Optical characteristic measuring method and optical characteristic measuring device

Abstract

To measure the optical characteristic of a coupled multicore fiber by the bidirectional OTDR, irrespective of the core-to-core coupling size.SOLUTION: An optical characteristic measurement method according to the present disclosure includes: connecting two single-core fibers (SCF) in series, mode field diameters (MFD) and relative refractive index differences of which are mutually different respectively and known; performing a bidirectional OTDR on an optical fiber transmission path formed by connecting one of the SCFs and one core of a coupled multicore fiber (MCF) in series and acquiring two intensity distributions of respective backscattering lights; calculating, for one discretional point in each of the two SCFs and each of middle points in the longitudinal direction of the coupled MCF, arithmetic mean intensities of the two intensities of the respective backscattering lights which are indicated with a logarithmic scale; and calculating at least one of the MFD, a specific refractive index difference and wavelength dispersion of the coupled MCF from the three arithmetic mean intensities, and the respective MFDs and respective relative refractive index differences of the two SCFs.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for evaluating the characteristics of a coupled multi-core fiber used in optical communications and the like, and to a method and apparatus for measuring a mode field diameter, a relative refractive index difference, and chromatic dispersion. [Background technology]

[0002] Due to the spread of a wide variety of Internet services, the amount of traffic flowing through a single optical fiber is increasing rapidly year by year. However, the transmission capacity that can be transmitted through an optical fiber is limited, and it is predicted that the currently widely used single-mode fiber (SMF) will not be able to handle the future increase in traffic. To overcome this situation, spatial multiplexing transmission systems using optical fiber with multiple cores (hereinafter referred to as multicore fiber) are being considered.

[0003] In recent years, in order to achieve both high-density spatial multiplexing and a reduction in the group delay difference between propagation modes, multicore fibers in which the modes between cores are randomly coupled by random bending or twisting in the longitudinal direction of the optical fiber (hereinafter referred to as coupled multicore fibers) have attracted attention. As with general-purpose single mode fibers, the mode field diameter, relative refractive index difference, and chromatic dispersion are important optical properties of coupled multicore fibers. To develop and commercialize coupled multicore fiber technology, a technique that can easily evaluate these optical properties is required.

[0004] Non-Patent Document 1 discloses a method for measuring the mode field diameter, relative refractive index difference, and chromatic dispersion in an SMF by bidirectional OTDR (Optical Time Domain Reflectometry). This method is widely used as a technique for evaluating the optical characteristics of an SMF because it can easily measure the optical characteristics of an optical fiber from one end non-destructively. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] 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. [Non-Patent Document 2] M. Ohashi et al., “Longitudinal fiber parameter measurements of multi-core fiber using OTDR,” Optics Express, vol. 22, no. 24, pp. 30137-30147, 2014. [Non-Patent Document 3] M. Ohashi et al., “OTDR technique for measuring crosstalk and fiber parameters in multi-core fibers,” Proc. IEEE 6th International Conference on Photonics, 2016. [Non-Patent Document 4] S. Kobayashi et al., “Characteristics of optical fibers in infrared wavelength region,” Review of the Electrical Communication Laboratories, vol. 26, no. 3-4, pp. 453-467, 1978. [Non-Patent Document 5] K. Nakajima et al., “Chromatic dispersion distribution measurement along a single-mode optical fiber,” Journal of Lightwave Technology, vol. 15, no. 7, pp. 1905-1101, 1997. Summary of the Invention [Problem to be solved by the invention]

[0006] For example, Non-Patent Document 2 discloses a method for measuring the mode field diameter, relative refractive index difference, and chromatic dispersion in a coupled multicore fiber by a bidirectional OTDR method. In this method, the mode field diameter, relative refractive index difference, and chromatic dispersion are obtained by assuming that the crosstalk between cores is negligibly small.

[0007] However, the method of Non-Patent Document 2 can be applied only to coupled multicore fibers in which inter-core coupling is negligibly small, and has a problem in that it cannot measure the mode field diameter, relative refractive index difference, and chromatic dispersion of a coupled multicore fiber in which inter-core coupling cannot be negligible.

[0008] In order to solve the above problems, an object of the present invention is to provide an optical characteristic measuring method and an optical characteristic measuring device that can measure the mode field diameter, relative refractive index difference, and chromatic dispersion of a coupled multicore fiber by using a bidirectional OTDR, regardless of the magnitude of inter-core coupling. [Means for solving the problem]

[0009] In order to achieve the above object, the optical characteristic measuring method and the optical characteristic measuring device disclosed herein use an optical fiber transmission line in which two single-core fibers having different optical characteristics and known optical characteristics are connected in series with a coupled multi-core fiber having unknown optical characteristics. The unknown optical characteristics of the coupled multi-core fiber are calculated using the backscattered light intensity distributions acquired at one end and the other end of the optical fiber transmission line and the known optical characteristics.

[0010] Specifically, the optical property measuring method according to the present disclosure includes: 1. A method for measuring optical characteristics of a coupled multi-core fiber, comprising the steps of: forming an optical fiber transmission line by connecting in series a first single-core fiber and a second single-core fiber, the first single-core fiber and the second single-core fiber having different mode field diameters and relative refractive index differences and the mode field diameters and the relative refractive index differences being known, and connecting one core of the coupled multi-core fiber in series to an unconnected end of the second single-core fiber; inputting a first test optical pulse from one end of the optical fiber transmission line; measuring a distribution of backscattered light intensity due to the first test light pulse with respect to distance from the one end; inputting a second test optical pulse from the other end of the optical fiber transmission line; measuring a distribution of backscattered light intensity due to the second test light pulse with respect to distance from the one end; calculating an arithmetic mean intensity of the backscattered light intensity due to the first test light pulse and the backscattered light intensity due to the second test light pulse, the backscattered light intensity being displayed on a logarithmic scale, for each of an arbitrary point on the first single-core fiber, an arbitrary point on the second single-core fiber, and a midpoint in a longitudinal direction of the coupled multi-core fiber; calculating at least one of a mode field diameter, a relative refractive index difference, and a chromatic dispersion of the coupled multicore fiber from the three arithmetic mean intensities, each of the mode field diameters of the first single-core fiber and the second single-core fiber, and each of the relative refractive index differences of the first single-core fiber and the second single-core fiber; Do the following.

[0011] Specifically, the optical property measuring device according to the present disclosure includes: a test optical pulse generator for generating a test optical pulse; an input / output unit that inputs a first test optical pulse to one end of an optical fiber transmission line and outputs backscattered light due to the first test optical pulse, and inputs a second test optical pulse to the other end of the optical fiber transmission line and outputs backscattered light due to the second test optical pulse, the input / output unit being configured to input a first test optical pulse to one end of the optical fiber transmission line and output backscattered light due to the second test optical pulse, the first test optical pulse being different from the second test optical pulse in mode field diameter and relative refractive index difference, the mode field diameter and relative refractive index difference being known, and a coupled multicore fiber having one of its cores connected in series to the second single-core fiber; a measuring device that measures a distribution of the intensity of backscattered light due to the first test light pulse and the intensity of backscattered light due to the second test light pulse output by the input / output device with respect to a distance from the one end; a calculator that calculates an arithmetic mean intensity of the intensity of backscattered light due to the first test light pulse and the intensity of backscattered light due to the second test light pulse, displayed on a logarithmic scale, for each of an arbitrary point on the first single-core fiber, an arbitrary point on the second single-core fiber, and a midpoint in the longitudinal direction of the coupled multi-core fiber, and calculates at least one of a mode field diameter, a relative refractive index difference, and a chromatic dispersion of the coupled multi-core fiber from the three arithmetic mean intensities, each of the mode field diameters of the first single-core fiber and the second single-core fiber, and each of the relative refractive index differences of the first single-core fiber and the second single-core fiber; Equipped with.

[0012] For example, in the optical characteristic measuring method and the optical characteristic measuring apparatus according to the present disclosure, The mode field diameter of the coupled multi-core fiber may be calculated using the formula C1.

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[0013] For example, in the optical characteristic measuring method and the optical characteristic measuring apparatus according to the present disclosure, The relative refractive index difference of the coupled multi-core fiber may be calculated using the formula C2.

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[0014] For example, in the optical characteristic measuring method and the optical characteristic measuring apparatus according to the present disclosure, When calculating the chromatic dispersion of the coupled multicore fiber, calculating a mode field diameter of the coupled multicore fiber using a formula C3; calculating the relative refractive index difference of the coupled multicore fiber using equation C4; calculating material dispersion from the relative refractive index difference of the coupled multicore fiber; calculating a wavelength dependency of a mode field diameter of the coupled multicore fiber from mode field diameters of the coupled multicore fiber acquired for a plurality of wavelengths λ; calculating waveguide dispersion from the wavelength dependence; The chromatic dispersion may be calculated by adding the material dispersion and the waveguide dispersion.

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[0015] The optical characteristic measuring method and optical characteristic measuring device disclosed herein use the backscattered light intensity distribution and the known optical characteristics acquired at one end and the other end of an optical fiber transmission line in which two single-core fibers having different and known optical characteristics and a coupled multi-core fiber having unknown optical characteristics are connected in series, and are capable of measuring the mode field diameter, relative refractive index difference, and chromatic dispersion of the coupled multi-core fiber by a bidirectional OTDR regardless of the magnitude of inter-core coupling.

[0016] The above-mentioned inventions can be combined as much as possible. Effect of the Invention

[0017] According to the present disclosure, it is possible to provide an optical characteristic measuring method and an optical characteristic measuring device that can measure the mode field diameter, relative refractive index difference, and chromatic dispersion of a coupled multicore fiber regardless of the magnitude of inter-core coupling by using a bidirectional OTDR. [Brief description of the drawings]

[0018] [Figure 1] 1 shows an example of a schematic configuration of an optical characteristic measuring device according to the present invention. [Diagram 2] 1 shows an example of a schematic configuration of an optical characteristic measuring device according to the present invention. [Diagram 3] 1 shows an example of a schematic configuration of an optical characteristic measuring device according to the present invention. [Figure 4]1 shows an example of a procedure for the optical property measuring method according to the present invention. [Diagram 5] 1 shows an example of a procedure for the optical property measuring method according to the present invention. [Figure 6] 1A to 1C are diagrams illustrating an optical characteristic measuring method according to the present invention. [Figure 7] 1A to 1C are diagrams illustrating an optical characteristic measuring method according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present invention 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 are assumed to indicate the same components.

[0020] (Embodiment 1) FIG. 1 shows an example of a schematic configuration of an optical characteristic measuring device according to this embodiment. The optical property measuring apparatus according to the present disclosure comprises: a test optical pulse generator 11 for generating a test optical pulse; an input / output unit 12 that inputs a first test light pulse to one end A of an optical fiber transmission line 10 and outputs backscattered light due to the first test light pulse, and inputs a second test light pulse to the other end B of the optical fiber transmission line 10 and outputs backscattered light due to the second test light pulse; the input / output unit 12 comprises a first single-core fiber 10-1 and a second single-core fiber 10-2, which are connected in series and have different mode field diameters and relative refractive index differences and which are known from each other; and a coupled multi-core fiber 10-3, one of whose cores is connected in series to the second single-core fiber 10-2; a measuring device 13 for measuring the distribution of the intensity of backscattered light due to the first test light pulse and the intensity of backscattered light due to the second test light pulse outputted from the input / output device 12 with respect to the distance from the one end A; and a calculator 14 that calculates an arithmetic mean intensity of the intensity of the backscattered light due to the first test light pulse and the intensity of the backscattered light due to the second test light pulse, displayed on a logarithmic scale, for each of an arbitrary point on the first single-core fiber 10-1, an arbitrary point on the second single-core fiber 10-2, and a midpoint in the longitudinal direction of the coupled multi-core fiber 10-3, and calculates at least one of the mode field diameter, the relative refractive index difference, and the chromatic dispersion of the coupled multi-core fiber 10-3 from the three arithmetic mean intensities, the mode field diameters of the first single-core fiber 10-1 and the second single-core fiber 10-2, and the relative refractive index differences of the first single-core fiber 10-1 and the second single-core fiber 10-2.

[0021] An example of the configuration of an optical characteristic measuring device according to this embodiment will be specifically described with reference to FIGS. 2 and 3. FIG.

[0022] 2 shows an example of the test optical pulse generator 11. The test optical pulse generator 11 includes a light source 11-1, an optical intensity modulator 11-2, and a pulse generator 11-3.

[0023] The light source 11-1 outputs continuous light of a wavelength to be used as test light.

[0024] The pulse generator 11-3 sends a pulse signal to the optical intensity modulator 11-2. The pulse generator 11-3 may also output a trigger signal to the measuring instrument 13 to determine the timing for starting measurement of the backscattered light intensity distribution.

[0025] The optical intensity modulator 11-2 pulses the continuous light output from the light source 11-1 in accordance with a signal from a pulse generator 11-3 to generate a test optical pulse. The optical intensity modulator 11-2 may be, for example, an acousto-optic modulator equipped with an acousto-optic switch that pulse-drives an acousto-optic element.

[0026] The input / output unit 12 includes, for example, an optical circulator that controls the propagation direction of light.

[0027] An example of the measuring instrument 13 is shown in Fig. 3. The measuring instrument 13 includes a photoreceiver 13-1, an A / D (analog / digital) converter 13-2, and a signal processing unit 13-3.

[0028] The photoreceiver 13-1 is connected to the input / output unit 12 via a single-core fiber, and receives the backscattered light generated in the optical transmission line 10 via the input / output unit 12.

[0029] The A / D converter 13-2 converts the electrical signal from the photoreceiver 13-1 into digital data, and inputs the digital data to the signal processor 13-3.

[0030] The signal processing unit 13-3 obtains the intensity distribution of the backscattered light from the input digital data.

[0031] The calculator 14 performs a calculation process to calculate the mode field diameter, the relative refractive index difference, and the chromatic dispersion from the distribution of the backscattered light intensity acquired by the signal processor 13-3. Details of this calculation process will be described later in step S07.

[0032] The signal processing unit 13-3 and the arithmetic unit 14 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.

[0033] An example of the procedure of the optical property measuring method according to this embodiment is shown in FIG. 4 and FIG. The optical characteristic measuring method according to the present embodiment includes the steps of: An optical characteristic measuring method for measuring optical characteristics of a coupled multi-core fiber 10-3, comprising the steps of: A first single-core fiber 10-1 and a second single-core fiber 10-2, which have different mode field diameters and relative refractive index differences and are known, are connected in series, and one core of a coupled multi-core fiber 10-3 is connected in series to an unconnected end of the second single-core fiber 10-2 to form an optical fiber transmission line 10 (step S00); A first test optical pulse is input from one end A of the optical fiber transmission line 10 (step S01); Measuring a distribution of backscattered light intensity by the first test light pulse with respect to distance from one end A (steps S02 and S03); A second test optical pulse is input from the other end B of the optical fiber transmission line 10 (step S04); Measuring a distribution of backscattered light intensity by the second test light pulse with respect to distance from the one end A (steps S05 and S06); calculating an arithmetic mean intensity of the backscattered light intensity due to the first test light pulse and the backscattered light intensity due to the second test light pulse, which are displayed on a logarithmic scale, for each of an arbitrary point on the first single-core fiber 10-1, an arbitrary point on the second single-core fiber 10-2, and a midpoint in the longitudinal direction of the coupled multi-core fiber 10-3 (substep S07-1); At least one of the mode field diameter, the relative refractive index difference, and the chromatic dispersion of the coupled multi-core fiber 10-3 is calculated from the three arithmetic mean intensities, the mode field diameters of the first single-core fiber 10-1 and the second single-core fiber 10-2, and the relative refractive index differences of the first single-core fiber 10-1 and the second single-core fiber 10-2 (substeps S07-2 to S07-7). Steps S00 to S07 will be described in detail below.

[0034] (Step S00) As shown in FIG. 1, a first single-core fiber 10-1 and a second single-core fiber 10-2, which have different mode field diameters and relative refractive index differences and whose mode field diameters and relative refractive index differences are known, are connected in series, and one core of a coupled multi-core fiber 10-3 is connected in series to the unconnected end of the second single-core fiber 10-2 to form an optical fiber transmission line 10.

[0035] (Step S01) As described above, the test optical pulse generator 11 converts the continuous light output from the light source 11-1 into a test optical pulse, and inputs the first test optical pulse from one end A of the optical fiber transmission line 10 via the input / output unit 12.

[0036] (Step S02) The measuring instrument 13 receives the backscattered light generated in the optical fiber transmission line 10 by the first test light pulse input in step S01 via the input / output unit 12 connected to one end A, as described above.

[0037] (Step S03) The measuring instrument 13 measures the distribution of the intensity of the backscattered light due to the first test light pulse received in step S02 with respect to the distance from one end A of the optical fiber transmission line 10.

[0038] (Step S04) As described above, the test optical pulse generator 11 converts the continuous light output from the light source 11-1 into a test optical pulse, and inputs the second test optical pulse from the other end B of the optical fiber transmission line 10 via the input / output unit 12.

[0039] (Step S05) The measuring instrument 13 receives the backscattered light generated in the optical fiber transmission line 10 by the second test light pulse input in step S04 via the input / output device 12 connected to the other end B, as described above.

[0040] (Step S06) The measuring instrument 13 measures the distribution of the intensity of the backscattered light due to the second test light pulse received in step S05 with respect to the distance from the one end A of the optical fiber transmission line 10.

[0041] Alternatively, steps S04 to S06 may be performed first to measure the distribution of the backscattered light intensity due to the second test light pulse, and then steps S01 to S03 may be performed to measure the distribution of the backscattered light intensity due to the first test light pulse.

[0042] (Step S07) In steps S03 and S06, the calculator 14 calculates the mode field diameter, the relative refractive index difference, and the chromatic dispersion of the coupled multi-core fiber by using the distribution of the backscattered light intensity by the first test light pulse and the distribution of the backscattered light intensity by the second test light pulse measured by the signal processor 13-3. This step may further include sub-steps S07-1 to S07-7 shown in Fig. 5. Sub-steps S07-1 to S07-7 will be described below.

[0043] (Substep S07-1) In steps S03 and S06, the calculator 14 extracts the backscattered light intensity due to the first test light pulse and the backscattered light intensity due to the second test light pulse at a midpoint in the longitudinal direction of the coupled multi-core fiber 10-3 from the distribution of the backscattered light intensity due to the first test light pulse and the distribution of the backscattered light intensity due to the second test light pulse measured by the signal processing unit 13-3, and displays them on a logarithmic scale. The backscattered light intensity due to the first test light pulse and the backscattered light intensity due to the second test light pulse displayed on the logarithmic scale are added and divided by 2 to obtain an arithmetic mean intensity. Similarly, the calculator 14 obtains an arithmetic mean intensity for each of an arbitrary point of the first single-core fiber 10-1 and an arbitrary point of the second single-core fiber 10-2.

[0044] The arithmetic mean intensity calculated directly from the measured backscattered light intensity can be expressed using a formula if the formula for the distribution of the backscattered light intensity is known. The formula expressing the arithmetic mean intensity will be explained by taking a coupled multicore fiber having two cores as an example. Here, the distance from one end A of the optical fiber transmission line 10 is represented by z. The coupled multicore fiber has two cores, which are referred to as core 1 and core 2. The loss coefficients α and Rayleigh scattering coefficients α of core 1 and core 2 are sand the backscattered light capture rate B have the same properties with respect to the distance z, and the power coupling coefficient h can be considered to be uniform in the longitudinal direction. The power coupling coefficient h is a parameter that indicates the magnitude of inter-core coupling. Here, the explanation will be given assuming that the second single-core fiber 10-2 is connected in series to the core 1, but the same applies to the case of the core 2.

[0045] In step S01, the intensity of the first test optical pulse input from one end A of the optical fiber transmission line 10 is set to P0. In step S04, the intensity of the second test optical pulse input from the other end B of the optical fiber transmission line 10 is set to P1.

[0046] In the above settings, the distribution of the backscattered light intensity due to the first test light pulse in the core 1 obtained in step S03 can be expressed by the following equation 1.

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[0047] The backscattered light intensity distribution of the second test light pulse in the core 1 obtained in step S06 can be expressed by Equation 2.

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[0048] The arithmetic mean intensity I(z) can be expressed by Equation 3 using Equation 1 and Equation 2.

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[0049] If the term having the power combining coefficient h of the arithmetic mean intensity I(z) in Equation 3 is M(z), then I(z) can be expressed using M(z) as in Equation 3a, and M(z) can be expressed as in Equation 3b. Note that in Equation 3a, I(z) is expressed by one equation for the entire range of 0≦z≦L0+L.

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[0050] Two OTDR waveforms acquired by bidirectional OTDR and their arithmetic mean intensities are shown in Fig. 7. In Fig. 7, the length of the first single-core fiber 10-1 is L1, the length of the second single-core fiber 10-2 is L2, and L0 = L1 + L2. As shown in Fig. 7, the arithmetic mean intensity I(z) takes a constant value independent of the distance z in the ranges of 0 ≤ z ≤ L1 and L1 ≤ z ≤ L0. Note that in Fig. 7, the α of the first single-core fiber 10-1 and the second single-core fiber 10-2 is s In the range of L0≦z≦L0+L, the arithmetic mean intensity I(z) takes a value according to the distance z. h = L0 + L / 2 arithmetic mean intensity I(z h ), it becomes possible to calculate the mode field diameter and the relative refractive index difference with high accuracy, as will be described later.

[0051] In the above, the case where the loss coefficient α of each core of the optical fiber transmission line 10 is uniform has been shown. However, when the loss coefficient α of each core is not uniform, the loss coefficient α can be expressed by Equation 4.

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[0052] (Substep S07-2) The three arithmetic mean intensities acquired in sub-step S07-1, the mode field diameter of the first single-core fiber 10-1, and the mode field diameter of the second single-core fiber 10-2 are used in equation 7 (see Non-Patent Document 3) to calculate the mode field diameter of the coupled multi-core fiber 10-3.

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[0053] (Substep S07-3) The three arithmetic average intensities acquired in sub-step S07-1, the relative refractive index difference of the first single-core fiber 10-1, and the relative refractive index difference of the second single-core fiber 10-2 are used in equation 8 (see non-patent document 3) to calculate the relative refractive index difference of the coupled multi-core fiber 10-3.

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[0054] The arithmetic mean intensity I(z h ) in Equation 7 and Equation 8, the accuracy of calculation of the mode field diameter and the relative refractive index difference is improved regardless of the magnitude of the inter-core coupling. In order to accurately calculate the mode field diameter and the relative refractive index difference regardless of the magnitude of the inter-core coupling, the difference in arithmetic mean intensity, that is, I(z h It is necessary that a term having a power coupling coefficient h is not included in I(z1)-I(z1) or I(z2)-I(z1) (Non-Patent Document 3). Then, the arithmetic mean intensity I(z h ), I(z h This is because it is possible to delete a term having the power combining coefficient h from (z1)-I(z1). As described above, z1 is the distance from one end A to an arbitrary point on the first single-core fiber 10-1, and z2 is the distance from one end A to an arbitrary point on the second single-core fiber 10-2, and each satisfies 0≦z≦L0.

[0055] Hereinafter, the reason why, when the arithmetic mean intensity at the midpoint in the longitudinal direction of the coupled multi-core fiber 10-3 is used, the difference in the arithmetic mean intensity does not include a term having the power coupling coefficient h will be described.

[0056] The term having the power combining coefficient h of the arithmetic mean intensity is written as M(z) in Equation 3b, so that the term having the power combining coefficient h in the difference of the arithmetic mean intensity I(z), for example, I(z)-I(z1), is expressed as the difference of M(z), for example, M(z)-M(z1).

[0057] As shown in the formula 3b, M(z) is constant regardless of the distance z when 0≦z≦L0. Therefore, for z1 and z2 that satisfy 0≦z≦L0, M(z2)-M(z1) is 0, and I(z2)-I(z1) does not include a term having the power coupling coefficient h. On the other hand, when L0≦z≦L0+L, M(z) becomes a function of the distance z. Here, when z1 is the midpoint in the longitudinal direction of the coupled multi-core fiber 10-3, h =L0+L / 2 is substituted for M(z) as shown in Equation 6.

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[0058] Therefore, by using the arithmetic mean intensity at the midpoint in the longitudinal direction of the coupled multi-core fiber 10-3, an arbitrary point in the first single-core fiber 10-1, and an arbitrary point in the second single-core fiber 10-2, the difference in the arithmetic mean intensity, that is, I(z h The term having the power coupling coefficient h can be deleted from I(z1)-I(z1) and I(z2)-I(z1), and the mode field diameter and the relative refractive index difference can be calculated with high accuracy using Equations 7 and 8.

[0059] (Substep S07-4) The relative refractive index difference Δ(z h) and Equation 9 (see Non-Patent Document 4), which represents Sellmeier's dispersion relation describing the wavelength dependence of the refractive index n, are used to calculate the wavelength dependence of the refractive index of the cores of the coupled multi-core fiber 10-3.

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[0060] The wavelength dependence of the refractive index of the core calculated by Equation 9 is used in Equation 10 (see Non-Patent Document 3) to obtain the material dispersion D m where c represents the speed of light.

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[0061] (Substep S07-5) Steps S01 to S06 are repeated multiple times by changing the wavelength of the test light pulse, the distribution of backscattered light intensity for multiple wavelengths is measured, and the mode field diameter of the coupled multi-core fiber 10-3 for multiple wavelengths is calculated by sub-steps S07-1 and S07-2.

[0062] The wavelength dependence of the mode field diameter of the coupled multi-core fiber 10-3 is calculated using the mode field diameter of the coupled multi-core fiber 10-3 at multiple wavelengths and equation 11 (see Non-Patent Document 5), which represents an approximation formula expressing the wavelength dependence of the mode field radius.

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[0063] (Substep S07-6) The wavelength dependency of the mode field diameter of the coupled multi-core fiber 10-3 calculated in sub-step S07-5 is used in Equation 12 (see Non-Patent Document 3), which is an approximation formula for the waveguide dispersion, to calculate the waveguide dispersion.

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[0064] (Substep S07-7) As shown in Equation 14, the material dispersion Dm calculated in substep S07-4 and the waveguide dispersion D w The chromatic dispersion is calculated by adding

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[0065] As described above, the optical characteristic measuring method and optical characteristic measuring device disclosed herein use the backscattered light intensity distributions and the known optical characteristics acquired at one end and the other end of an optical fiber transmission line in which two single-core fibers having different optical characteristics and known optical characteristics and a coupled multi-core fiber having unknown optical characteristics are connected in series, and are capable of measuring the mode field diameter, relative refractive index difference, and chromatic dispersion of the coupled multi-core fiber by a bidirectional OTDR regardless of the magnitude of inter-core coupling. [Industrial Applicability]

[0066] The optical property measuring method and the optical property measuring apparatus according to the present disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0067] 10: Optical fiber transmission line 10-1: First single-core fiber 10-2: Second single-core fiber 10-3: Coupled multicore fiber 11: Test optical pulse generator 11-1:Light source 11-2: Optical Intensity Modulator 11-3: Pulse generator 12: Input / Output Device 13: Measuring instrument 13-1:Receiver 13-2: A / D converter 13-3: Signal processing section 14: Arithmetic unit 101: Optical property measuring device

Claims

1. An optical characteristic measuring method for measuring optical characteristics of a coupled multicore fiber without measuring the magnitude of inter-core coupling, comprising the steps of: forming an optical fiber transmission line by connecting in series a first single-core fiber and a second single-core fiber, the first single-core fiber and the second single-core fiber having different mode field diameters and relative refractive index differences and the mode field diameters and the relative refractive index differences being known, and connecting one core of the coupled multi-core fiber in series to an unconnected end of the second single-core fiber; inputting a first test optical pulse from one end of the optical fiber transmission line; measuring a distribution of backscattered light intensity due to the first test light pulse with respect to distance from the one end; inputting a second test optical pulse from the other end of the optical fiber transmission line; measuring a distribution of backscattered light intensity due to the second test light pulse with respect to distance from the one end; calculating an arithmetic mean intensity of the backscattered light intensity due to the first test light pulse and the backscattered light intensity due to the second test light pulse, the backscattered light intensity being displayed on a logarithmic scale, for each of an arbitrary point on the first single-core fiber, an arbitrary point on the second single-core fiber, and a midpoint in a longitudinal direction of the coupled multi-core fiber; calculating a mode field diameter of the coupled multicore fiber from the three arithmetic mean intensities and the mode field diameters of the first single-core fiber and the second single-core fiber using a formula C1; A method for measuring optical properties. [Number C1] Here, z ｈ is the distance from the one end to the midpoint, z １ is the distance from the one end to any one point of the first single-core fiber, z ２ is the distance from the one end to any one point of the second single-core fiber, I(z ｈ ) is the arithmetic mean intensity with respect to the midpoint, I(z １ ) is the arithmetic mean intensity at any one point of the first single-core fiber, I(z ２ ) is the arithmetic mean intensity at any one point of the second single-core fiber, λ is the wavelength, w(λ, z ｈ ) is the mode field diameter with respect to the midpoint, w(λ, z １ ) is the mode field diameter at any one point of the first single-core fiber, w(λ, z ２ ) is the mode field diameter at any one point of the second single-core fiber.

2. An optical characteristic measuring method for measuring optical characteristics of a coupled multicore fiber without measuring the magnitude of inter-core coupling, comprising: forming an optical fiber transmission line by connecting in series a first single-core fiber and a second single-core fiber, the first single-core fiber and the second single-core fiber having different mode field diameters and relative refractive index differences and the mode field diameters and the relative refractive index differences being known, and connecting one core of the coupled multi-core fiber in series to an unconnected end of the second single-core fiber; inputting a first test optical pulse from one end of the optical fiber transmission line; measuring a distribution of backscattered light intensity due to the first test light pulse with respect to distance from the one end; inputting a second test optical pulse from the other end of the optical fiber transmission line; measuring a distribution of backscattered light intensity due to the second test light pulse with respect to distance from the one end; calculating an arithmetic mean intensity of the backscattered light intensity due to the first test light pulse and the backscattered light intensity due to the second test light pulse, the backscattered light intensity being displayed on a logarithmic scale, for each of an arbitrary point on the first single-core fiber, an arbitrary point on the second single-core fiber, and a midpoint in a longitudinal direction of the coupled multi-core fiber; calculating a relative refractive index difference of the coupled multicore fiber from the three arithmetic average intensities and the respective relative refractive index differences of the first single-core fiber and the second single-core fiber using a formula C2; A method for measuring optical properties. [Number C2] Here, z ｈ is the distance from the one end to the midpoint, z １ is the distance from the one end to any one point of the first single-core fiber, z ２ is the distance from the one end to any one point of the second single-core fiber, I(z ｈ ) is the arithmetic mean intensity with respect to the midpoint, I(z １ ) is the arithmetic mean intensity at any one point of the first single-core fiber, I(z ２ ) is the arithmetic mean intensity at any one point of the second single-core fiber, Δ(z ｈ ) is the relative refractive index difference with respect to the midpoint, Δ(z １ ) is the relative refractive index difference at any one point of the first single-core fiber, Δ(z ２ ) is the relative refractive index difference at any one point of the second single-core fiber, k is a constant.

3. An optical characteristic measuring method for measuring optical characteristics of a coupled multicore fiber without measuring the magnitude of inter-core coupling, comprising: forming an optical fiber transmission line by connecting in series a first single-core fiber and a second single-core fiber, the first single-core fiber and the second single-core fiber having different mode field diameters and relative refractive index differences and the mode field diameters and the relative refractive index differences being known, and connecting one core of the coupled multi-core fiber in series to an unconnected end of the second single-core fiber; inputting a first test optical pulse from one end of the optical fiber transmission line; measuring a distribution of backscattered light intensity due to the first test light pulse with respect to distance from the one end; inputting a second test optical pulse from the other end of the optical fiber transmission line; measuring a distribution of backscattered light intensity due to the second test light pulse with respect to distance from the one end; calculating an arithmetic mean intensity of the backscattered light intensity due to the first test light pulse and the backscattered light intensity due to the second test light pulse, the backscattered light intensity being displayed on a logarithmic scale, for each of an arbitrary point on the first single-core fiber, an arbitrary point on the second single-core fiber, and a midpoint in a longitudinal direction of the coupled multi-core fiber; calculating a mode field diameter of the coupled multicore fiber from the three arithmetic mean intensities and the mode field diameters of the first single-core fiber and the second single-core fiber using a formula C3; calculating a relative refractive index difference of the coupled multicore fiber from the three arithmetic average intensities and the respective relative refractive index differences of the first single-core fiber and the second single-core fiber using Equation C4; calculating material dispersion from the relative refractive index difference of the coupled multicore fiber; calculating a wavelength dependency of a mode field diameter of the coupled multicore fiber from mode field diameters of the coupled multicore fiber acquired for a plurality of wavelengths λ; calculating waveguide dispersion from the wavelength dependence; calculating the chromatic dispersion of the coupled multicore fiber by adding the material dispersion and the waveguide dispersion; A method for measuring optical properties. [Number C3] [Number C4] Here, z ｈ is the distance from the one end to the midpoint, z １ is the distance from the one end to any one point of the first single-core fiber, z ２ is the distance from the one end to any one point of the second single-core fiber, I(z ｈ ) is the arithmetic mean intensity with respect to the midpoint, I(z １ ) is the arithmetic mean intensity at any one point of the first single-core fiber, I(z ２ ) is the arithmetic mean intensity at any one point of the second single-core fiber, λ is the wavelength, w(λ, z ｈ ) is the mode field diameter with respect to the midpoint, w(λ, z １ ) is the mode field diameter at any one point of the first single-core fiber, w(λ, z ２ ) is a mode field diameter at any one point of the second single-core fiber, Δ(z ｈ ) is the relative refractive index difference with respect to the midpoint, Δ(z １ ) is the relative refractive index difference at any one point of the first single-core fiber, Δ(z ２ ) is the relative refractive index difference at any one point of the second single-core fiber, k is a constant.

4. An optical characteristic measuring device for measuring optical characteristics of a coupled multicore fiber without measuring the magnitude of inter-core coupling, comprising: a test optical pulse generator for generating a test optical pulse; an input / output unit that inputs a first test optical pulse to one end of an optical fiber transmission line and outputs backscattered light due to the first test optical pulse, and inputs a second test optical pulse to the other end of the optical fiber transmission line and outputs backscattered light due to the second test optical pulse, the input / output unit being configured to input a first test optical pulse to one end of the optical fiber transmission line and output backscattered light due to the second test optical pulse, the first test optical pulse being configured to input a second test optical pulse to the other end of the optical fiber transmission line, the second test optical pulse being configured to output backscattered light due to the second test optical pulse; a measuring device that measures a distribution of the intensity of backscattered light due to the first test light pulse and the intensity of backscattered light due to the second test light pulse output by the input / output device with respect to a distance from the one end; a calculator that calculates an arithmetic mean intensity of the intensity of backscattered light due to the first test light pulse and the intensity of backscattered light due to the second test light pulse, which are displayed on a logarithmic scale, for each of an arbitrary point on the first single-core fiber, an arbitrary point on the second single-core fiber, and a midpoint in the longitudinal direction of the coupled multi-core fiber, and calculates a mode field diameter of the coupled multi-core fiber from the three arithmetic mean intensities and each of the mode field diameters of the first single-core fiber and the second single-core fiber using a formula C5; An optical characteristic measuring apparatus comprising: 【Number C5】 Here, z ｈ is the distance from the one end to the midpoint, z １ is the distance from the one end to any one point of the first single-core fiber, z ２ is the distance from the one end to any one point of the second single-core fiber, I(z ｈ ) is the arithmetic mean intensity with respect to the midpoint, I(z １ ) is the arithmetic mean intensity at any one point of the first single-core fiber, I(z ２ ) is the arithmetic mean intensity at any one point of the second single-core fiber, λ is the wavelength, w(λ, z ｈ ) is the mode field diameter with respect to the midpoint, w(λ, z １ ) is the mode field diameter at any one point of the first single-core fiber, w(λ, z ２ ) is the mode field diameter at any one point of the second single-core fiber.

5. An optical characteristic measuring device for measuring optical characteristics of a coupled multicore fiber without measuring the magnitude of inter-core coupling, comprising: a test optical pulse generator for generating a test optical pulse; an input / output unit that inputs a first test optical pulse to one end of an optical fiber transmission line and outputs backscattered light due to the first test optical pulse, and inputs a second test optical pulse to the other end of the optical fiber transmission line and outputs backscattered light due to the second test optical pulse, the input / output unit being configured to input a first test optical pulse to one end of the optical fiber transmission line and output backscattered light due to the second test optical pulse, the first test optical pulse being configured to input a second test optical pulse to the other end of the optical fiber transmission line, the second test optical pulse being configured to output backscattered light due to the second test optical pulse; a measuring device that measures a distribution of the intensity of backscattered light due to the first test light pulse and the intensity of backscattered light due to the second test light pulse output by the input / output device with respect to a distance from the one end; a calculator that calculates an arithmetic mean intensity of the intensity of backscattered light due to the first test light pulse and the intensity of backscattered light due to the second test light pulse, which are displayed on a logarithmic scale, for each of an arbitrary point on the first single-core fiber, an arbitrary point on the second single-core fiber, and a midpoint in the longitudinal direction of the coupled multi-core fiber, and calculates a relative refractive index difference of the coupled multi-core fiber from the three arithmetic mean intensities and each of the relative refractive index differences of the first single-core fiber and the second single-core fiber using equation C6; An optical characteristic measuring apparatus comprising: 【Number C6】 Here, z ｈ is the distance from the one end to the midpoint, z １ is the distance from the one end to any one point of the first single-core fiber, z ２ is the distance from the one end to any one point of the second single-core fiber, I(z ｈ ) is the arithmetic mean intensity with respect to the midpoint, I(z １ ) is the arithmetic mean intensity at any one point of the first single-core fiber, I(z ２ ) is the arithmetic mean intensity at any one point of the second single-core fiber, Δ(z ｈ ) is the relative refractive index difference with respect to the midpoint, Δ(z １ ) is the relative refractive index difference at any one point of the first single-core fiber, Δ(z ２ ) is the relative refractive index difference at any one point of the second single-core fiber, k is a constant.

6. An optical characteristic measuring device for measuring optical characteristics of a coupled multicore fiber without measuring the magnitude of inter-core coupling, comprising: a test optical pulse generator for generating a test optical pulse; an input / output unit that inputs a first test optical pulse to one end of an optical fiber transmission line and outputs backscattered light due to the first test optical pulse, and inputs a second test optical pulse to the other end of the optical fiber transmission line and outputs backscattered light due to the second test optical pulse, the input / output unit being configured to input a first test optical pulse to one end of the optical fiber transmission line and output backscattered light due to the second test optical pulse, the first test optical pulse being configured to input a second test optical pulse to the other end of the optical fiber transmission line, the second test optical pulse being configured to output backscattered light due to the second test optical pulse; a measuring device that measures a distribution of the intensity of backscattered light due to the first test light pulse and the intensity of backscattered light due to the second test light pulse output by the input / output device with respect to a distance from the one end; calculating an arithmetic mean intensity of the intensities of backscattered light due to the first test light pulse and the backscattered light due to the second test light pulse, the intensities being displayed on a logarithmic scale, for each of an arbitrary point on the first single-core fiber, an arbitrary point on the second single-core fiber, and a midpoint in the longitudinal direction of the coupled multi-core fiber; calculating a mode field diameter of the coupled multicore fiber from the three arithmetic mean intensities and the mode field diameters of the first single-core fiber and the second single-core fiber using Equation C7; calculating a relative refractive index difference of the coupled multicore fiber from the three arithmetic average intensities and the respective relative refractive index differences of the first single-core fiber and the second single-core fiber using Equation C8; calculating material dispersion from the relative refractive index difference of the coupled multicore fiber; calculating a wavelength dependency of a mode field diameter of the coupled multi-core fiber from the mode field diameters of the coupled multi-core fiber acquired for a plurality of wavelengths λ; Calculating the waveguide dispersion from the wavelength dependence; a calculator for calculating the chromatic dispersion of the coupled multi-core fiber by adding the material dispersion and the waveguide dispersion; An optical characteristic measuring apparatus comprising: [Number C7] [Number C8] Here, z ｈ is the distance from the one end to the midpoint, z １ is the distance from the one end to any one point of the first single-core fiber, z ２ is the distance from the one end to any one point of the second single-core fiber, I(z ｈ ) is the arithmetic mean intensity with respect to the midpoint, I(z １ ) is the arithmetic mean intensity at any one point of the first single-core fiber, I(z ２ ) is the arithmetic mean intensity at any one point of the second single-core fiber, λ is the wavelength, w(λ, z ｈ ) is the mode field diameter with respect to the midpoint, w(λ, z １ ) is the mode field diameter at any one point of the first single-core fiber, w(λ, z ２ ) is a mode field diameter at any one point of the second single-core fiber, Δ(z ｈ ) is the relative refractive index difference with respect to the midpoint, Δ(z １ ) is the relative refractive index difference at any one point of the first single-core fiber, Δ(z ２ ) is the relative refractive index difference at any one point of the second single-core fiber, k is a constant.

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