JP2012202827A - Mode coupling measuring method and measuring device for multi-core optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and a device for measuring the value of a mode coupling coefficient of a multi-core fiber and a lengthwise distribution together nondestructively.SOLUTION: An optical combiner is used which couples N single-core optical fibers to one multi-core fiber having N cores. An optical pulse is made incident on one core of the multi-core fiber from one OTDR device, and optical power scattered back by the N cores is measured. The value of a coupling coefficient from the incident core to other cores and the lengthwise distribution thereof are measured from ratios of scattered light power to the incident core and other cores, and further mode coupling between arbitrary cores can be simultaneously measured by placing the N OTDR devices in synchronous operation.

Description

  The present invention relates to a method and apparatus for measuring the size of mode coupling between cores in a multi-core optical fiber in which a plurality of cores are provided in one optical fiber, and the distribution in the longitudinal direction thereof.

  In today's optical fiber transmission lines that support global high-capacity information and communication infrastructures, with the increase in input optical power due to the rapid increase in information traffic in recent years, thermal destruction (optical fiber fuse) due to excess optical power and nonlinear optical effects have been introduced. The physical limitations are pointed out. The optical fiber used in the backbone optical communication infrastructure around the world today is standard with one core per fiber. Recently, however, in order to overcome the physical limitations of existing optical fibers, there is a great interest in spatial multiplexing transmission using a multi-core fiber in which a plurality of cores are provided on one optical fiber.

As an example of the multi-core fiber, a cross section of a 7-core fiber is shown in FIGS. All have a shape in which six cores are symmetrically arranged in a hexagonal shape around the central core. As shown in FIG. 1A, the structure includes a plurality of high-refractive-index cores in which GeO ₂ is added in the same clad (solid core type multi-core fiber), as shown in FIG. 1B. There are two types (multi-core photonic crystal fiber) in which a plurality of silica cores (portions having no holes) are provided in the fiber cross section in the fiber cross section.

K. Takenaga, S. Tanigawa, N. Guan, S. Matsuo, K. Saitoh, and M. Koshiba, “Reduction of Crosstalk by Quasi-Homogeneous Solid Multi-Core Fiber,” Optical Fiber Communication Conference (OFC 2010), OWK7 , March 2010. T. Hayashi, T. Nagashima, O. Shimakawa, T. Sasaki, and E. Sasaoka, “Crosstalk variation of multi-core fiber due to fiber bend,” European Conference on Optical Communication (ECOC 2010), We.8.F .6, September 2010. M. Nakazawa, M. Tokuda, and Y. Negishi, “Measurement of polarization mode coupling along a polarization-maintaining optical fiber using a backscattering technique,” Opt. Lett., Vol. 8, no. 10, pp. 546-548 , October 1983.

  In spatial multiplexing transmission using a multicore fiber, the most important issue is how to suppress mode coupling between different cores. That is, since the waveguide mode of each core has its field distribution base oozing into the clad (called an evanescent component), when this overlaps with the field distribution of other cores, mode coupling occurs between the two cores. As a result, when many cores are provided at high density, crosstalk occurs between signals propagating through different cores. This is a major obstacle to increasing the density of multi-core fibers.

  There have been many reports on crosstalk between cores as an important evaluation item for multicore fibers. According to recent research, it has been reported that crosstalk significantly increases when fluctuations in the shape and arrangement of the cores exist in the longitudinal direction or when the fiber is bent (Non-Patent Documents 1 and 2). ). Therefore, if it is possible to measure the longitudinal distribution at the same time as the mode coupling between the cores, extremely useful knowledge can be obtained as the basic transmission characteristics of the fiber.

  However, in the conventional crosstalk measurement, light is input to one core, the output optical power from each core is measured at the output end, and the crossing is performed from the magnitude of the power transferred from the input core to the other core. The talk was being evaluated. For this reason, N-core fibers require N-1 times for one core and N × (N-1) times of crosstalk measurement for the entire core, which makes measurement complicated. It was.

  Furthermore, in order to evaluate the distribution of the mode coupling coefficient over the longitudinal direction, it is necessary to cut the fiber at each point and measure the input / output optical power for each core to obtain the mode coupling ratio. Therefore, it has been difficult to measure the longitudinal distribution of mode coupling in a batch and nondestructively.

  The present invention is intended to solve such problems, and an object of the present invention is to provide a method and an apparatus for collectively and nondestructively measuring the size of the mode coupling coefficient and the distribution in the longitudinal direction in a multicore fiber. .

  In order to achieve such an object, in the mode coupling measurement apparatus of the present invention, an optical combiner that couples N single-core optical fibers (SCF) to one N-core MCF is used to provide one OTDR (Optical). A light pulse is incident on one core of the MCF from a time domain (Reflectometer) device. Then, the light back scattered by the N cores is input to N OTDR devices, and the mode coupling between the cores is measured.

Specifically, an optical pulse is incident on one core of the MCF, and light returning to each core at the incident end is branched into N SCFs via the optical combiner. These are input to N OTDR devices to measure the powers P _1, P _2, ..., _PN, and the scattered light power P ₁ of the incident core and the other powers P _2, P ₃ ,. the ratio of the _{P N P 2 / P 1,} P 3 / P 1, obtaining the ··· _P N / _{P 1.} Thereby, the magnitude of the coupling coefficient from the incident core to the other core and the distribution in the longitudinal direction thereof are measured.

  In particular, by synchronously operating N OTDR devices, the magnitude of the mode coupling coefficient and the distribution in the longitudinal direction can be measured simultaneously for any two cores.

  According to the present invention, the longitudinal state of mode coupling in a multi-core fiber can be measured simultaneously and non-destructively for a plurality of cores. As a result, it is possible to easily and efficiently evaluate the crosstalk characteristics of the manufactured fiber. Furthermore, in actual multi-core fiber transmission, when crosstalk between cores occurs due to external factors such as fiber stress and temperature change, it is easy to identify the position where mode coupling occurs in the fiber. Is possible. From the above, the present invention can provide important knowledge for evaluating the transmission performance of multi-core fibers.

Example of cross-sectional structure of multi-core fiber Example of configuration of multi-core fiber mode coupling measurement device An example of optical combiner Explanation of measurement principle of mode coupling according to this embodiment Schematic diagram of measurement results of backscattered light power according to this example Relationship between backscattered light power ratio and mode coupling coefficient Optical pulse excitation to measure mode coupling between arbitrary cores

  An example of an embodiment of the present invention is shown in FIG. From one of the N OTDR devices, an optical pulse is incident on one of the cores of the multicore fiber 4 to be measured via the single core fiber 2 and the optical combiner 3. The configuration of the OTDR apparatus 1 is the same as that used for the measurement of a conventional single core fiber, and includes a pulse light source, a light receiver, and a signal processing unit.

  An example of the optical combiner 3 is shown in FIG. As shown in the figure, each core of N single-core fibers is coupled to N cores of a multi-core fiber. Conversely, the scattered light returning to the N cores of the multi-core fiber is also used to branch into N single core fibers.

The measurement principle of mode coupling according to this embodiment will be described with reference to FIG. Now, from one OTDR device, an optical pulse with power P ₀ and pulse width Δτ is incident on one of the cores of the multi-core fiber 4 (core 1 in the figure). At this time, a part of the light propagating through the core 1 is scattered backward by Rayleigh scattering in the fiber and returns to the input end. Further, when light leaks from the core 1 to other cores (cores 2 to 7 in the figure) due to mode coupling, the backscattered light returns to the input end of each core. The scattered light returning to the input end is branched for each core, and the power (P ₁ , P ₂ ,...) Is measured by N OTDR devices.

Here, the time width .DELTA..tau of light pulses, in accordance with the spatial measurement resolution ΔL of mode coupling is determined by Δτ = 2ΔL / v _g. However v _g is the group velocity of the light pulse in the fiber. Although the spatial resolution can be improved by narrowing the pulse width, there is a trade-off in that the dynamic range cannot be secured because the power is reduced for a narrow pulse width. Therefore, it is necessary to set the pulse width to an appropriate value according to the spatial resolution and the measurement length.

FIG. 5 schematically shows how the scattered light power is measured over time with N OTDR devices. P ₁ is the scattered light power in the incident core, and the loss in the longitudinal direction accompanying the propagation of the core 1 can be evaluated from the time change as in the conventional OTDR measurement. That is, the loss in the longitudinal direction can be measured by converting the measurement time t from the incidence of the light pulse into the length L = v _g t / 2.

On the other hand, P ₂ , P ₃ ... Are scattered light powers in other cores, and this magnitude reflects mode coupling. That is, the mode coupling of the measured fiber can be measured nondestructively from the change in the longitudinal direction of the ratios P ₂ / P ₁ , P ₃ / P ₁ ... With the scattered light power P ₁ to the incident core. .

で記述される。ここでｈ_１，ｍはモード結合係数、α_１，α_ｍは各モードの損失係数である。ＯＴＤＲによってｈ_１，ｍを測定する方法は、非特許文献３に記載されている。 For example, the mode coupling between core 1 and core m is the power coupling equation

It is described by. Here, h _{1 and m} are mode coupling coefficients, and α ₁ and α _m are loss coefficients of the respective modes. A method for measuring h _{1, m} by OTDR is described in Non-Patent Document 3.

で関係付けられる。ここで右辺第１項の係数２は光パルスの往復に伴うものである。またＫは定数であり、Ｐ_ｍ／Ｐ_１を長さＬの関数として描いたときのＬ＝０での切片より求められる。その結果ｈ_１，ｍは、パワーの比Ｐ_ｍ／Ｐ_１をＬの関数として描いたときの直線の傾きより求めることが出来る。その様子を図６に示す。 From the analysis results of Non-Patent Document 3, the powers P ₁ and P _m , the mode coupling coefficient h _{1, m} , and the fiber length L are

Are related. Here, the coefficient 2 in the first term on the right side is associated with the reciprocation of the optical pulse. K is a constant, and is obtained from an intercept at L = 0 when P _m / P ₁ is drawn as a function of length L. As a result, h _{1 and m} can be obtained from the slope of a straight line when the power ratio P _m / P ₁ is drawn as a function of L. This is shown in FIG.

P _m / P ₁ is linear with respect to L if h _{1, m} is constant regardless of distance, but P _m / P ₁ deviates from linear when h _{1, m} changes locally. . From the amount of change from the straight line, the mode coupling distribution in the longitudinal direction can be evaluated. As described above, the major feature of this method is that not only the magnitude of the mode coupling coefficient but also the change in the longitudinal direction can be measured by OTDR.

In the present invention, P ₂ / P ₁ , P ₃ / P ₁ ..., P _N / P ₁ can be measured simultaneously by operating N OTDR devices in synchronization. As a result, the mode coupling between the core 1 and the cores 2 to N can be collectively evaluated by one measurement. As a result, the efficiency of the mode coupling measurement of the multi-core fiber is greatly improved.

Furthermore, as shown in FIG. 7, mode coupling hm _{, n} between arbitrary cores is collectively performed by inputting optical pulses from N OTDR devices one by one to the multi-core fiber with a time interval T. It becomes possible to measure. The interval T may be set longer than the time required for one optical pulse to make one round trip through the measured fiber.

と書くことが出来る。ここで右辺のｈ_ｍ，ｎＰ_ｍの項はコアｎからコアｍへモードが結合することによるＰ_ｍの増加を、−ｈ_ｎ，ｍＰ_ｎの項はコアｍからコアｎへのモード結合によるＰ_ｍの減衰を表している。またα_ｍはコアｍの損失係数である。コアｍからｎへのモード結合とｎからｍへのモード結合係数は等しいことに注意すると、ｈ_ｍ，ｎ＝ｈ_ｎ，ｍと置いて下式の電力結合方程式を得る。

If the mode coupling coefficient h _{m, n} is obtained, the state in which signals of optical power P _m (z) (m = 1, 2,.

Can be written. Here, the term h _{m, n} P _{m on} the right side indicates the increase in P _m due to the mode coupling from the core n to the core m, and the term −h _{n, m} P _n represents the mode coupling from the core m to the core n. _Represents the attenuation of P _m by. Α _m is the loss factor of the core m. Note that the mode coupling coefficient from the core m to n is equal to the mode coupling coefficient from n to m, and h _{m, n} = h _{n, m} is obtained to obtain the following power coupling equation.

初期条件Ｐ（０）が与えられれば、式（５）の連立微分方程式の解より距離ｚ伝搬後の各コアのパワー分布Ｐ（ｚ）を求めることが出来る。 When equation (4) is displayed as a matrix, the following equation is obtained.

If the initial condition P (0) is given, the power distribution P (z) of each core after propagation of the distance z can be obtained from the solution of the simultaneous differential equations of the equation (5).

の解で与えられる。式（６）はλについてのＮ次の代数方程式であり、そのＮ個の解λ_１，λ_２，・・・，λ_Ｎが行列Ｈの固有値を与える。固有値λ_１，λ_２，・・・，λ_Ｎはモード結合係数に関係している。固有値が求まれば、各固有値λ_ｍに対応する固有ベクトルｓ_ｍが

の解より求められる。以上の計算により得られたＮ個の固有値および固有ベクトルを用いて、式（５）の解は

で与えられる。ここでｃ_１，ｃ_２，・・・，ｃ_Ｎは初期条件Ｐ（０）より決定される定数である。 If H is a constant matrix that does not depend on z, the analytic solution of equation (5) is the eigenvalues λ ₁ , λ ₂ ,..., Λ _N of the matrix H and the eigenvectors s ₁ , s ₂ ,. It can be determined using s _N. The eigenvalues of the matrix H are characteristic equations

Given by the solution. Equation (6) is an Nth-order algebraic equation for λ, and its N solutions λ ₁ , λ ₂ ,..., Λ _N give the eigenvalues of the matrix H. The eigenvalues λ ₁ , λ ₂ ,..., Λ _N are related to the mode coupling coefficient. If the eigenvalues is obtained, is eigenvector s _m corresponding to each eigenvalue λ _m

It is obtained from the solution of Using the N eigenvalues and eigenvectors obtained by the above calculation, the solution of equation (5) is

Given in. Here, c ₁ , c ₂ ,..., C _N are constants determined from the initial condition P (0).

  If the matrix H is a function of z, such as when the mode coupling coefficient changes in the longitudinal direction, the solution P (z) can be obtained by using a numerical approximation method such as a successive approximation method or a difference.

として求められる。 If P (z) is obtained by the above calculation, the mode coupling magnitude (crosstalk) between the core m and the core n is

As required.

  As described above, the matrix H includes all the information related to the mode coupling of the fiber to be measured, and can provide extremely useful knowledge for use as a fiber transmission line.

  As described in detail above, by using N OTDR devices, the mode coupling coefficient between cores in a multi-core fiber can be evaluated in a simple manner. The present invention is particularly characterized in that the longitudinal distribution of mode coupling can be measured nondestructively, and can provide an important index for evaluating the performance of the manufactured fiber. Moreover, since the mode coupling between arbitrary cores can be collectively measured by injecting an optical fiber into each core at an appropriate timing, it is possible to efficiently evaluate the characteristics of the fiber in a short time. If a multi-core fiber with low crosstalk can be realized based on this measurement, spatial multiplexing transmission of the optical fiber becomes possible, which can greatly contribute to the increase in capacity of the optical communication system.

1 OTDR device 2 Single core fiber 3 Optical combiner 4 Multicore fiber

Claims (4)

  N OTDR devices and an optical combiner that couples N single-core fibers to one multi-core fiber having N cores, and one multi-core fiber from one OTDR device through the optical combiner. A mode coupling measurement method, comprising: measuring a mode coupling coefficient between an incident core and each core by using a light pulse incident on one core and backscattered light on N cores. A light pulse is incident on one core of a multi-core fiber, and the light returning to the incident end of each core is branched into N single-core fibers via the optical combiner, and the power P is obtained using N OTDR devices. _{1, P} 2, ···, measured _{P N,} the ratio _P 2 _/ P ₁ of the scattered light power for incident core and another core, _{P 3} / _P 1, by obtaining the ··· _P N / _{P 1} 2. The method of measuring mode coupling according to claim 1, wherein the magnitude of the coupling coefficient from the incident core to the other core and the distribution in the longitudinal direction thereof are measured.   The mode coupling measurement according to claim 1, wherein the mode coupling coefficient between arbitrary cores and the distribution in the longitudinal direction thereof are collectively measured by synchronizing N OTDR devices. Method.   N OTDR devices and an optical combiner that couples N single-core fibers to one multi-core fiber having N cores, and one multi-core fiber from one OTDR device through the optical combiner. A mode coupling measuring apparatus for measuring a mode coupling coefficient between an incident core and each core using light that is incident on one core and backscattered on N cores.

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