Method and device for measuring power coupling coefficient of multi-core optical fiber

Abstract

The purpose of the present invention is to provide: a method for measuring a power coupling coefficient in consideration of asymmetry between cores; and a device therefor.　The device according to the present invention comprises: a measurement apparatus 11 that causes, at one end of a multi-core optical fiber, test light to be incident on an incident core that is one of the cores, and measures, at the one end of the multi-core optical fiber, the back-scattered light intensity of back-scattered light output from the incident core and an adjacent core adjacent thereto; and a calculation unit 12 that calculates a ratio between the back-scattered light intensity of the incident core and the back-scattered light intensity of the adjacent core from a distant position at which the back-scattered light intensity ratio becomes stationary in the longitudinal direction from the one end of the multi-core optical fiber, and calculates a power coupling coefficient between the cores by substituting, to a calculation formula, the ratio and the back-scattered light intensities of the incident core and the adjacent core at an arbitrary point in the longitudinal direction from the one end of the multi-core optical fiber.

Description

Method and apparatus for measuring the power coupling coefficient of a multi-core optical fiber

[0001] The present disclosure relates to a method and apparatus for measuring the power coupling coefficient of a multi-core optical fiber.

[0002] Research and development of space division multiplexing (SDM) transmission technology is being conducted as a means of increasing the transmission capacity per optical fiber, and multi-core optical fibers (MCFs) are attracting attention. For both uncoupled and coupled MCFs, the power coupling coefficient, which indicates the strength of coupling, is an important parameter. Toward the realization of SDM transmission, evaluation of MCF characteristics is important, and a technique for measuring the power coupling coefficient is required. Non-Patent Document 1 discloses a method for measuring the power coupling coefficient of homogeneous core MCFs with uniform core characteristics.

[0003] M. Ohashi, K. Kawazu, A. Nakamura and Y. Miyoshi, “Simple backscattered power technique for measuring crosstalk of multi-core fibers,” Proc. 17th Opto-Electron. Commun. Conf. , pp. 357-358, 2012.

[0004] However, when the characteristics of each core of an MCF are different (heterogeneous-core MCF), the power coupling coefficient between the cores is not symmetrical, and the method of Non-Patent Document 1 cannot be applied. In other words, there is a problem that it is difficult to measure the power coupling coefficient between cores of a heterogeneous-core MCF. In order to solve the above problem, an object of the present invention is to provide a power coupling coefficient measurement method and apparatus that take into account the asymmetry between cores.

[0005] In order to achieve the above object, the power coupling coefficient measurement method according to the present invention is configured to approximately determine the power coupling coefficient by using the intensity ratio at a distance where the backscattered light intensity ratio becomes constant.

[0006] Specifically, a power coupling coefficient measurement method according to the present invention is a method for measuring the power coupling coefficient of a multi-core optical fiber in which the power coupling coefficients between cores are asymmetric, and is characterized by: injecting test light into an incident core, which is one of the cores, at one end of the multi-core optical fiber; measuring backscattered light intensities of backscattered light output from the incident core and an adjacent core adjacent to the incident core, at the one end of the multi-core optical fiber; calculating a ratio between the backscattered light intensity of the incident core and the backscattered light intensity of the adjacent core from a distance where the backscattered light intensity ratio becomes steady in the longitudinal direction from the one end of the multi-core optical fiber; and substituting the backscattered light intensities of the incident core and the adjacent core and the ratio at an arbitrary point in the longitudinal direction from the one end of the multi-core optical fiber into a calculation formula to calculate the power coupling coefficient between cores.

[0007] Furthermore, a power coupling coefficient measurement device according to the present invention is a device for measuring the power coupling coefficient of a multi-core optical fiber in which the power coupling coefficient between cores is asymmetric, and includes: a measuring device that inputs test light into an incident core, which is one of the cores, at one end of the multi-core optical fiber, and measures backscattered light intensities of backscattered light output from the incident core and an adjacent core adjacent to the incident core at the one end of the multi-core optical fiber; and a computing unit that calculates a ratio between the backscattered light intensity of the incident core and the backscattered light intensity of the adjacent core from a distance where the backscattered light intensity ratio becomes steady in the longitudinal direction from the one end of the multi-core optical fiber, and calculates the power coupling coefficient between cores by substituting the backscattered light intensities of the incident core and the adjacent core and the ratio at an arbitrary point in the longitudinal direction from the one end of the multi-core optical fiber into a calculation formula.

[0008] In the case of heterogeneous core MCF, the power coupling coefficient between the two cores of interest is asymmetric, and theoretically, it is not possible to determine these power coupling coefficients from the ratio of the backscattered light intensities. However, the power coupling coefficient measurement method of the present invention can measure the backscattered light intensity P ｂｓ１ Backscattered light intensity P of adjacent core ｂｓ２The ratio of these is the power coupling coefficient h from the incident core to the adjacent core. １２ Power coupling coefficient h from adjacent core to incident core ２１ The power coupling coefficient can be approximately calculated by using the following equation:

[0009] Therefore, the present invention can provide a power coupling coefficient measurement method and device that takes into account the asymmetry between cores.

[0010] The above calculation formula is number C1. However, h ２１ is the power coupling coefficient from the adjacent core to the incident core, h １２ is the power coupling coefficient from the incident core to the adjacent core, r is the ratio, z is the distance (m) from the one end to the arbitrary point, P ｂｓ１ (z) is the backscattered light intensity (dB) from the arbitrary point measured at the incident core, P ｂｓ２ (z) is the backscattered light intensity (dB) from the arbitrary point measured at the adjacent core.

[0011] The computing unit can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network.

[0012] The above inventions can be combined as much as possible.

[0013] The present invention can provide a power coupling coefficient measurement method and device that takes into account the asymmetry between cores.

[0014] FIG. 1 is a diagram for explaining a power coupling coefficient. FIG. 2 is a diagram for explaining a power coupling coefficient measurement method according to the present invention. FIG. 3 is a diagram for explaining a power coupling coefficient measurement device according to the present invention. FIG. 4 is a diagram for explaining backscattered light intensity from each core of an optical fiber 50 under test, acquired by a measuring instrument 11 of a power coupling coefficient measurement device according to the present invention. FIG. 5 is a diagram for explaining the power coupling coefficient of a heterogeneous-core MCF (2 cores) measured by a power coupling coefficient measurement device according to the present invention. FIG. 6 is a diagram for explaining points to note regarding backscattered light intensity. FIG. 7 is a diagram for explaining the power coupling coefficient measured by a power coupling coefficient measurement device according to the present invention.

[0015] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The preferred embodiments described below are examples of the present invention, and the present invention is not limited to the preferred embodiments. In this specification and the drawings, components having the same reference numerals are intended to represent the same components.

[0016] (Comparative Example) As a comparative example, a method for measuring the power coupling coefficient in an MCF with the same core type will be described. FIG. 1 is a diagram for explaining the comparative example. The MCF will be described using a two-core fiber. When a light intensity P ０ When test light of this magnitude is incident, the relationship between the optical intensities of the cores at a position z in the longitudinal direction from the incident end is described by the power coupling equation in equation (1). Because the cores are of the same type, the power coupling coefficients of each core can be set to the same value, h. Here, P １ (z) is the light intensity at the position z of the incident core (core1), P ２ (z) is the optical intensity at position z of the adjacent core (core 2), α １ is the loss per unit length of the incident core (core1), α ２ is the loss per unit length of the adjacent core (core2), and

[0017] The solution to the power equation can be expressed as equation (2), where α １ = α ２ = α. The backscattered light intensity (P ｂｓ１ (z), P ｂｓ２ (z)) can be expressed as follows, where α ｓ (z) is the scattering coefficient at position z, and B(z) is the capture rate at position z. Therefore, the power coupling coefficient h can be calculated from the ratio of the backscattered light intensities.

[0018] (Example) Next, a method for measuring the power coupling coefficient in a heterogeneous core MCF will be described. Fig. 2 is a diagram for explaining this example. The MCF will be described using a two-core fiber. When a light intensity P ０When test light of this magnitude is incident, the relationship between the optical intensities of the cores at a position z in the longitudinal direction from the incident end is described by the power coupling equation in equation (5). In the case of heterogeneous cores, the power coupling coefficients of the cores are different (h １２ , h ２１ ).

[0019] The solution to the power equation can be expressed as equation (6), where α １ = α ２ = α. Details of the derivation of equation (6) are described in Appendix 1. Then, the backscattered light intensity (P ｂｓ１ (z), P ｂｓ２ (z)) can be expressed as follows, where α ｓ (z) is the scattering coefficient at position z, and B(z) is the capture rate at position z.

[0020] From equation (7), the relationship between the ratio of backscattered light intensities and the power coupling coefficient is as follows: Here, normally it is not possible to solve h because there are two variables, but if z is large enough, z = z ０ In this case, the power coupling coefficient h can be approximately calculated by utilizing the fact that the intensity ratio of the backscattered light of each core becomes constant. That is, by approximating as in equation (9), equation (8) can be expressed as the following equation. From the above, the power coupling coefficient of each core of the heterogeneous coupled MCF can be calculated as follows.

[0021] 3 is a diagram illustrating an apparatus 301 for measuring the power coupling coefficient of an MCF, which is an optical fiber 50 under test. The apparatus 301 includes a measuring device 11, a computing unit 12, and an optical branching component 13. The optical branching component 13 can input light from the measuring device 11 to any core of the MCF and can input light from any core of the MCF to the measuring device 11. The optical branching component 13 is, for example, a fan-in / fan-out component.

[0022] The MCF of the optical fiber under test 50 has an asymmetrical power coupling coefficient between cores. The measuring instrument 11 includes: at one end of the multi-core optical fiber, inputting test light into an input core, which is one of the cores; and at one end of the multi-core optical fiber, measuring the backscattered light intensity (P ｂｓ１ (z), P ｂｓ２ (z)) and the measuring instrument 11 is, for example, an OFDR (Frequency Domain Reflectometry).

[0023] The calculator 12 calculates a ratio r between the backscattered light intensity of the incident core and the backscattered light intensity of the adjacent core from a distance where the backscattered light intensity ratio becomes steady in the longitudinal direction from one end of the multi-core optical fiber, and calculates the power coupling coefficient (h) between the cores by substituting the backscattered light intensity of the incident core and the adjacent core at an arbitrary point z in the longitudinal direction from one end of the multi-core optical fiber and the ratio into a calculation formula (11). ２１ , h １２ ) is calculated. Note that "far" means the region where the backscattered light intensity ratio is constant. For example, in the case of the optical fiber in Figure 4, it is the region where z ≥ 50 m. Note that the "far" region differs depending on the optical fiber, so it does not always mean that 50 m or more is far.

[0024] The device 301 uses a circuit as shown in FIG. 3(a) to measure the backscattered light intensity P ｂｓ１ Next, the device 301 measures the backscattered light intensity P from the adjacent core using a circuit such as that shown in FIG. ｂｓ２ (z) is measured. Reference numeral 14 denotes an optical circulator.

[0025] 4 is a diagram illustrating the intensity of backscattered light from each core of the optical fiber 50 under test, acquired by the measuring instrument 11. The horizontal axis represents the position z (m), and the vertical axis represents the intensity (dB) of the backscattered light. For example, when z=z ０ At 100 m, P ｂｓ１ (z ０ )=-109.062dB, P ｂｓ２ (z ０) = -110.609 dB. Using this value, the ratio r in equation (9) can be calculated as follows:

[0026] By applying the ratio r in equation (12) to equation (11), the power coupling coefficient h at the position z is calculated. １２ and ２１ 5 is a diagram illustrating the power coupling coefficient of a heterogeneous-core MCF (two cores) calculated by equation (11) using the ratio r in equation (12). The horizontal axis represents the position z, and the vertical axis represents the power coupling coefficient at each position. The white line represents h １２ , the black line is h ２１ is.

[0027] (Effects of the Invention) The device 301 measures the backscattered light intensity from each of the first core and the second core when test light is incident on the first core of the MCF, and calculates the backscattered light intensity P ｂｓ１ The backscattered light intensity P of the second core ｂｓ２ The ratio of the power coupling coefficient h from the first core to the second core at a distance is １２ Power coupling coefficient h from the second core to the first core for ２１ By using the fact that the power coupling coefficient h is equal to the ratio of １２ and h ２１ The device 301 can measure the power coupling coefficient between cores of an MCF in which the characteristics of each core are different, which was previously impossible to measure.

[0028] [Supplementary Note 1] The derivation of the general solution of the power combining equation will be explained. Equation (5) can be rearranged to obtain equation (A1). Here, a = α １ +h ２１ , b=h １２ , c=α ２ +h １２ , d=h ２１ Then, equation (A1) becomes: By rearranging the first equation of equation (A2) for P2(z) and substituting it into the second equation of equation (A2), the following equation is obtained. The solution to the characteristic equation of equation (A3) is: where: Then, equation (A4) can be written as follows:

[0029] The initial condition P １ (0) = P ０ , P ２ Since (0)=0, the solution of the power combining equation (A3) is as follows: Here, the loss coefficient is equal for each core (α １ ≒α ２ = α), From the above, the solution of the power combining equation (A5) is obtained as shown in equation (A6).

[0030] [Supplementary Note 2] A note on the measurement results of the backscattered light intensity in Fig. 4 will be explained. As in Fig. 5 and Fig. 6, the position z on the horizontal axis includes the fiber device portion within the apparatus 301, and z = 0 m is not the origin of the MCF. For this reason, in Fig. 5, in this example, the region from z = 2 m onwards shows the results for the MCF.

[0031] [Supplementary Note 3] A note on the power coupling coefficient calculated by the calculator 12 is provided below. The power coupling coefficient varies depending on the state of the optical fiber, and therefore actually varies in the longitudinal direction as shown by the solid line in FIG. 7 . Meanwhile, what contributes to the backscattered light intensity from an arbitrary position z (the dotted line in FIG. 7 ) is the integral of the power coupling coefficient from positions 0 to z (the hatched area in FIG. 7 ). Because the device 301 calculates the power coupling coefficient from the backscattered light intensity ratio, this value is the integral of the power coupling coefficient from positions 0 to z. In other words, what is calculated by equation (11) is the averaged power coupling coefficient from positions 0 to z. Note that the averaged power coupling coefficient measured by the device 301 is inversely proportional to z, and therefore diverges at z = 0. However, because no coupling occurs at z = 0, the value at z = 0 has no physical significance.

[0032] 11: Measuring instrument 12: Computing unit 13: Optical branching component 14: Optical circulator 15: Optical fiber to be measured (MCF) 301: Device

Claims

1. A method for measuring a power coupling coefficient of a multi-core optical fiber in which the power coupling coefficient between cores is asymmetric, comprising: injecting test light into an incident core, which is one of the cores, at one end of the multi-core optical fiber; measuring backscattered light intensities of backscattered light output from the incident core and an adjacent core adjacent to the incident core, at one end of the multi-core optical fiber; calculating a ratio of the backscattered light intensity of the incident core to the backscattered light intensity of the adjacent core from a distance where the backscattered light intensity ratio becomes steady in the longitudinal direction from one end of the multi-core optical fiber; and substituting the backscattered light intensities of the incident core and the adjacent core at an arbitrary point in the longitudinal direction from one end of the multi-core optical fiber and the ratio into a calculation formula to calculate the power coupling coefficient between cores.

2. The method for measuring a power coupling coefficient according to claim 1, characterized in that the calculation formula is the formula C1. However, ２１ is the power coupling coefficient from the adjacent core to the incident core, h １２ is the power coupling coefficient from the incident core to the adjacent core, r is the ratio, z is the distance (m) from the one end to the arbitrary point, P ｂｓ１ (z) is the backscattered light intensity (dB) from the arbitrary point measured at the input core, P ｂｓ２ (z) is the backscattered light intensity (dB) from the arbitrary point measured at the adjacent core.

3. An apparatus for measuring a power coupling coefficient of a multi-core optical fiber in which the power coupling coefficient between cores is asymmetric, comprising: a measuring device that performs the following at one end of the multi-core optical fiber: injecting test light into an incident core, which is one of the cores, and measuring backscattered light intensities of backscattered light output from the incident core and an adjacent core adjacent to the incident core, at one end of the multi-core optical fiber; and a calculator that performs the following: calculating a ratio of the backscattered light intensity of the incident core to the backscattered light intensity of the adjacent core from a distance where the backscattered light intensity ratio becomes steady in the longitudinal direction from one end of the multi-core optical fiber, and substituting the backscattered light intensities of the incident core and the adjacent core at an arbitrary point in the longitudinal direction from one end of the multi-core optical fiber and the ratio into a calculation formula to calculate the power coupling coefficient between cores.

4. The power coupling coefficient measuring device according to claim 3, wherein the calculation formula is the formula C1. However, ２１ is the power coupling coefficient from the adjacent core to the incident core, h １２ is the power coupling coefficient from the incident core to the adjacent core, r is the ratio, z is the distance (m) from the one end to the arbitrary point, P ｂｓ１ (z) is the backscattered light intensity (dB) from the arbitrary point measured at the input core, P ｂｓ２ (z) is the backscattered light intensity (dB) from the arbitrary point measured at the adjacent core.

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