Spatial mode dispersion measuring device and measuring method

Abstract

To provide a space mode dispersion measurement device and a measurement method which can measure a space mode dispersion of an integration-type multicore fiber with an inexpensive device and by a small number of measurement steps.SOLUTION: The space mode dispersion measurement of the present invention is not measuring an integration-type multicore fiber alone as a measurement target but measuring a transmission path formed by serially connecting two single core fibers with a known structure parameter to the integration-type multicore fiber as the measurement target. The space mode dispersion of the integration multicore fiber is calculated by comparison between backward scattering light from a single core fiber and backward scattering light from the integration multicore fiber.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a measurement device and a measurement method for measuring spatial mode dispersion of a coupled multicore fiber. [Background technology]

[0002] A coupled multicore fiber is one of the promising optical fibers as a medium for realizing future high-capacity optical communications. Spatial mode dispersion in a coupled multicore fiber is an important parameter related to the load of signal processing required to restore a transmitted signal received by a photodetector. Known methods for measuring spatial mode dispersion include the wavelength sweep method (see, for example, Non-Patent Document 1) and optical frequency domain reflectometry (OFDR) (see, for example, Non-Patent Document 2). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] T. Sakamoto et al., “Fiber twisting- and bending-induced adiabatic / nonnadiabatic super-mode transition in coupled multicore fiber,” Journal of Lightwave Technology, vol. 34, no. 4, pp. 1228-1237, 2016. [Non-patent document 2] S. Ohno et al., “Distributed spatial mode dispersion measurement along strongly coupled multicore fibers based on the correlation analysis of Rayleigh backscattering amplitudes,” Optics Express, vol. 25, no. 24, pp. 29650-29658, 2017. [Non-patent document 3] Ohashi et al., “Measurement of fiber parameters of pure silica core fibers based on the OTDR technique,” ​​Optics Express, vol. 29, no. 10, pp. 15078-, 2021. [Non-patent document 4] Ohashi et al., “Simple backscattered power technique for measuring crosstalk of multi-core fibers,” in Proc. 17th Opto-Electronics and Communications Conference, P1_25, 2012. [Non-Patent Document 5] CD Poole, “Statistical treatment of polarization dispersion in single-mode fiber,” Optics Letters, vol. 13, no. 8, pp. 687-689, 1998. [Non-patent document 6] A. Rossaro et al., “Spatially resolved chromatic dispersion measurement by a bidirectional OTDR technique,” ​​Journal of Lightwave Technology, vol. 7, no. 3, pp. 475-483, 2001. Summary of the Invention [Problem to be solved by the invention]

[0004] However, the method of Non-Patent Document 1 requires a large number of measurement steps because it is necessary to measure the light intensity output from one core of a coupled multi-core fiber and the light intensity output from all cores. Moreover, the method of Non-Patent Document 2 requires only a small number of measurement steps because it measures backscattered light generated when light is incident on one core of a coupled multi-core fiber, but the measuring equipment is very expensive. In other words, the disclosed methods have a problem in that it is difficult to measure the spatial mode dispersion of a coupled multi-core fiber using an inexpensive device with a small number of measurement steps.

[0005] In order to solve the above problems, an object of the present invention is to provide a spatial mode dispersion measuring device and a measuring method that can measure the spatial mode dispersion of a coupled multicore fiber using an inexpensive device and with a small number of measurement steps. [Means for solving the problem]

[0006] In order to achieve the above object, the spatial mode dispersion measurement device according to the present invention is an inexpensive optical time domain reflectometry (OTDR) measurement device that calculates the spatial mode dispersion of a coupled multicore fiber from parameters measured in a simple procedure.

[0007] Specifically, the spatial mode dispersion measurement device according to the present invention is a spatial mode dispersion measurement device for measuring the spatial mode dispersion of a coupled multicore fiber, and includes the following steps: a test light input unit that inputs an optical pulse to a transmission line connected so that one core of the coupled multicore fiber and each core of the two single-core fibers are in series and the single-core fibers are adjacent to each other; a receiving unit that receives backscattered light output from the transmission path; a calculation unit for calculating the spatial mode dispersion; Equipped with The receiving unit measuring a first backscattered light returning to the one core of the coupled multi-core fiber and a second backscattered light returning to another core of the coupled multi-core fiber by an optical pulse input to the one core of the coupled multi-core fiber at both ends of the transmission line; and measuring third backscattered light returning to the core of the single-core fiber due to an optical pulse input to the core of the single-core fiber between both ends of the transmission path; The calculation unit calculating a core radius and a relative refractive index difference of the coupled multicore fiber using an arithmetic average of the first backscattered light and the third backscattered light, the core radius and the relative refractive index difference of the single-core fiber, which are known, and the total length of the transmission line, which is known; calculating a differential group delay time from the core radius, the relative refractive index difference, and a known inter-core distance of the coupled multicore fiber; calculating a power coupling coefficient from an intensity ratio of the first backscattered light and the second backscattered light; and Calculating the spatial mode dispersion from the differential group delay, the power coupling coefficient, and the known fiber length of the coupled multicore fiber. It is characterized by:

[0008] Furthermore, a spatial mode dispersion measurement method according to the present invention is a spatial mode dispersion measurement method for measuring spatial mode dispersion of a coupled multicore fiber, comprising the steps of: forming a transmission line by connecting one core of the coupled multicore fiber and each core of the two single-core fibers so that they are in series and the single-core fibers are adjacent to each other; inputting an optical pulse into the one core of the coupled multicore fiber among both ends of the transmission line, and measuring a first backscattered light returning to the one core of the coupled multicore fiber and a second backscattered light returning to the other core of the coupled multicore fiber; inputting an optical pulse into the core of the single-core fiber between both ends of the transmission path, and measuring third backscattered light returning to the core of the single-core fiber; calculating a core radius and a relative refractive index difference of the coupled multicore fiber using an arithmetic average of the first backscattered light and the third backscattered light, the core radius and the relative refractive index difference of the single-core fiber, which are known, and the total length of the transmission line, which is known; calculating a differential group delay time from the core radius, the relative refractive index difference, and a known inter-core distance of the coupled multicore fiber; calculating a power coupling coefficient from an intensity ratio of the first backscattered light and the second backscattered light; and Calculating the spatial mode dispersion from the differential group delay, the power coupling coefficient, and the known fiber length of the coupled multicore fiber. It is characterized by:

[0009] The spatial mode dispersion measurement according to the present invention does not measure the coupled multicore fiber to be measured alone, but measures a transmission line in which two single-core fibers with known structural parameters are connected in series to the coupled multicore fiber to be measured. The spatial mode dispersion of the coupled multicore fiber is calculated by comparing the backscattered light from the single-core fibers with the backscattered light from the coupled multicore fiber.

[0010] Therefore, the present invention can provide a spatial mode dispersion measuring device and a measuring method that can measure the spatial mode dispersion of a coupled multi-core fiber using an inexpensive device and with a small number of measurement steps.

[0011] The calculation unit of the spatial mode dispersion measuring device 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. [Effects of the Invention]

[0012] The present invention can provide a spatial mode dispersion measuring device and a measuring method that can measure the spatial mode dispersion of a coupled multi-core fiber using an inexpensive device and with a small number of measurement steps. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram illustrating a spatial mode dispersion measuring device according to the present invention. [Figure 2] 1 is a flowchart illustrating a spatial mode dispersion measuring method according to the present invention. [Figure 3] 1A and 1B are diagrams illustrating a spatial mode dispersion measuring method according to the present invention. [Figure 4] 1 shows an OTDR waveform measured by the spatial mode dispersion measuring device according to the present invention. [Figure 5] 1 is a schematic diagram illustrating the calculation of a spatial mode dispersion measuring method according to the present invention; [Figure 6] FIG. 1 is a diagram illustrating structural imperfection loss in an optical transmission line. [Figure 7] 1 is a diagram illustrating the inter-core intensity ratio of backscattered light measured by the spatial mode dispersion measurement device according to the present invention. FIG. [Figure 8] FIG. 2 is a diagram illustrating a measurement system. [Figure 9] 10A and 10B are diagrams illustrating an OTDR waveform of an optical pulse coupled to another core. DETAILED DESCRIPTION OF THE INVENTION

[0014] 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.

[0015] 1 is a diagram illustrating a spatial mode dispersion measurement apparatus 301 according to this embodiment. The spatial mode dispersion measurement apparatus 301 is a measurement apparatus for measuring the spatial mode dispersion of a coupled multi-core fiber 50a, a test light input unit 10 that inputs an optical pulse to a transmission line 50 connected so that one core (core #m in FIG. 1) of the coupled multicore fiber 50a and each core #o of two single-core fibers (50b1, 50b2) are in series and the single-core fibers (50b1, 50b2) are adjacent to each other; a receiving unit (20) that receives backscattered light output from a transmission line (50); a calculation unit 30 for calculating spatial mode dispersion; Equipped with. The receiving unit 20 measures a first backscattered light returning to one core (core #m in the case of FIG. 1) of the coupled multi-core fiber 50a and a second backscattered light returning to another core (core #n in the case of FIG. 1) of the coupled multi-core fiber 50a due to an optical pulse input to one core (core #m in the case of FIG. 1) of the coupled multi-core fiber 50a at both ends of the transmission line 50. Furthermore, the receiving unit 20 measures a third backscattered light returning to core #o of the single-core fiber 50b2 due to an optical pulse input to core #o of the single-core fiber 50b2 at both ends of the transmission line 50. the calculation unit 30 calculates the core radius a and the relative refractive index difference Δ of the coupled multicore fiber 50a using the arithmetic mean of the first backscattered light and the third backscattered light, the core radius and the relative refractive index difference of the single-core fibers (50b1, 50b2) that are known, and the total length of the transmission line 50 that is known; calculating a differential group delay time G from the core radius a, the relative refractive index difference Δ, and the known inter-core distance D of the coupled multi-core fiber 50a; calculating a power coupling coefficient h from the intensity ratio of the first backscattered light and the second backscattered light; and The differential group delay G, the power coupling coefficient h, and the fiber length l of the coupled multi-core fiber 50a are known. a and calculating the spatial mode dispersion from the

[0016] The test light input unit 10 has a pulse light source 11, an optical circulator 12, an optical switch 13, and an input / output device 14. The pulse light source 11 outputs an optical pulse of an arbitrary wavelength. The optical circulator 12 passes the optical pulse from the pulse light source 11 toward the optical transmission line 50. The input / output device 14 can input an optical pulse to each of multiple cores appearing at one end A of the optical transmission line 50. For example, the input / output device 14 is a fan-in / fan-out device for a multicore fiber. The optical switch 13 performs path selection to select one of the backscattered light beams (core #m or #n) returning to one end A of the optical transmission line 50.

[0017] The receiving unit 20 has an opto-electrical converter 21 and an AD converter 22. When an optical pulse is input to one of the cores (for example, core #m) appearing at one end A of the optical transmission line 50, the optical pulse propagates through the optical transmission line 50, causing backscattered light (Rayleigh scattered light) to be generated in core #m. Furthermore, since the optical transmission line 50 is a coupled multi-core fiber 50a, the optical pulse is also coupled to core #n, causing backscattered light (Rayleigh scattered light) to be generated in core #n as well. The backscattered light is received by the opto-electrical conversion unit 21 via the input / output device 14, the optical switch 13, and the optical circulator 12. The opto-electrical converter 21 is, for example, a photodiode. The opto-electrical converter 21 converts the light intensity of the received backscattered light (light intensity relative to the distance from one end A of the optical transmission line 50) into an electrical signal. The AD converter 22 converts the analog electrical signal into a digital signal.

[0018] The calculation unit 30 includes a waveform analysis unit 31 and a crosstalk calculation unit 32. The operation of the calculation unit 30 will be described later.

[0019] 2 is a flowchart illustrating a method for measuring the spatial mode dispersion of the coupled multi-core fiber 50a using the spatial mode dispersion measurement device 301. forming a transmission line 50 by connecting one core (core #m in the case of FIG. 1 ) of the coupled multicore fiber 50a and each core #o of two single-core fibers (50b1, 50b2) in series and adjacent to each other (step S01); inputting an optical pulse to the one core (core #m in the case of FIG. 1 ) of the coupled multi-core fiber 50a at one end A side of both ends of the transmission line 50, and measuring a first backscattered light returning to the one core (core #m in the case of FIG. 1 ) of the coupled multi-core fiber 50a and a second backscattered light returning to the other core (core #n in the case of FIG. 1 ) of the coupled multi-core fiber 50a (step S02); At the other end B of the transmission line 50, an optical pulse is input to the core #o of the single-core fiber 50b2, and a third backscattered light returning to the core #o of the single-core fiber 50b2 is measured (step S03); and Calculating spatial mode dispersion from the first to third backscattered lights (step S04). It is characterized by:

[0020] 1 , the coupled multicore fiber 50a, the single-core fiber 50b1, and the single-core fiber 50b2 are connected in this order so that the central axes of the core #m of the coupled multicore fiber 50a, the core #o of the single-core fiber 50b1, and the core #o of the single-core fiber 50b2 are aligned. Here, of both ends of the transmission line 50, the end on the coupled multicore fiber 50a side is referred to as one end A, and the opposite end is referred to as the other end B. The length of the transmission line 50 is 0 [km] at one end A, z0 [km] at the connection point between the coupled multicore fiber 50a and the single-core fiber 50b1, z1 [km] at the connection point between the single-core fiber 50b1 and the single-core fiber 50b2, and z2 [km] at the other end B.

[0021] 3(1) and (2) are diagrams for explaining step S02. First, the spatial mode dispersion measuring device 301 is connected to one end A of the transmission line 50 . In step S02-1, the spatial mode dispersion measurement apparatus 301 inputs an optical pulse to the core #m of the coupled multicore fiber 50a. Then, in step S02-2, the spatial mode dispersion measurement apparatus 301 receives backscattered light generated in the core #m of the coupled multicore fiber 50a, the core #o of the single-core fiber 50b1, and the core #o of the single-core fiber 50b2. In step S02-3, the spatial mode dispersion measurement apparatus 301 acquires the light intensity distribution (OTDR waveform) P1(λ,z)[W] of the backscattered light in the longitudinal direction of the transmission line 50 from the backscattered light received by the spatial mode dispersion measurement apparatus 301 (see FIG. 4(1)).

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[0022] Next, the optical switch 13 is switched to the side of core #n of the coupled multi-core fiber 50a. In step S02-4, the spatial mode dispersion measurement device 301 inputs an optical pulse to core #m of the coupled multi-core fiber 50a. Then, in step S02-5, the spatial mode dispersion measurement device 301 receives backscattered light generated in core #n of the coupled multi-core fiber 50a by the optical pulse coupled to core #n. In step S02-6, the spatial mode dispersion measurement device 301 acquires, from the backscattered light received by the spatial mode dispersion measurement device 301, a light intensity distribution (OTDR waveform) P2(λ,z)[W] of the backscattered light in the longitudinal direction of the transmission line 50 (only the coupled multi-core fiber 50a) (see FIG. 4(2)).

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[0023] FIG. 3(3) is a diagram illustrating step S03. Following step S02, the spatial mode dispersion measuring apparatus 301 is reconnected to the other end B of the transmission line 50. At this time, the connection end of the input / output device 14 on the optical circulator 12 side is connected to the core #o of the single-core fiber 50b2. In step S03-1, the spatial mode dispersion measurement apparatus 301 inputs an optical pulse to the core #o of the single-core fiber 50b2. Then, in step S03-2, the spatial mode dispersion measurement apparatus 301 receives backscattered light generated in the core #m of the coupled multicore fiber 50a, the core #o of the single-core fiber 50b1, and the core #o of the single-core fiber 50b2. In step S03-3, the spatial mode dispersion measurement apparatus 301 acquires the light intensity distribution (OTDR waveform) P3(λ,z)[W] of the backscattered light in the longitudinal direction of the transmission line 50 from the backscattered light received by the spatial mode dispersion measurement apparatus 301 (see FIG. 4(3)).

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[0024] In step S04, the calculation unit 30 calculates spatial mode dispersion from each OTDR waveform in Fig. 4. Fig. 5 is an image diagram illustrating the calculation performed by the calculation unit 30. Step S04 is made up of steps S10, S20, and S30.

[0025] First, step S10 will be described. In step S10, the core radius a and the relative refractive index difference Δ of the coupled multicore fiber 50a are calculated using the arithmetic mean of the first backscattered light P1(λ, z) and the third backscattered light P3(λ, z) (step S11), the core radii and relative refractive index differences of the single-core fibers (50b1, 50b2), which are known, and the total length L of the transmission line 50, which is known (steps S12 and S13).

[0026] In step S11, the common logarithms of the first backscattered light P1(λ,z) ​​and the third backscattered light P3(λ,z) ​​are calculated, and their arithmetic mean is calculated.

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[0027] Figure 6 shows the distribution of I(λ, z) values ​​in the z direction, representing the structural mismatch loss [dB]. Here, we define the following normalized structural mismatch loss based on the structural mismatch loss at position z0: I n (λ, z) = I(λ, z) - I(λ, z0)

[0028] As shown in equation (4), I(λ, z) contains the capture efficiency B(λ, z) of equation (1a), which contains information on the structural parameters of the optical fiber. Therefore, from equations (4) and (1a), the core radius a and relative refractive index difference Δ of the coupled multi-core fiber 50a can be calculated as follows (steps S12 and S13):

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[0029] Next, step S20 will be described. In step S20, the power coupling coefficient h is calculated from the intensity ratio η between the first backscattered light P1(λ,z) ​​and the second backscattered light P2(λ,z) ​​(steps S21 and S22).

[0030] In step S21, the power ratio η between the first backscattered light P1(λ,z) ​​and the second backscattered light P2(λ,z) ​​is calculated. Figure 7 shows the distribution of the power ratio η in the z direction. In the region where hz<<1, the power ratio η and the power coupling ratio h have the following relationship:

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[0031] Therefore, in step S22, the power combining coefficient h is calculated as follows:

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[0032] Finally, step S30 will be described. In step S30, the differential group delay G is calculated from the core radius a, relative refractive index difference Δ, and known inter-core distance D of the coupled multi-core fiber 50a (step S31), and the spatial mode dispersion is calculated from the differential group delay G, power coupling coefficient h, and known fiber length z0 of the coupled multi-core fiber 50a (step S32).

[0033] In step S31, the electric field distribution is calculated by numerical analysis using the core diameter a obtained in step S12, the relative refractive index difference Δ obtained in step S13, the known wavelength λ of the optical pulse, and the known inter-core distance D (see, for example, reference 1). Then, the mode coupling coefficient κ is calculated by the overlap integral of the electric field distribution. Note that if the refractive index distribution is step-type, the mode coupling coefficient may be calculated using an approximate formula f or the like (see, for example, reference 2). κ=f(λ,a,Δ,D) Reference 1: Junichi Sakai, “Waveguide Optics”, Kyoritsu Publishing, ISBN978-4-320-08616-6 Reference 2: Katsunari Okamoto, "Fundamentals of Optical Waveguides", Corona Publishing, ISBN978-4-339-00602-5

[0034] In equations (2) and (3) of Non-Patent Document 1, if the propagation constants of the cores are equal (the core structures are the same), Δβ becomes zero in equation (3), and therefore equation (2) becomes δβ=2κ Here, δβ is the difference in propagation constants between the supermodes of core #m and core #n. Both the propagation constant difference δβ and the mode coupling coefficient κ depend on the optical frequency ω of the optical pulse.

[0035] The differential group delay G can be obtained from the propagation constant difference δβ using the following equation:

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[0036] In step S32, the spatial mode dispersion is calculated. The width of the optical pulse output from the far end when an impulse is incident on one core of a coupled two-core fiber can be derived using the same procedure as for the polarization mode dispersion in a single-mode optical fiber (see, for example, Non-Patent Document 5). Then, the spatial mode dispersion Δτ can be expressed by the following equation:

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[0037] The spatial mode dispersion Δτ can be calculated by substituting the differential group delay calculated in step S31, the power coupling coefficient h calculated in step S22, and the known fiber length z0 of the coupled multi-core fiber 50a into equation (9).

[0038] [Appendix 1] The core radius a and the relative refractive index difference Δ of the coupled multi-core fiber 50a in equation (5) will now be explained in more detail. The measurement system is shown in Figure 8. The reference fiber is a pure silica core fiber, and the fiber under test is also a pure silica core fiber. The mode field diameter (MFD), relative refractive index difference Δ, core diameter (2a), and effective area Aeff of the reference fiber are assumed to be known. The total length of the fiber is L. In this case, the backscattered light from a distance z when OTDR measurement is performed from A1 can be described by equation (101).

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[0039] [Appendix 2] A supplementary explanation of the calculation of the power coupling coefficient is provided below. Figure 9 shows the distribution (OTDR waveform) of the intensity of backscattered light output from one core of a coupled multicore fiber when an optical pulse is input to that core along the fiber length. When a coupled multi-core fiber is strongly coupled, the slope (in dB) of the OTDR waveform in the Hz>>1 region represents the average loss (corresponding to the loss explained as the loss coefficient of a pure silica single-core fiber in equations (1) to (3)) of two modes (light propagating through core #m and light coupled from core #m to #n and propagating through core #n). In other words, the OTDR waveform in Figure 9 can be described by the following equation.

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[0040] 10: Test light input section 11: Pulsed light source 12: Optical circulator 13: Optical switch 14: Input / output devices 20: Receiving unit 21: Photoelectric conversion unit 22: AD converter 30: Arithmetic section 31: Waveform analysis section 32: Spatial mode dispersion calculation unit 50: Optical transmission line 50a: Coupled multicore fiber 50b1, 50b2: Single-core fiber (reference fiber)

Claims

1. A spatial mode dispersion measurement device for measuring spatial mode dispersion of a coupled multi-core fiber, comprising: a test light input unit that inputs an optical pulse to a transmission line connected so that one core of the coupled multicore fiber and each core of the two single-core fibers are in series and the single-core fibers are adjacent to each other; a receiving unit that receives backscattered light output from the transmission path; a calculation unit for calculating the spatial mode dispersion; Equipped with The receiving unit measuring a first backscattered light returning to the one core of the coupled multicore fiber and a second backscattered light returning to another core of the coupled multicore fiber by an optical pulse input to the one core of the coupled multicore fiber at both ends of the transmission line; and measuring third backscattered light returning to the core of the single-core fiber due to an optical pulse input to the core of the single-core fiber between both ends of the transmission path; The calculation unit calculating a core radius and a relative refractive index difference of the coupled multicore fiber using an arithmetic average of the first backscattered light and the third backscattered light, the core radius and the relative refractive index difference of the single-core fiber, which are known, and the total length of the transmission line, which is known; calculating a differential group delay time from the core radius, the relative refractive index difference, and a known inter-core distance of the coupled multicore fiber; calculating a power coupling coefficient between the one core and the other core of the coupled multicore fiber from an intensity ratio between the first backscattered light and the second backscattered light; and Calculating the spatial mode dispersion from the differential group delay, the power coupling coefficient, and the known fiber length of the coupled multicore fiber. A spatial mode dispersion measuring device characterized by:

2. A spatial mode dispersion measurement method for measuring spatial mode dispersion of a coupled multi-core fiber, comprising: forming a transmission line by connecting one core of the coupled multicore fiber and each core of the two single-core fibers so that they are in series and the single-core fibers are adjacent to each other; inputting an optical pulse into the one core of the coupled multicore fiber out of both ends of the transmission path, and measuring a first backscattered light returning to the one core of the coupled multicore fiber and a second backscattered light returning to the other core of the coupled multicore fiber; inputting an optical pulse into the core of the single-core fiber between both ends of the transmission path, and measuring third backscattered light returning to the core of the single-core fiber; calculating a core radius and a relative refractive index difference of the coupled multicore fiber using an arithmetic average of the first backscattered light and the third backscattered light, the core radius and the relative refractive index difference of the single-core fiber, which are known, and the total length of the transmission line, which is known; calculating a differential group delay time from the core radius, the relative refractive index difference, and a known inter-core distance of the coupled multicore fiber; calculating a power coupling coefficient between the one core and the other core of the coupled multicore fiber from an intensity ratio between the first backscattered light and the second backscattered light; and Calculating the spatial mode dispersion from the differential group delay, the power coupling coefficient, and the known fiber length of the coupled multicore fiber. A spatial mode dispersion measurement method comprising:

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