーパーモードの伝搬定数とクラッドの屈折率差が０．０００５以上であるように構成される、請求項１～５のうちのいずれか一項に記載のマルチコアファイバ。 The high-order supermode propagation constant that can be transmitted and the refractive index difference of the cladding are 0.0005 or more in a predetermined transmission wavelength band, according to any one of claims 1 to 5. The described multi-core fiber.
  7. 　所定の伝送波長帯域において、伝送に用いるモード群間の伝搬定数の差が０．０００５以上であるように構成される、請求項１～５のうちのいずれか一項に記載のマルチコアファイバ。 The multi-core fiber according to any one of claims 1 to 5, wherein a difference in propagation constant between mode groups used for transmission is 0.0005 or more in a predetermined transmission wavelength band.
  8. 　前記列方向において互いに隣り合うコア同士の間隔が、前記行方向において互いに隣り合うコア同士の間隔のおよそ√３倍である、請求項１～７のうちのいずれか一項に記載のマルチコアファイバ。 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.
  9. 　ｎ＝２である、請求項１及び３～８のうちのいずれか一項に記載のマルチコアファイバ。 The multi-core fiber according to any one of claims 1 and 3 to 8, wherein n = 2.
  10. 　ｍ＝２である、請求項１，２，及び４～８のうちのいずれか一項に記載のマルチコアファイバ。 The multi-core fiber according to any one of claims 1, 2, and 4 to 8, wherein m = 2.
  11. 　ｍ行×ｎ列（ｍ，ｎは２以上の整数）のマトリックス状に配置された前記複数のコアが形成される領域を複数有し、
    　前記複数の領域は、それぞれの領域を伝搬する光同士がそれぞれ非結合となるように配置される、請求項１～１０のうちのいずれか一項に記載のマルチコアファイバ。     a plurality of regions in which the plurality of cores arranged in a matrix of m rows × n columns (m and n are integers of 2 or more) are formed;
    The multi-core fiber according to any one of claims 1 to 10, wherein the plurality of regions are arranged so that light propagating through the respective regions is uncoupled.
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