Method for measuring torsion in coupled multi-core fibers

Abstract

The present disclosure provides a method for measuring torsion in coupled multi-core fibers, the method including: causing test light to be incident on a first end surface of a first core provided in a coupled multi-core fiber; measuring the intensity of backscattered light resulting from the test light being scattered by the coupled multi-core fiber and emitted from the first end surface of a second core provided in the coupled multi-core fiber; and measuring torsion in the coupled multi-core fiber on the basis of the distance between points at which the intensity of the backscattered light increases.

Description

Method for measuring twist in coupled multicore fibers

[0001] The present disclosure relates to a method for measuring twist in a coupled multicore fiber.

[0002] Research and development of space division multiplexing (SDM) transmission technology has been conducted to increase the transmission capacity per optical fiber, and coupled multi-core fibers (hereinafter, multi-core fibers may be abbreviated as MCFs) have attracted attention as transmission media.

[0003] It is known that optical characteristics such as the coupling characteristics between cores of coupled MCFs change due to disturbances such as bending and twisting. Twisting in particular has a significant effect on changes in optical characteristics, so a technique for measuring twisting is necessary to ensure the quality of SDM transmission.

[0004] A method for measuring the twist of uncoupled MCFs has been proposed (Non-Patent Document 1). In Non-Patent Document 1, the twist is measured by the change in the backscattering pattern when there is no twist and when there is a twist. Since this pattern change corresponds to the strain caused by the twist, the amount of strain applied to the fiber can be measured from the pattern change. In the case of uncoupled multicore fibers, the twist of the entire fiber is measured by measuring the strain for each core.

[0005] G. Yin, L. Lu, L. Zhou, C. Shao, Q. Fu, J. Zhang, and T. Zhu, “Distributed directional torsion sensing based on an optical frequency domain reflectometer and a helical multicore fiber,” Opt. Express. , vol. 28, no. 11, pp. 16140-16150, 2020. T. Sakamoto, T. Mori, M. Wada, T. Yamamoto, F. Yamamoto, and K. Nakajima, “Strongly-coupled multi-core fiber and its optical characteristics for MIMO transmission systems,” Opt. Fiber Technology. , vol. 35, pp. 8-18, 2017.

[0006] When light between cores is randomly coupled, as in a coupled MCF, the light propagating through each core mixes randomly, so what can be measured is a signal that is a mixture of backscattering patterns due to distortion in all cores, and it is not possible to measure distortion for each core. Therefore, the method in Non-Patent Document 1 can measure distortion due to twisting, but it cannot measure twisting of the entire fiber.

[0007] Therefore, an object of the present disclosure is to make it possible to measure the twist of a coupled MCF.

[0008] In a coupled MCF, coupling between cores occurs when the propagation constants between the cores match (Non-Patent Document 2). Therefore, in this disclosure, the coupling between cores in a coupled MCF is used to measure the twist of the coupled MCF.

[0009] Specifically, the measurement device and method of the present disclosure measure the twist in a coupled MCF based on the distance between points where the intensity of light coupled from a first core to a second core in the coupled MCF is greatest.

[0010] The measurement device according to the present disclosure may include a backscattered light waveform measuring device that applies test light to a first end face of the first core of the coupled MCF and measures the intensity of backscattered light output from the first end face of the second core. In this case, the measurement device may measure the twist in the coupled MCF based on the distance between points where the intensity of the backscattered light is greatest.

[0011] The present disclosure utilizes the fact that the distance between two points where the backscattered light intensity changes sharply coincides with the distance between two points where the propagation constants between cores coincide due to twisting. For example, the measurement device of the present disclosure may measure twist based on the fact that the twist per distance between the points corresponds to a rotation angle according to the number of cores in the coupled multicore fiber.

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

[0013] According to the present disclosure, the twist of a coupled MCF can be measured.

[0014] FIG. 1 is a diagram illustrating a twisted state of a coupled MCF; FIG. 2 is an explanatory diagram showing an example of a change in the difference in propagation constant between cores when a coupled MCF is twisted; FIG. 3 is an explanatory diagram of the measurement principle of the present disclosure; FIG. 4 is an explanatory diagram of a backscattering waveform measured in the present disclosure; FIG. 5 is a configuration example of a measurement device of the present disclosure; FIG. 6 is a flowchart showing an example of a measurement method of the present disclosure; FIG. 7 is an example of a measurement result obtained using the measurement method of the present disclosure; and FIG. 8 is an explanatory diagram of the measurement principle of the present disclosure when the number of cores is three.

[0015] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0016] An installed optical fiber is twisted at a certain rate. Therefore, when a coupled MCF 91 having two cores in a cladding is bent or twisted at a radius R, the positions of the cores C1 and C2 in the cross section of the optical fiber change, as shown in FIG. 1 . The change in the positions of the cores C1 and C2 in the cross section of the coupled MCF 91 can be considered as rotation. Therefore, the twist rate of the coupled MCF 91 corresponds to the amount of rotation of the cores C1 and C2. Therefore, in this disclosure, the amount of rotation per unit length [rad / m] is measured as the twist rate.

[0017] Figure 2 shows the change in the propagation constant difference between cores in the longitudinal direction when a two-core coupled MCF 91 is bent, and the state (rotation) of the fiber cross section at that time. When a two-core coupled MCF 91 is bent, the refractive index of the outer core and the inner core changes relative to the bending radius in the longitudinal direction, and the propagation constant also changes accordingly. Therefore, as shown in Figure 2, the propagation constant difference between the cores changes periodically.

[0018] In Figure 2, as indicated by circles, the difference in propagation constants between cores C1 and C2 becomes zero at a half-rotation period of the two-core coupled MCF 91. It is known that when this inter-core propagation constant difference becomes zero, light couples between the cores at that point (Non-Patent Document 2). In a three- or more-core coupled MCF 91, the inter-core propagation constant difference also becomes zero at a rotation angle corresponding to the number of cores, as in the two-core coupled MCF 91. Therefore, in the present disclosure, the distance between the points where light couples between the cores, i.e., the coupling section Dh, is measured, and the twist per coupling section Dh is measured based on the fact that it corresponds to the rotation angle corresponding to the number of cores in the coupled MCF 91.

[0019] Fig. 3 shows the concept of a method for measuring twist. For clarity, the radius R and twist are omitted from the drawing. In reality, at least one of bending and twisting is applied, as shown in Fig. 1. The coupled MCF 91 is an example of a coupled optical fiber including two cores.

[0020] Test light is incident on core C1 from the left side of Figure 3. The test light propagates through core C1 from left to right. There are points where the difference in propagation constant between core C1 and core C2 is 0. The first point where the difference in propagation constant is 0 is designated as D1. At point D1, part of the test light propagating through core C1 transfers to core C2 and travels through core C2 to the right.

[0021] The next point where the difference in propagation constants is 0 is designated as D2. Here, the same phenomenon occurs, and the test light moves from core C1 to core C2 and travels to the right in Figure 3. The test light propagates through cores C1 and C2 to the right in Figure 3.

[0022] When test light propagates through an optical fiber, Rayleigh scattering occurs due to particles that are sufficiently small compared to the wavelength, and fluctuations in density and composition. As a result, backscattered light propagates to the left through core C2 due to Rayleigh scattering, and is output from the end face of core C2 in Figure 3.

[0023] In the present disclosure, test light is incident on one core C1 of the coupled MCF 91, and backscattered light from a core C2 adjacent to the core C1 onto which the test light is incident (adjacent core) is measured using the method shown in FIG.

[0024] 4 shows an overview of the measured backscattered light intensity. The measured backscattered light intensity waveform has the distance in the optical fiber on the horizontal axis and the intensity of the backscattered light on the vertical axis. There are two points D1 and D2 where the backscattered light intensity changes sharply. These points D1 and D2 are where the difference in propagation constant between core C1 and core C2 is zero. In the two-core coupled MCF 91 of this embodiment, the distance between points D1 and D2 corresponds to half a turn of the twist of the coupled MCF 91. In the present disclosure, the twist per unit length is measured using the coupling section Dh defined by points D1 and D2.

[0025] (Overall Configuration) Fig. 5 shows an embodiment of the measurement device of the present disclosure. A measurement device 90 of this embodiment includes a frequency domain reflectometer 92, a circulator 93, a fan-in / fan-out device 94, and an arithmetic processing unit 95. The frequency domain reflectometer 92 is connected to a first end face 91A of a coupled MCF 91. The frequency domain reflectometer 92 can be any backscattered light waveform measuring device with high distance resolution.

[0026] 6 is a flowchart showing an example of the twist measurement method of the present disclosure. First, the frequency domain reflectometer 92 measures the intensity of backscattered light from the adjacent core C2 of the coupled MCF 91 (S11). The frequency domain reflectometer 92 incidents test light on a first end facet of the first core C1 of the coupled MCF 91, and measures the intensity of the backscattered light that is scattered by the coupled MCF 91 and emitted from the first end facet of the second core C2 of the coupled MCF 91.

[0027] The frequency domain reflectometer 92 inputs test light to a first end facet 91A of the core C1 of the coupled MCF 91. In this disclosure, the core C1 to which the test light is input may be referred to as the input core or the first core. The test light output from the frequency domain reflectometer 92 passes through a circulator 93 and a fan-in / fan-out device 94 and is input to the core C1.

[0028] The frequency domain reflectometer 92 measures the intensity of backscattered light emitted from the first end face 91A of the core C2 adjacent to the core C1. In this disclosure, the core C2 adjacent to the core C1 may be referred to as an adjacent core or a second core. The core C2 may be any core to which the propagating light of the core C1 couples.

[0029] The backscattered light intensity from the core C2 passes through a fan-in / fan-out device 94 and a circulator 93, and is measured by a frequency domain reflectometer 92. By using the circulator 93, the backscattered light intensity from the adjacent core C2 can be measured using a compact configuration.

[0030] Fig. 7 shows an example of the backscattered light intensity obtained in step S11. Fig. 7 shows the actual backscattered light intensity measured for coupled MCF 91 using the measurement system in Fig. 5. In step S12, the calculation processing unit 95 detects two points D1 and D2 where the backscattered light intensity changes sharply from the distance distribution of the backscattered light intensity. As shown in Fig. 7, the two points D1 and D2 where the backscattered light intensity changes sharply were confirmed in the actual measurement.

[0031] Next, the arithmetic processing unit 95 calculates the torsion in the coupled MCF based on the distance between the points where the intensity of the backscattered light increases (S12, S13). For example, the arithmetic processing unit 95 detects two points where the intensity of the backscattered light changes sharply (S12), and calculates the torsion rate from the distance between the two detected points (S13).

[0032] In step S13, the calculation processing unit 95 calculates the twist rate using the coupling section Dh. As shown in FIG. 7, the distance between points D1 and D2 is 2.86 m. In other words, the twisted coupling section Dh is 2.86 m. Therefore, in the two-core coupled MCF 91 of this embodiment, the twist rate [rad / m] can be calculated using the half rotation amount π using the following equation: π / 2.86=1.10

[0033] Although the above-described embodiment illustrates an example in which the coupled MCF has two cores, the present disclosure can be applied to a coupled MCF having any number of cores greater than or equal to two. For example, FIG. 8 shows an example of application to a three-core coupled MCF. Test light is incident on core C1 of the coupled MCF 91, and the test light transitions to each core at points where the difference in propagation constant between core C1 and core C2 and core C3 is zero. For example, the test light is coupled to core C2 at points D1 and D2 where the difference in propagation constant between core C2 and core C3 is zero, and the test light is coupled to core C3 at points D3 and D4 where the difference in propagation constant between core C3 and core C3 is zero.

[0034] The frequency domain reflectometer 92 measures the backscattered light from the cores C2 and C3. The processor 95 then executes step S12 shown in Fig. 6 for each of the cores C2 and C3. This detects points D1 and D2 in the core C2 and points D1 and D2 in the core C3.

[0035] In step S13, the calculation processing unit 95 calculates the bonded section Dh1 in core C2 from points D1 and D2 in core C2, and calculates the bonded section Dh2 in core C3 from points D1 and D2 in core C3. The calculation processing unit 95 then calculates a first torsion rate using the bonded section Dh1, and calculates a second torsion rate using the bonded section Dh2. The calculation processing unit 95 then uses the first and second torsion rates to determine the torsion rate of the bonded MCF 91. At this time, the calculation processing unit 95 may calculate the average of the first and second torsion rates, or may select either one of them.

[0036] As described above, this embodiment can measure the torsion of the coupled MCF 91. Although the above-described embodiments have shown examples of two and three cores, the torsion ratio of the coupled MCF 91 can be obtained by performing similar measurements even when the number of cores is four or more.

[0037] In the present disclosure, an example has been shown in which a backscattered light waveform measuring device is connected to the first end face 91A of the coupled MCF 91. By employing such a configuration, it is possible to measure the twist of the coupled MCF 91 even when the coupled MCF 91 is installed for use in service. Furthermore, the backscattered light waveform measuring device can determine the longitudinal distance of the coupled MCF 91 at which the twist rate is measured. Therefore, it is possible to determine the twist rate at which the coupled MCF 91 is installed. Note that the present disclosure is not limited to the twist rate of the coupled MCF 91, and any method capable of quantifying the twist of the coupled MCF 91 can be employed to evaluate the twist of the coupled MCF 91.

[0038] If an unexpected twist occurs in an installed optical fiber, the optical characteristics will not be as designed. In particular, since the coupled MCF 91 is used for large-capacity communications, any unexpected twist may result in a degradation of communication quality. In this regard, the present disclosure makes it possible to accurately grasp the twist of the coupled MCF 91, thereby achieving the characteristics as designed and improving communication services using the coupled MCF.

[0039] 90: Measuring device 91: Coupled MCF 92: Frequency domain reflectometer 93: Circulator 94: Fan-in / Fan-out device 95: Processing unit

Claims

1. A measurement device that measures a twist in a coupled multicore fiber based on the distance between points where the intensity of light coupled from a first core to a second core in the coupled multicore fiber is high.

2. The measurement device according to claim 1, wherein the twist is measured based on the fact that the twist per unit distance between the points corresponds to a rotation angle according to the number of cores of the coupled multicore fiber.

3. The measurement device according to claim 1, further comprising a backscattered light waveform measuring device that incidents test light on a first end face of the first core provided in the coupled multicore fiber and measures the intensity of backscattered light emitted from the first end face of the second core, and measures the twist in the coupled multicore fiber based on the distance between points where the intensity of the backscattered light is greatest.

4. A measurement method comprising: measuring a twist in a coupled multicore fiber based on a distance between points at which the intensity of light coupled from a first core to a second core in the coupled multicore fiber is high.

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