Optical fiber testing apparatus and optical fiber testing method

Abstract

The objective of the present invention is to provide an optical fiber testing device and an optical fiber testing method with which it is possible to measure a distance dependency of crosstalk between cores of an uncoupled multicore fiber during bidirectional transmission.　An optical fiber testing device 301 according to the present invention is provided with: a measuring instrument 10 which inputs a light pulse into one core from one end A of an uncoupled multicore fiber 50 and measures a first light intensity of backscattered light output from the one core at the one end A, and inputs a light pulse into another core from the one end A of the uncoupled multicore fiber 50 and measures a second light intensity of backscattered light output from the one core at the one end A; and a calculator 20 which calculates a distance dependency of crosstalk between the cores when performing bidirectional transmission, from the first light intensity and the second light intensity.

Description

Technical Field

[0001] The present disclosure relates to an optical fiber test apparatus and an optical fiber test method for measuring crosstalk in uncoupled multi-core fibers.

Background Art

[0002] Uncoupled multi-core fiber is one of the promising optical fibers as a medium for realizing future high-capacity optical communication. Crosstalk between cores is an important parameter that limits the transmission capacity. Therefore, in order to ensure the desired transmission capacity, a method for evaluating the magnitude and longitudinal distribution of crosstalk between cores of uncoupled multi-core fibers is required.

[0003] Non-Patent Document 1 and Non-Patent Document 2 disclose a method for measuring the longitudinal distribution of crosstalk between cores when the signal transmission directions of each core in an uncoupled multi-core fiber are the same (unidirectional transmission). Non-Patent Document 3 proposes a method (bidirectional transmission) of making the signal transmission directions of adjacent cores alternate in order to reduce the influence of crosstalk between cores. Also, a method for measuring crosstalk between cores when such bidirectional transmission is performed is disclosed.

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Non-Patent Document 2

[0005] However, Non-Patent Document 1 and Non-Patent Document 2 do not disclose a method for measuring inter-core crosstalk when a non-bonded multi-core fiber is operated in bidirectional transmission. Further, Non-Patent Document 3 discloses obtaining inter-core crosstalk during bidirectional transmission operation of the entire non-bonded multi-core fiber to be measured, but does not disclose a method for obtaining the longitudinal distribution (distance dependence) of inter-core crosstalk. That is, currently, there is a problem that it is difficult to measure the distance dependence of inter-core crosstalk when bidirectional transmission is performed with a non-bonded multi-core fiber.

[0006] Therefore, an object of the present invention is to provide an optical fiber test apparatus and an optical fiber test method capable of measuring the distance dependence of inter-core crosstalk of a non-bonded multi-core fiber during bidirectional transmission in order to solve the above problems. [Means for Solving the Problems]

[0007] To achieve the above objective, the optical fiber testing apparatus according to the present invention measures the light intensity of backscattered light caused by a test light pulse incident on the core from one end of an uncoupled multicore fiber, and calculates the distance dependence of intercore crosstalk from that light intensity.

[0008] Specifically, the optical fiber testing apparatus according to the present invention is Inputting an optical pulse from one end of an uncoupled multicore fiber to one core, and measuring the first optical intensity of the backscattered light output from the one core at the one end, and A measuring instrument that inputs an optical pulse from one end of the uncoupled multicore fiber to one of two cores including the first core, and measures the second optical intensity of the backscattered light output from the other of the two cores at the one end, A computing unit that calculates the dependence of the inter-core crosstalk distance between the two cores of the uncoupled multicore fiber on the first and second light intensities when bidirectional transmission is performed with different light transmission directions between the two cores, It is equipped with.

[0009] Furthermore, the optical fiber testing method according to the present invention is A light pulse is input from one end of an uncoupled multicore fiber to one core, and the first light intensity of the backscattered light output from the one core is measured at the one end. An optical pulse is input from one end of the uncoupled multicore fiber to one of the two cores, including the first core, and the second optical intensity of the backscattered light output from the other of the two cores at the one end is measured, and The dependence of the inter-core crosstalk distance between the two cores of the uncoupled multicore fiber on the inter-core crosstalk distance when bidirectional transmission is performed with different optical transmission directions between the two cores is calculated from the first optical intensity and the second optical intensity. To do so.

[0010] As the first method for calculating the distance dependence of inter-core crosstalk, The light intensity of the light pulse that has passed through the first core of the uncoupled multicore fiber is calculated from the first light intensity as the signal light intensity. The leakage light intensity is defined as the product of the Rayleigh scattering coefficient, the backscatter light capture rate, and the integral value of the second light intensity integrated in the longitudinal direction of the uncoupled multicore fiber, and The ratio of the signal light intensity to the leaked light intensity is made dependent on the inter-core crosstalk distance. It is characterized by the following.

[0011] As a second method for calculating the distance dependence of inter-core crosstalk, Calculating the crosstalk between the two cores of the uncoupled multicore fiber when unidirectional transmission is performed in which the direction of optical transmission is the same between the two cores, from the first optical intensity and the second optical intensity. Calculating the power coupling coefficient from the aforementioned crosstalk, The loss coefficient is calculated from the light intensity of the light pulse incident on the first core from one end of the uncoupled multicore fiber and the first light intensity, and The inter-core crosstalk distance dependence is calculated by substituting the Rayleigh scattering coefficient, the backscattered light capture rate, and the loss coefficient into the power coupling equation of number C1. It is characterized by the following.

number

[0012] As described above, the present invention provides an optical fiber testing apparatus and an optical fiber testing method that can measure the distance dependence of intercore crosstalk in uncoupled multicore fibers during bidirectional transmission. Furthermore, the above inventions can be combined as much as possible. [Effects of the Invention]

[0013] The present invention provides an optical fiber testing apparatus and an optical fiber testing method that can measure the distance dependence of intercore crosstalk in uncoupled multicore fibers during bidirectional transmission. [Brief explanation of the drawing]

[0014] [Figure 1] This is a diagram illustrating the optical fiber testing apparatus according to the present invention. [Figure 2] This is a diagram illustrating the optical fiber testing method according to the present invention. [Figure 3] This figure illustrates the waveform of backscattered light obtained by the optical fiber testing apparatus according to the present invention. [Figure 4] This figure illustrates a method for calculating loss values ​​from the waveform of backscattered light obtained by the optical fiber testing apparatus according to the present invention. [Figure 5] This figure illustrates a method for calculating the cumulative value of backscattered light from the waveform of backscattered light obtained by the optical fiber testing apparatus according to the present invention. [Figure 6] This figure illustrates an example of the optical fiber testing apparatus according to the present invention. [Figure 7] This diagram illustrates the effects of the optical fiber testing apparatus according to the present invention. [Figure 8] This diagram illustrates how to obtain the Rayleigh scattering coefficient and capture rate. [Figure 9] This diagram illustrates the measurement principle of the optical fiber testing apparatus according to the present invention. [Figure 10] This diagram illustrates the measurement principle of the optical fiber testing apparatus according to the present invention. [Modes for carrying out the invention]

[0015] Embodiments of the present invention will be described with reference to the attached drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to these embodiments. In this specification and in the drawings, components with the same reference numerals refer to the same components.

[0016] Figure 1 is a diagram illustrating the optical fiber testing apparatus 301 according to the present invention. The optical fiber testing apparatus 301 measures crosstalk during bidirectional transmission using an uncoupled multicore fiber 50 as the optical fiber under test. The optical fiber testing apparatus 301 An optical pulse is input from one end A of an uncoupled multicore fiber 50 to one core (e.g., #m), and the first optical intensity of the backscattered light output from the one core at one end A is measured (first measurement), and A measuring instrument 10 performs the following: input an optical pulse from one end A of an uncoupled multicore fiber 50 to one of two cores (for example, #m or #n) including the core 1, and measure the second optical intensity of the backscattered light output from the other of the two cores (core #n if the core to which the optical pulse was input is #m, and core #m if the core to which the optical pulse was input is #n) at one end A (second measurement), A computing unit 20 that calculates the dependence of the inter-core crosstalk distance between the two cores of an uncoupled multicore fiber 50 on the first and second light intensities when bidirectional transmission is performed with different optical transmission directions between the two cores, It is equipped with. In the case of a multi-core optical fiber with three or more cores, the "two cores" mentioned above refer to adjacent cores.

[0017] Figure 2 is a flowchart illustrating the optical fiber testing method performed by the optical fiber testing apparatus 301. This method is: Step S01: An optical pulse is input from one end A of an uncoupled multicore fiber 50 to one core, and the first optical intensity of the backscattered light output from the one core at one end A is measured. Step S02 involves inputting an optical pulse from one end A of the uncoupled multicore fiber 50 to one of the two cores including the core 1, and measuring the second optical intensity of the backscattered light output from the other of the two cores at one end A, and Step S03: Calculate the dependence of the inter-core crosstalk distance between the two cores of the uncoupled multicore fiber 50 on the first and second optical intensities when bidirectional transmission is performed with different optical transmission directions between the two cores. To do so.

[0018] The measuring instrument 10 comprises a test light generation unit 11 that generates an optical pulse, an input / output unit 12 that inputs the optical pulse to an uncoupled multicore fiber 50 and captures the backscattered light from the uncoupled multicore fiber 50, and a receiving unit 13 that measures the intensity of the backscattered light. The measuring instrument 10 performs steps S01 and S02. The test light generation unit 11 and the input / output unit 12 perform steps m11, m12, m21, and m22, and the receiving unit 13 performs steps m13 and m23.

[0019] The input / output unit 12 includes, for example, an optical circulator 12a, an optical switch 12b, and an input / output device 12c. The optical switch 12b selects the core (#m or #n) of the uncoupled multicore fiber 50 into which the optical pulse is injected, and selects the core (#m or #n) of the uncoupled multicore fiber 50 from which the backscatter to be captured is emitted. The receiving unit 13 includes, for example, a photoelectric converter 13a that receives the backscattered light and converts it into an electrical signal, and an AD converter 13b that converts the electrical signal from analog to digital.

[0020] The arithmetic unit 20 performs step S03. The arithmetic unit 20 includes, for example, a waveform analysis unit 20a that analyzes the waveform of the electrical signal converted into a digital signal, and a crosstalk calculation unit 20b that calculates the crosstalk. The calculations performed by the arithmetic unit 20 will be explained in the following embodiment.

[0021] (Example 1) This embodiment is a method for calculating inter-core crosstalk in an uncoupled multi-core fiber during bidirectional transmission using the integration of backscattered light. Step S01: The measuring instrument 10 injects an optical pulse into the core #m from one end A of the uncoupled multicore fiber 50 and measures the optical intensity of the backscattered light 1 from the core #m at one end A. The backscattered light 1 is the backscattered light intensity from the incident core. Step S02: The measuring instrument 10 injects an optical pulse into core #n from one end A of the uncoupled multicore fiber 50 and measures the intensity of the backscattered light 2 from core #m at end A. The backscattered light 2 is the backscattered light intensity from the adjacent core. If the loss coefficients of each core of the uncoupled multicore fiber 50 can be considered equal, the backscattered light from core #n caused by the optical pulse injected into core #m may also be considered as the backscattered light 2. By performing steps S01 and S02, the measuring instrument 10 can obtain the light intensity distribution shown in Figure 3.

[0022] The arithmetic unit 20 is, The light intensity of the light pulse that has passed through the first core of the uncoupled multicore fiber 50 is defined as the signal light intensity P. signal This is calculated from the first light intensity (backscattered light 1). The product of the Rayleigh scattering coefficient, the backscattered light capture rate, and the integral value obtained by integrating the second light intensity (backscattered light 2) over the longitudinal distance of the uncoupled multicore fiber 50 is the leakage light intensity P. bs To do so, and Signal light intensity P signal and leakage light intensity P bs The ratio of the inter-core crosstalk distance is made dependent on the inter-core crosstalk distance. It is characterized by the following.

[0023] Step S03: The arithmetic unit 20 performs the following calculation using the light intensity distribution in Figure 3. Step m31: The arithmetic unit 20 calculates the core #m loss value over the entire length of the uncoupled multicore fiber 50 from the backscattered light 1, as shown in Figure 4. Since the product of the power coupling coefficient and fiber length is usually well less than 1 for an uncoupled multicore fiber, the difference in intensity between the near end and the far end can be considered as the loss value. Step m32: From the loss value of core #m calculated above, the signal light intensity P of the optical pulse output from core #m at the other end B is calculated. signalCalculate it. Note that the definition of the inter-core crosstalk of the uncoupled multi-core fiber during bidirectional transmission is as described in Appendix 2. In this case, an optical pulse must be incident on core #m at the other end B, and the optical intensity of the optical pulse at core #m at one end A must be measured. However, whether it is an optical pulse incident on one end A and emitted from the other end B or an optical pulse incident on the other end B and emitted from one end A (in the reverse direction), the signal optical intensity P signal will be the same value. Therefore, in this calculation, this concept is utilized, and for the optical pulse incident on core #m at one end A and emitted from core #m at the other end B, the signal optical intensity P signal is obtained. Step m33: Obtain the product of the Rayleigh scattering coefficient α s of the uncoupled multi-core fiber 50 and the backscattered light capture rate B by any of the methods described in Appendix 1. Step m34: As shown in FIG. 5, by integrating the optical intensity of the backscattered light 2 in the distance z direction, the cumulative value (leakage optical intensity) P bs of the backscattered light 2 is calculated.

Equation

Equation

[0024] The calculator 20 obtains the distance dependence of the inter-core crosstalk when bidirectional transmission is performed by calculating XT b for each distance z.

[0025] (Example 2) This embodiment is a method for calculating inter-core crosstalk during bidirectional transmission from inter-core crosstalk during unidirectional transmission of an uncoupled multi-core fiber. Step S01: The measuring instrument 10 injects an optical pulse into the core #m from one end A of the uncoupled multicore fiber 50 and measures the optical intensity of the backscattered light 1 from the core #m at one end A. The backscattered light 1 is the backscattered light intensity from the incident core. Step S02: The measuring instrument 10 injects an optical pulse into core #m from one end A of the uncoupled multicore fiber 50 and measures the backscattered light 2 from core #n at end A. The backscattered light 2 is the backscattered light intensity from the adjacent core. By performing steps S01 and S02, the measuring instrument 10 can obtain the light intensity distribution shown in Figure 3.

[0026] The arithmetic unit 20 is, The crosstalk between the two cores of the uncoupled multicore fiber 50 is calculated from the first light intensity (backscattered light 1) and the second light intensity (backscattered light 2) when unidirectional transmission is performed in which the direction of light transmission is the same between the two cores. Calculate the power coupling coefficient h from the aforementioned crosstalk, The loss factor α is calculated from the light intensity of the optical pulse incident on one core (e.g., #m) from one end A of the uncoupled multicore fiber 50 and the first light intensity, and The inter-core crosstalk distance dependence is calculated by substituting the Rayleigh scattering coefficient, backscattered light capture rate, and loss coefficient α into the power coupling equation of number C1. It is characterized by the following.

number

[0027] Step S03: The arithmetic unit 20 performs the following calculation using the light intensity distribution in Figure 3. Step m41: The crosstalk XT between the two cores when unidirectional transmission is performed can be calculated from the ratio of backscattered light 1 to backscattered light 2. The arithmetic unit 20 calculates the inter-core crosstalk XT during unidirectional transmission from the light intensity distribution in Figure 3 and calculates the power coupling coefficient h according to the method described in Non-Patent Literature 2. Step m42: Obtain the product of the Rayleigh scattering coefficient and backscattered light capture rate of the uncoupled multicore fiber 50 using one of the methods described in Appendix 1. Note that steps m41 and m42 may be performed in any order. Step m43: Substitute the various parameters obtained in steps m41 and m42 into equation (C1) which represents the inter-core crosstalk derived from the power coupling equation, and calculate the inter-core crosstalk XT in the case of bidirectional transmission. b This is obtained. The method for deriving equation (C1) from the power coupling equation is explained in Appendix 2.

[0028] The arithmetic unit 20 calculates XT for every distance z. b By calculating this, the inter-core crosstalk distance dependence in the case of bidirectional transmission is obtained.

[0029] [Examples] Opposite transmission crosstalk XT with optical fiber test equipment 301 b Experiments were conducted to confirm whether it was possible to measure the crosstalk XT of the opposite transmission using the methods of Example 1 and Example 2. b The crosstalk XT obtained by the power meter method is calculated and the crosstalk XT of the opposing transmission is obtained. b This was compared with the previous setup. The experimental setup is shown in Figure 6. The configuration in Figure 6(A) corresponds to the optical fiber testing apparatus 301. Figure 6(B) shows the configuration using the power meter method. A 4CF (SN: 4CMCF2110-01) manufactured by Furukawa Electric Co., Ltd. was used as the uncoupled multicore fiber 50.

[0030] Figure 7 illustrates the experimental results. Figure 7(A) illustrates the OTDR waveform measured with the configuration shown in Figure 6(A). The wavelength of the light pulse is 1550 nm, and the pulse width is 1 μs. The dashed line is the waveform of backscattered light 1 obtained from the core into which the light pulse was incident. The solid line is the waveform of backscattered light 2 obtained from the core adjacent to the core into which the light pulse was incident.

[0031] Figure 7(B) illustrates the distance dependence of crosstalk calculated from the OTDR waveform. The solid line shows the result of the inter-core crosstalk distance dependence during unidirectional transmission. The dashed line shows the result of the inter-core crosstalk distance dependence during bidirectional transmission, calculated directly from the backscattered light intensity described in Example 1. The dotted line shows the result of the inter-core crosstalk distance dependence during bidirectional transmission, calculated from the fiber parameters described in Example 2. Both results were approximately the same.

[0032] Furthermore, in Figure 7(B), the circles represent the crosstalk values ​​during bidirectional transmission obtained by the power meter method. The crosstalk XT at the far end obtained by the calculation methods of Example 1 and Example 2. b This was in close agreement with the crosstalk obtained by the power meter method. From these results, it was confirmed that the distance dependence of crosstalk during bidirectional transmission measured with the optical fiber test device 301 is reliable.

[0033] [Appendix 1] Method for obtaining Rayleigh scattering coefficient and capture rate (Method 1) If the core's mode field diameter is known, the backscattered light capture rate B is calculated from the mode field diameter using equation (11).

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[0034] (Method 2) Using an optical fiber with known Rayleigh scattering coefficient and capture rate as a reference fiber, the Rayleigh scattering coefficient and capture rate of the optical fiber under test are obtained by bidirectional OTDR (see Reference A). Figure 8(A) illustrates this method. A core 51a of a reference fiber 51 with a known Rayleigh scattering coefficient and capture rate is connected to one core (e.g., #m) of an uncoupled multicore fiber 50. Test light is incident on this test system from both sides (reference fiber 51 side and the other end B side) and the OTDR waveform is measured. From this OTDR waveform, the waveform of the structural imperfection component I(z) as shown in Figure 8(B) is obtained. The structural imperfection component I(z) can be expressed by the following equation. [Number 13] I(z) = 10log[α s (z)B(z)]+a0 Here, a0 is a constant determined by the input power and losses. In the waveform in Figure 8(B), interval z s The structural imperfection component I(z) of the reference fiber 51 is a known value. Therefore, interval z t The structural imperfection component I(z) of the uncoupled multicore fiber 50 can be obtained as a relative value to the structural imperfection component I(z) of the reference fiber 51. In other words, although the individual values ​​of the Rayleigh scattering coefficient and capture rate of the uncoupled multicore fiber 50 are unknown, the Rayleigh scattering coefficient α s The product of this and the capture rate B can be calculated from the structural irregularity component I(z) of the reference fiber 51. (Reference A) Kazuhide Nakajima et al., “Chromatic Dispersion Distribution Measurement Along a Single-Mode Optical Fiber”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 15, NO. 7, JULY 1997

[0035] [Appendix 2] Crosstalk Evaluation Techniques for Uncoupled Multicore Fibers (1) Definition of crosstalk Generally, crosstalk refers to the optical power P of a signal intended for transmission. signal And the light power P of the signal intended to block noise This is the ratio. In unidirectional transmission, crosstalk XT is the power ratio (XT = P) between the signal light incident from core #m at one end A, output from core #m at the other end B, and the leaked light output from the adjacent core #n. noise / P signal ) (Figure 9(A)). On the other hand, crosstalk XT in bidirectional transmission b When light leakage from non-adjacent cores is sufficiently small, the signal light P that enters from core #m at the other end B is output from core #m at one end A. signal And another signal light that entered from an adjacent core #n of end A is reflected light P that is output from core #m of end A. bs Power ratio (XT b =P bs / P signal ) (Figure 9(B)). (2) Relationship between crosstalk and fiber parameters Here, we formulate the relationship between crosstalk and fiber parameters in a two-core fiber (referred to as cores #m and #n). The fiber loss is equal in each core, and furthermore, various parameters (fiber loss α, power coupling coefficient h, backscattered light capture rate B, Rayleigh scattering coefficient α) are assumed to be equal. s It is assumed that the θ is uniform along the longitudinal direction of the optical fiber. The fiber length is L (m). Fresnel reflection is assumed to be absent. Under these assumptions, the following relationship holds between adjacent cores even in a multicore fiber with three or more cores. i) In the case of one-way transmission The optical intensity of each core at position z in a two-core fiber can be described by the following power coupling equation.

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[0036] 10: Measuring instrument 11: Test light generation unit 12: Input / output section 12a: Optical Circulator 12b: Optical switch 12c: Input / Output Devices 13: Receiving Unit 13a: Photoelectric conversion unit 13b: AD converter 20: Arithmetic unit 20a: Waveform analysis section 20b: Crosstalk calculation unit 50: Optical fiber under test (uncoupled multicore fiber) 51: Reference fiber 301: Optical fiber testing equipment

Claims

1. A light pulse is input to one core from one end of an uncoupled multicore fiber, and the first light intensity of the backscattered light output from the one core is measured at the one end, and A measuring instrument that inputs an optical pulse from one end of the uncoupled multicore fiber to one of two cores including the first core, and measures the second optical intensity of the backscattered light output from the other of the two cores at the one end, A computing unit that calculates the dependence of the inter-core crosstalk distance between the two cores of the uncoupled multicore fiber on the first and second light intensities when bidirectional transmission is performed with different light transmission directions between the two cores, An optical fiber testing apparatus comprising, The aforementioned arithmetic unit, The light intensity of the light pulse that has passed through the first core of the uncoupled multicore fiber is the signal light intensity P. ｓｉｇｎａｌ This is calculated from the first light intensity. Rayleigh scattering coefficient α of the uncoupled multicore fiber obtained by any method ｓ , the backscatter light capture rate B of the uncoupled multicore fiber obtained by any method, and the second light intensity P in the interval Δz at any point. ｋ Let's assume that the leakage light intensity P is calculated using equation 1. ｂｓ Calculating, and The inter-core crosstalk distance dependence XT is given by equation 2. ｂ Calculating A fiber optic testing device characterized by the following features. [Math 1] [Math 2]

2. A light pulse is input to one core from one end of an uncoupled multicore fiber, and the first light intensity of the backscattered light output from the one core is measured at the one end, and A measuring instrument that inputs an optical pulse from one end of the uncoupled multicore fiber to one of two cores including the first core, and measures the second optical intensity of the backscattered light output from the other of the two cores at the one end, A computing unit that calculates the dependence of the inter-core crosstalk distance between the two cores of the uncoupled multicore fiber on the first and second light intensities when bidirectional transmission is performed with different light transmission directions between the two cores, An optical fiber testing apparatus comprising, The aforementioned arithmetic unit, To calculate the crosstalk between the two cores of the uncoupled multicore fiber when unidirectional transmission is performed in which the direction of optical transmission is the same between the two cores, from the first optical intensity and the second optical intensity. Calculating the power coupling coefficient from the aforementioned crosstalk, The loss coefficient is calculated from the light intensity of the light pulse incident on the first core from one end of the uncoupled multicore fiber and the first light intensity, and The inter-core crosstalk distance dependence is calculated by substituting the Rayleigh scattering coefficient, the backscattered light capture rate, and the loss coefficient into the power coupling equation of number C1. A fiber optic testing device characterized by the following features. [Math C1] However, α is the loss coefficient, α ｓ is the Rayleigh scattering coefficient, B is the backscattered light capture rate, h is the power coupling coefficient, and L is the fiber length of the uncoupled multicore fiber.

3. A light pulse is input from one end of an uncoupled multicore fiber to one core, and the first light intensity of the backscattered light output from the one core is measured at the one end. A light pulse is input from one end of the uncoupled multicore fiber to one of the two cores, including the first core, and the second light intensity of the backscattered light output from the other of the two cores at the one end is measured, and The dependence of the inter-core crosstalk distance between the two cores of the uncoupled multicore fiber on the inter-core crosstalk distance when bidirectional transmission is performed with different optical transmission directions between the two cores is calculated from the first optical intensity and the second optical intensity. A method for testing optical fibers, In the calculation of the inter-core crosstalk distance dependence, The light intensity of the light pulse that has passed through the first core of the uncoupled multicore fiber is the signal light intensity P. ｓｉｇｎａｌ This is calculated from the first light intensity. The Rayleigh scattering coefficient α of the unbonded multi-core fiber obtained by any method ｓ , the backscattered light capture rate B of the unbonded multi-core fiber obtained by any method, and the second light intensity P in the interval Δz at any point ｋ is defined as, and the leakage light intensity P is calculated using Equation (1) ｂｓ and The inter-core crosstalk distance dependence XT is given by equation 2. ｂ Calculating A fiber optic testing method characterized by the following. [Math 1] [Math 2]

4. A light pulse is input from one end of an uncoupled multicore fiber to one core, and the first light intensity of the backscattered light output from the one core is measured at the one end. A light pulse is input from one end of the uncoupled multicore fiber to one of the two cores, including the first core, and the second light intensity of the backscattered light output from the other of the two cores at the one end is measured, and The dependence of the inter-core crosstalk distance between the two cores of the uncoupled multicore fiber on the inter-core crosstalk distance when bidirectional transmission is performed with different optical transmission directions between the two cores is calculated from the first optical intensity and the second optical intensity. A method for testing optical fibers, In the calculation of the inter-core crosstalk distance dependence, To calculate the crosstalk between the two cores of the uncoupled multicore fiber when unidirectional transmission is performed in which the direction of optical transmission is the same between the two cores, from the first optical intensity and the second optical intensity. Calculating the power coupling coefficient from the aforementioned crosstalk, The loss coefficient is calculated from the light intensity of the light pulse incident on the first core from one end of the uncoupled multicore fiber and the first light intensity, and The inter-core crosstalk distance dependence is calculated by substituting the Rayleigh scattering coefficient, the backscattered light capture rate, and the loss coefficient into the power coupling equation of number C1. A fiber optic testing method characterized by the following. [Math C1] However, α is the loss coefficient, α ｓ is the Rayleigh scattering coefficient, B is the backscattered light capture rate, h is the power coupling coefficient, and L is the fiber length of the uncoupled multicore fiber.

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