Method for measuring crosstalk between spatial channels and device for measuring crosstalk between spatial channels

Abstract

The spatial inter-channel crosstalk measurement method includes a first step to a fifth step. In the first step, a light reflection suppression unit for suppressing the reflection of the test light is formed or provided on the second incident surface of the space division multiplexing optical fiber. In the second step, the test light is incident on the first spatial channel at the first incident surface. In a third step, at least a portion of the test light is subjected to backward Rayleigh scattering in the space division multiplexing fiber. In a fourth step, a first optical power, which is the power of the light emitted from the first spatial channel at the first entrance / exit surface, of the at least part of the light, and a second optical power, which is the power of the light emitted from the second spatial channel at the first entrance / exit surface, of the at least part of the light, are detected. In a fifth step, the magnitude of crosstalk between the first spatial channel and the second spatial channel is calculated on the basis of the first optical power and the second optical power.

Description

Technical Field

[0001] The present disclosure relates to a method and apparatus for measuring spatial crosstalk between channels. This application claims priority based on Japanese application No. 2023-021827, filed on February 15, 2023, and incorporates herein the entire contents of the Japanese application. Background Art

[0002] Patent Document 1 and Non-Patent Document 1 disclose methods for measuring inter-core crosstalk in multi-core optical fibers. In these methods, inter-core crosstalk is measured by injecting measurement light into a core at one end of the multi-core optical fiber and detecting the power of the measurement light emitted from that core and other cores at the other end of the multi-core optical fiber.

[0003] Non-Patent Document 2 discloses a method for measuring inter-core crosstalk in multi-core optical fibers using an OTDR (Optical Time Domain Reflectometer). This method measures inter-core crosstalk by injecting pulsed light into a core at one end of the multi-core optical fiber and detecting the temporal variation in the power of Rayleigh backscattered light emitted from that core and other cores at that end.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: International Publication No. 2012 / 115162

[0007] Non-patent literature

[0008] Non-patent document 1: Tetsuya Hayashi et al., "Characterization of Crosstalk in Ultra-Low-Crosstalk Multi-Core Fiber", Journal of Lightwave Technology, Vol.30, No.4, (2012)

[0009] Non-Patent Document 2: Masataka Nakazawa et al., “Nondestructive measurement of mode couplings along a multi-core fiber using a synchronous multi-channel OTDR”, Optics Express, Vol. 20, No. 11, (2012) Summary of the Invention

[0010] One aspect of the present disclosure relates to a method for measuring crosstalk between spatial channels, which includes steps 1 to 5. In the first step, a light reflection suppression portion for suppressing the reflection of the test light is formed or provided on the second entrance and exit surface of the space-division multiplexing optical fiber. In the second step, the test light is incident on the first spatial channel at the first entrance and exit surface. In the third step, at least a portion of the test light is subjected to backward Rayleigh scattering in the space-division multiplexing optical fiber. In the fourth step, a first optical power and a second optical power are detected, wherein the first optical power is the power of the light emitted from the first spatial channel at the first entrance and exit surface in at least a portion of the light, and the second optical power is the power of the light emitted from the second spatial channel at the first entrance and exit surface in at least a portion of the light. In the fifth step, based on the first optical power and the second optical power, the magnitude of the crosstalk between the first spatial channel and the second spatial channel is calculated. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 This is a diagram showing the configuration of a measuring device according to one embodiment of the present disclosure.

[0012] Figure 2 A diagram showing a cross section perpendicular to the central axis of a multi-core optical fiber.

[0013] Figure 3 It is a diagram showing the structure of a three-port optical coupler (optical circulator).

[0014] Figure 4 This is a diagram showing the structure of a modified example of the measuring device.

[0015] Figure 5 It is a diagram showing the structure of another modified example of the measuring device.

[0016] Figure 6 3 is a cross-sectional view showing an example of a light reflection suppressing portion formed or provided on the second incident and exit surface.

[0017] Figure 7 3 is a cross-sectional view showing an example of a light reflection suppressing portion formed or provided on the second incident and exit surface.

[0018] Figure 8 Graph showing the relationship between the crosstalk measurement error caused by the reflected light at the second incident and exit surface and the angle of the second incident and exit surface.

[0019] Figure 9 Graph showing the relationship between the crosstalk measurement error caused by the reflected light at the second incident and exit surface and the angle of the second incident and exit surface.

[0020] Figure 10 3 is a cross-sectional view showing an example of a light reflection suppressing portion formed or provided on the second incident and exit surface.

[0021] Figure 11 3 is a cross-sectional view showing an example of a light reflection suppressing portion formed or provided on the second incident and exit surface.

[0022] Figure 12 3 is a cross-sectional view showing an example of a light reflection suppressing portion formed or provided on the second incident and exit surface.

[0023] Figure 13 3 is a cross-sectional view showing an example of a light reflection suppressing portion formed or provided on the second incident and exit surface.

[0024] Figure 14 3 is a cross-sectional view showing an example of a light reflection suppressing portion formed or provided on the second incident and exit surface.

[0025] Figure 15 3 is a cross-sectional view showing an example of a light reflection suppressing portion formed or provided on the second incident and exit surface.

[0026] Figure 16 3 is a cross-sectional view showing an example of a light reflection suppressing portion formed or provided on the second incident and exit surface.

[0027] Figure 17 : is a graph schematically showing the time waveform of the optical power of continuous light.

[0028] Figure 18 : is a graph schematically showing the time waveform of the optical power of the chopped light.

[0029] Figure 19 3 is a flowchart illustrating a method for measuring spatial inter-channel crosstalk according to this embodiment.

[0030] Figure 20 This is a flowchart showing a modified example of the method for measuring spatial inter-channel crosstalk.

[0031] Figure 21 This is a flowchart showing another modified example of the method for measuring spatial inter-channel crosstalk.

[0032] Figure 22 This is a flowchart showing yet another modified example of the method for measuring spatial inter-channel crosstalk.

[0033] Figure 23 This is a flowchart showing yet another modified example of the method for measuring spatial inter-channel crosstalk.

[0034] Figure 24 It is a diagram schematically showing the configuration of a measuring device according to a reference example. DETAILED DESCRIPTION

[0035] [Technical Problems to be Solved by the Present Disclosure]

[0036] For example, in conventional crosstalk measurements of space-division multiplexing optical fibers, such as multi-core optical fibers, as described in Patent Document 1 and Non-Patent Document 1, test light is injected into a certain spatial channel (e.g., a core) at the first end of the space-division multiplexing optical fiber, and the power of the test light emitted from this spatial channel and other spatial channels is detected at the second end of the multi-core optical fiber. However, it is sometimes difficult to connect a light source that outputs the test light to the first end of the space-division multiplexing optical fiber and a light detector to the second end of the space-division multiplexing optical fiber.

[0037] In contrast, using an OTDR measurement method utilizing backscattered Rayleigh light, as described in Non-Patent Document 2, allows for the injection and emission of test light only at the first end of a spatially divided multiplexed optical fiber. However, the power of backscattered Rayleigh light is very low. Therefore, when crosstalk is low, if pulsed light is used to perform positional analysis of the backscattered Rayleigh light component at each length position of the optical fiber, as in the OTDR measurement method, the level of the detection signal becomes extremely weak. Consequently, the noise level increases relative to the detection signal, reducing the accuracy of crosstalk measurement.

[0038] [Effects of the Present Disclosure]

[0039] According to the present disclosure, a spatial crosstalk measurement method and a spatial crosstalk measurement device can be provided that can connect a light source and a light detector that output test light to the first end of a spatial division multiplexing optical fiber and measure the magnitude of spatial crosstalk with high accuracy.

[0040] [Description of Embodiments of the Present Disclosure]

[0041] First, the contents of the embodiments of the present disclosure will be listed and described.

[0042] [1] One aspect of the present disclosure relates to a method for measuring crosstalk between spatial channels, including steps 1 to 5. In the first step, a light reflection suppression portion for suppressing the reflection of the test light is formed or provided on the second incident surface of the space-division multiplexing optical fiber. In the second step, the test light is incident on the first spatial channel at the first incident surface. In the third step, at least a portion of the test light is subjected to backward Rayleigh scattering in the space-division multiplexing optical fiber. In the fourth step, a first optical power and a second optical power are detected, wherein the first optical power is the power of the light emitted from the first spatial channel at the first incident surface in at least a portion of the light, and the second optical power is the power of the light emitted from the second spatial channel at the first incident surface in at least a portion of the light. In the fifth step, the magnitude of the crosstalk between the first spatial channel and the second spatial channel is calculated based on the first optical power and the second optical power.

[0043] In the measurement method of the above-mentioned [1], it is sufficient to connect the light source and the light detector that output the test light to the first incident surface of the space-division multiplexing optical fiber, and there is no need to connect either of them to the second incident surface. Therefore, even when it is difficult to approach both incident surfaces of the space-division multiplexing optical fiber at the same time, the light source and the light detector can be easily connected. Furthermore, by halving the number of connections during measurement, the crosstalk measurement of the space-division multiplexing optical fiber can be efficiently performed. In addition, in the measurement method of the above-mentioned [1], for example, the total amount of backscattered Rayleigh light is detected instead of positionally decomposing the backscattered Rayleigh light components, thereby increasing the level of the detection signal and improving the measurement accuracy of the crosstalk. In this case, if the light reflection suppression unit is not provided, part of the test light is reflected at the second incident surface, and the reflected test light may be mixed with the backscattered Rayleigh light. Since the power of the backscattered Rayleigh light is very small, in such a case, it may be impossible to improve the accuracy of the crosstalk measurement. In the measurement method described in [1], a light reflection suppression portion that suppresses reflection of the test light is formed or provided on the second incident surface. This reduces the amount of test light reflected at the second incident surface, thereby reducing the amount of reflected light mixed with the backscattered Rayleigh light. Consequently, crosstalk between channels can be measured with high accuracy.

[0044] [2] In the measurement method of [1] above, the test light may be continuous light or chopped light. The first optical power may be the sum of the optical power components emitted from the first spatial channel, of the return optical power components of the test light including backscattered light at each position in the longitudinal direction of the spatial division multiplexing optical fiber. The second optical power may be the sum of the optical power components emitted from the second spatial channel, of the return optical power components of the test light including backscattered light at each position in the longitudinal direction of the spatial division multiplexing optical fiber.

[0045] [3] In the fifth step of the measurement method of [1] or [2] above, the first optical power may be set to PW1, the second optical power may be set to PW2, and the average value of the transmission loss coefficients of the first spatial channel and the second spatial channel may be set to α (km -1 ), when the length of the space-division multiplexing optical fiber is set to L (km), the size of the crosstalk XT is calculated using the following mathematical formula (A).

[0046]

[0047] For example, by using such a calculation formula, the magnitude of the crosstalk can be calculated based on only the total power of the Rayleigh backscattered light from the first incident surface to the second incident surface.

[0048] [4] In the measurement methods of [1] to [3] above, it is also possible to: dB Defined as αdB (dB / km)=(10 / ln10)α, α dB L (dB) is 0.01 dB or more. This allows Rayleigh backscattered light having a power level sufficient for measurement to be returned to the first incident and exit surface.

[0049] [5] In the measurement methods of [1] to [4] above, the optical reflection suppressing unit may suppress reflection of the test light at the second incident surface of the space-division multiplexing optical fiber. Alternatively, when the wavelength of the test light is λ (μm), the average value of the refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is n, 0.5 times the average value of the mode field diameter of the first spatial channel and the second spatial channel is w (μm), and the return loss of the reflection at the second incident surface is RL (dB), RL satisfies the following mathematical formula (B).

[0050]

[0051] This makes it possible to reduce the measurement error caused by the reflection of the test light at the second incident and exit surface to 1 dB or less.

[0052] [6] In the measurement methods of [1] to [4] above, the light reflection suppressing unit may suppress reflection of the test light at the second incident and exit surface of the space-division multiplexing optical fiber. Alternatively, when the wavelength of the test light is λ (μm), the average value of the refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is n, and 0.5 times the average value of the mode field diameter of the first spatial channel and the second spatial channel is w (μm), the angle θ (degrees) formed by a plane connecting the centers of the first spatial channel and the second spatial channel and a plane orthogonal to the central axis of the space-division multiplexing optical fiber satisfies the following mathematical formula (C).

[0053]

[0054] This makes it possible to reduce the measurement error caused by the reflection of the test light at the second incident and exit surface to 1 dB or less.

[0055] [7] In the measurement methods of [1] to [6] above, the first step may include forming the light reflection suppression portion by cutting the space-division multiplexing optical fiber to form the second light-input and light-output surfaces. This makes it possible to easily form the light reflection suppression portion on the second light-input and light-output surfaces.

[0056] [8] In the measurement methods of [1] to [6] above, the first step may include a step of forming a light reflection suppression portion by polishing the second incident and output surfaces of the space-division multiplexing optical fiber. This facilitates forming the light reflection suppression portion on the second incident and output surfaces.

[0057] [9] In the measurement methods of [1] to [5] above, the first step may include providing a light reflection suppressing portion by bringing a material having a refractive index matching that of the space-division multiplexing optical fiber into contact with the second incident and output surfaces of the space-division multiplexing optical fiber. This facilitates providing the light reflection suppressing portion on the second incident and output surfaces.

[0058]

[10] In the measurement method of [9] above, when the average refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is set to n and the refractive index of the above-mentioned substance is set to n0, the relative refractive index difference Δ0 defined as Δ0 = |n-n0| / n0 and α dB The relationship of L (dB) satisfies the following mathematical formula (D).

[0059] Δ0≤1.14×10 -2 (α dB L) 0.473 …(D)

[0060] This makes it possible to reduce the crosstalk measurement error to 1 dB or less.

[0061]

[11] In the measurement methods of [1] to [4] above, the light reflection suppression unit may include another optical fiber having a cladding mainly composed of the same material as the cladding of the space-division multiplexing optical fiber. Alternatively, the first step includes the step of fusing the end face of the other optical fiber to the second input / output surface of the space-division multiplexing optical fiber. Alternatively, the other optical fiber after fusion splicing does not have both a spatial channel that matches the first spatial channel of the space-division multiplexing optical fiber and a spatial channel that matches the second spatial channel. Thus, the light reflection suppression unit can be easily provided on the second input / output surface.

[0062]

[12] In the measurement methods of [1] to

[11] above, the space-division multiplexing optical fiber may be a multi-core optical fiber or a multimode optical fiber.

[0063]

[13] In the measurement methods of [1] to

[12] above, the first spatial channel and the second spatial channel may be the first fiber core and the second fiber core, respectively, or the first mode and the second mode, respectively.

[0064]

[14] An apparatus for measuring crosstalk between spatial channels according to one aspect of the present disclosure is an apparatus for measuring crosstalk between spatial channels of a space-division multiplexing optical fiber, the space-division multiplexing optical fiber having a first entrance and exit surface and a second entrance and exit surface, and having N (N is an integer greater than or equal to 2) spatial channels. The measuring apparatus comprises a light source unit, a light reflection suppression unit, a light detection unit, and a calculation unit. The light source unit causes test light to be incident on each of the N spatial channels at the first entrance and exit surface. The light reflection suppression unit is formed or provided on the second entrance and exit surface to suppress reflection of the test light. The light detection unit detects a first optical power and a second optical power, the first optical power being the power of light emitted from the first spatial channel into which the test light is incident in at least a portion of the test light that undergoes backward Rayleigh scattering in the space-division multiplexing optical fiber, and the second optical power being the power of light emitted from a second spatial channel different from the first spatial channel in at least a portion of the light. The calculation unit calculates the magnitude of crosstalk between the first spatial channel and the second spatial channel based on the first optical power and the second optical power.

[0065] In the measuring device of the above-mentioned

[14] , it is sufficient to connect the light source unit and the light detection unit to the first incident surface of the space-division multiplexing optical fiber, and there is no need to connect either of them to the second incident surface. Therefore, even in a case where it is difficult to approach the two incident surfaces of the space-division multiplexing optical fiber at the same time, the light source and the light detector can be easily connected. Furthermore, by halving the number of connections during measurement, the crosstalk measurement of the space-division multiplexing optical fiber can be efficiently performed. In addition, in the measuring device of the above-mentioned

[14] , a light reflection suppression unit that suppresses the reflection of the test light is formed or provided on the second incident surface. As a result, it is possible to reduce the reflection of a part of the test light on the second incident surface, thereby reducing the mixing of the reflected light into the backward Rayleigh scattered light. Therefore, it is possible to measure the crosstalk between channels with high precision.

[0066]

[15] In the measurement device of

[14] above, the test light may be continuous light or chopped light. Alternatively, the first optical power may be the sum of the optical power components emitted from the first spatial channel, of the return optical power components of the test light including backscattered light at each position in the longitudinal direction of the spatial division multiplexing optical fiber. Alternatively, the second optical power may be the sum of the optical power components emitted from the second spatial channel, of the return optical power components of the test light including backscattered light at each position in the longitudinal direction of the spatial division multiplexing optical fiber.

[0067]

[16] In the calculation unit of the measuring device of

[14] or

[15] , it is also possible that, when the first optical power is set to PW1, the second optical power is set to PW2, and the average value of the transmission loss coefficients of the first spatial channel and the second spatial channel is set to α (km -1 ), when the length of the space-division multiplexing optical fiber is set to L (km), the size of the crosstalk XT is calculated using the following mathematical formula (E).

[0068]

[0069] For example, by using such a calculation formula, the magnitude of the crosstalk can be calculated based on only the total power of the Rayleigh backscattered light from the first incident surface to the second incident surface.

[0070]

[17] In the measuring apparatus of

[14] to

[16] above, it is also possible that, when the loss coefficients α of the first spatial channel and the second spatial channel are dB Defined as α dB (dB / km)=(10 / ln10)α, α dB L (dB) is 0.01 dB or more. This allows Rayleigh backscattered light having a power level sufficient for measurement to be returned to the first incident and exit surface.

[0071]

[18] In the measuring devices of

[14] to

[17] above, the light reflection suppression unit may suppress the reflection of the test light at the second incident and output surface of the space-division multiplexing optical fiber, and when the wavelength of the test light is set to λ (μm), the average value of the refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is set to n, 0.5 times the average value of the mode field diameter of the first spatial channel and the second spatial channel is set to w (μm), and the return loss of the reflection at the second incident and output surface is set to RL (dB), RL satisfies the following mathematical formula (F).

[0072]

[0073] This makes it possible to reduce the measurement error caused by the reflection of the test light at the second incident and exit surface to 1 dB or less.

[0074]

[19] In the measurement devices of

[14] and

[15] above, the light reflection suppressing unit may suppress reflection of the test light at the second incident and exit surface of the space-division multiplexing optical fiber. Alternatively, when the wavelength of the test light is λ (μm), the average value of the refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is n, and 0.5 times the average value of the mode field diameter of the first spatial channel and the second spatial channel is w (μm), the angle θ (degrees) formed by a plane connecting the centers of the first spatial channel and the second spatial channel and a plane orthogonal to the central axis of the space-division multiplexing optical fiber satisfies the following mathematical formula (G).

[0075]

[0076] This makes it possible to reduce the measurement error caused by the reflection of the test light at the second incident and exit surface to 1 dB or less.

[0077]

[20] In the measurement devices of

[14] to

[18] above, the light reflection suppressing unit may include a substance in contact with the second incident and output surface of the space-division multiplexing optical fiber, and the substance may have a refractive index that matches the refractive index of the space-division multiplexing optical fiber. This makes it easy to provide the light reflection suppressing unit on the second incident and output surface.

[0078]

[21] In the measuring device of

[20] , when the average refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is set to n and the refractive index of the above-mentioned substance is set to n0, the relative refractive index difference Δ0 defined as Δ0 = |n-n0| / n0 and α dB The relationship of L (dB) satisfies the following mathematical formula (H).

[0079] Δ0≤1.14×10 -2 (α dB L) 0.473 …(H)

[0080] This makes it possible to reduce the crosstalk measurement error to 1 dB or less.

[0081]

[22] In the measurement devices of

[14] to

[17] above, the light reflection suppressing unit may include another optical fiber having a cladding primarily composed of the same material as the cladding of the space-division multiplexing optical fiber, and the end face of the other optical fiber may be fused to the second input / output surface of the space-division multiplexing optical fiber. Alternatively, the other optical fiber may not have both a spatial channel that matches the first spatial channel of the space-division multiplexing optical fiber and a spatial channel that matches the second spatial channel. This makes it easy to provide the light reflection suppressing unit on the second input / output surface.

[0082]

[23] In the measuring devices of

[14] to

[22] above, the space-division multiplexing optical fiber may be a multi-core optical fiber or a multi-mode optical fiber.

[0083]

[24] In the measuring devices of

[14] to

[23] above, the first spatial channel and the second spatial channel may be the first fiber core and the second fiber core, respectively, or the first mode and the second mode, respectively.

[0084] [Details of the embodiments of the present disclosure]

[0085] Specific examples of the present embodiment will be described with reference to the accompanying drawings as needed. The present invention is not limited to these examples but is defined by the claims, which are intended to encompass all modifications within the meaning and scope equivalent to the claims. In the following description, identical elements are denoted by the same reference numerals when describing the drawings, and duplicate descriptions are omitted.

[0086] Figure 1This figure shows the structure of a measuring device 1A according to one embodiment of the present disclosure. The measuring device 1A is a device for measuring the crosstalk between spatial channels of a space division multiplexing (SDM) optical fiber. An SDM optical fiber is, for example, a multi-core fiber (MCF) or a multimode optical fiber. An SDM optical fiber has multiple spatial channels. For example, the multiple spatial channels are multiple cores. Alternatively, the multiple spatial channels can be, for example, multiple modes. The following description describes a device for measuring the crosstalk between cores of an MCF 10, which is an example of an SDM optical fiber.

[0087] First, the structure of the MCF 10 will be described. The MCF 10 includes a first incident and exit surface 10a and a second incident and exit surface 10b on the side opposite to the first incident and exit surface 10a. Figure 2 : is a diagram showing a cross section perpendicular to the central axis of MCF10. Figure 2 As shown, MCF10 has a glass fiber 11 and a coating resin 12 that covers the outer peripheral surface of the glass fiber 11. The glass fiber 11 has N (N is an integer greater than or equal to 2) cores as N spatial channels. In the example shown in the figure, the glass fiber 11 has four (i.e., N=4) cores 13a, 13b, 13c, and 13d. Furthermore, the glass fiber 11 has a cladding 14. Furthermore, the glass fiber 11 may also have a mark 15. In the example shown in the figure, in a cross section of the MCF10 that is perpendicular to the central axis, the cores 13a, 13b, 13c, and 13d are arranged at equal intervals on concentric circles centered on the central axis of the MCF10. The cladding 14 is a common cladding that surrounds the cores 13a, 13b, 13c, and 13d and the mark 15. The mark 15 has a refractive index different from that of the cladding 14.

[0088] Here, α(km -1 ) is the average value of the transmission loss coefficients of the cores 13a, 13b, 13c, and 13d, and L (km) is the length of the MCF 10. Furthermore, the loss coefficients α of the cores 13a, 13b, 13c, and 13d are dB Defined as α dB (dB / km) = (10 / ln10)α. At this time, the transmission loss in MCF10 is α dB L (dB) is, for example, 0.01 dB or more.

[0089] Refer again Figure 1 The measuring device 1A includes a light source unit 20A, a light detection unit 30A, an optical coupler unit 40 , a fan-in / fan-out (FIFO) 50 , and a calculation unit 60 .

[0090] The light source unit 20A emits test light that enters each of the fiber cores 13a, 13b, 13c, and 13d at the first incident / exit surface 10a. The light source unit 20A of this embodiment includes a single light source 21 and a first optical switch 22. The first optical switch 22 has at least one input port 22a and the same number of output ports 22b, 22c, 22d, and 22e as the number of fiber cores 13a, 13b, 13c, and 13d. The first optical switch 22 selectively optically couples the input port 22a to any of the output ports 22b, 22c, 22d, and 22e. The input port 22a is optically coupled to the light source 21.

[0091] The light detection unit 30A detects light emitted from each of the fiber cores 13a, 13b, 13c, and 13d at the first incident / exit surface 10a. The light detection unit 30A of this embodiment includes a single optical receiver (power meter) 31 and a second optical switch 32. The second optical switch 32 has at least one output port 32a and the same number of input ports 32b, 32c, 32d, and 32e as the number of fiber cores 13a, 13b, 13c, and 13d. The second optical switch 32 selectively optically couples the output port 32a to any one of the input ports 32b, 32c, 32d, and 32e. The output port 32a is optically coupled to the light receiver 31.

[0092] The optical coupler unit 40 includes three-port optical couplers 41, 42, 43, and 44, which are the same number as the cores 13a, 13b, 13c, and 13d. The three-port optical couplers 41, 42, 43, and 44 are, for example, optical circulators. Figure 3 4 is a diagram showing the structure of a three-port optical coupler (optical circulator) 41. The structures of the three-port optical couplers 42, 43, and 44 are the same as that of the three-port optical coupler 41. Figure 3 As shown, a three-port optical coupler 41 has a first port P1, a second port P2, and a third port P3. The three-port optical coupler 41 outputs light L1 input to the first port P1 from the second port P2 with low loss, and outputs light L2 input to the second port P2 from the third port P3 with low loss. Light L1 input to the first port P1 is rarely output from the third port P3. Light L2 input to the second port P2 is rarely output from the first port P1. Light input to the third port P3 is rarely output from both the first port P1 and the second port P2.

[0093] The insertion loss from the first port P1 to the second port P2 is, for example, 1 dB or less. The insertion loss from the first port P1 to the third port P3 is, for example, 30 dB or more or 40 dB or more. The insertion loss from the second port P2 to the third port P3 is, for example, 1 dB or less. The insertion loss from the second port P2 to the first port P1 is, for example, 30 dB or more or 40 dB or more. The insertion loss from the third port P3 to the first port P1 and the second port P2 is, for example, 30 dB or more or 40 dB or more.

[0094] Refer again Figure 1 The first ports P1 of the three-port optical couplers 41, 42, 43, and 44 are optically coupled to the output ports 22b, 22c, 22d, and 22e of the first optical switch 22. Thus, the first optical switch 22 can selectively optically couple the light source 21 to the first port P1 of any of the three-port optical couplers 41, 42, 43, and 44. The third ports P3 of the three-port optical couplers 41, 42, 43, and 44 are optically coupled to the input ports 32b, 32c, 32d, and 32e of the second optical switch 32. Thus, the second optical switch 32 can selectively optically couple the optical receiver 31 to the third port P3 of any of the three-port optical couplers 41, 42, 43, and 44. In the above description, a three-port optical coupler 41, 42, 43, and 44 is illustrated as a three-port optical coupler. However, the three-port optical coupler is not limited to this. A 1×2 optical fiber coupler or a 2×2 optical fiber coupler with one port subjected to reflection suppression termination can also be used. By using a fused optical fiber coupler as the optical fiber coupler, reflections within the optical fiber coupler can be suppressed. The optical fiber coupler can also be a waveguide optical fiber coupler.

[0095] If the three-port optical coupler is a 1×2 fiber coupler or a 2×2 fiber coupler in which one port has been treated with reflection-suppressing termination, the three-port optical coupler has a first port P1, a second port P2, and a third port P3. The three-port optical coupler outputs light L1 input to the first port P1 from the second port P2 with low loss, and outputs light L2 input to the second port P2 from the third port P3 with low loss. Light L1 input to the first port P1 is barely output from the third port P3. Light L2 input to the second port P2 is barely output from the first port P1, and light input to the third port P3 is also barely output from the second port P2 with low loss, but this has no significant effect on the measurement. Light input to the third port P3 is barely output from the first port P1. However, in a 1×2 fiber coupler or a 2×2 fiber coupler with one port treated with reflection suppression termination, the insertion loss between ports P1 and P2, where insertion loss is low, and the insertion loss between ports P2 and P3, where insertion loss is low, is higher than the insertion loss in an optical circulator. This is because, for example, when a 1×2 fiber coupler or a 2×2 fiber coupler is an optical power splitter with a 50:50 splitting ratio, a loss of approximately 3 dB occurs as a fundamental loss.

[0096] The insertion loss from the first port P1 to the second port P2 is, for example, 4 dB or less. The insertion loss from the first port P1 to the third port P3 is, for example, 40 dB or more or 50 dB or more. The insertion loss from the second port P2 to the third port P3 is, for example, 4 dB or less. The insertion loss from the third port P3 to the first port P1 is, for example, 40 dB or more or 50 dB or more.

[0097] FIFO 50 is an optical component that optically couples the cores 13a, 13b, 13c, and 13d at the first incident and output surface 10a of MCF 10 to the second ports P2 of the three-port optical couplers 41, 42, 43, and 44, respectively. FIFO 50 includes an input / output port 50a connected to MCF 10, and input / output ports 50b, 50c, 50d, and 50e connected to the second ports P2 of the three-port optical couplers 41, 42, 43, and 44, respectively. The input / output ports 50a of FIFO 50 are formed, for example, by etching and tapering the front ends of the same number of single-core optical fibers as the cores 13a, 13b, 13c, and 13d of MCF 10, and then bundling these front ends into a bundle. In this bundle, the spacing between the cores of the single-core optical fibers is the same as the spacing between the cores 13a, 13b, 13c, and 13d of MCF 10. The bundle at the front end contacts the first light-input and output surfaces 10a of the MCF 10, thereby optically coupling the first ends of the single-core optical fibers to the respective cores 13a, 13b, 13c, and 13d. The second ends of the single-core optical fibers form input and output ports 50b, 50c, 50d, and 50e, respectively, and are optically coupled to the second ports P2 of the three-port optical couplers 41, 42, 43, and 44.

[0098] The calculation unit 60 calculates the magnitude of inter-core crosstalk based on the detection results of the light detection unit 30A. The calculation unit 60 is composed of, for example, a computer including a CPU, memory, and storage. The storage device stores software for calculating the magnitude of inter-core crosstalk. The CPU reads and executes the software to calculate the magnitude of inter-core crosstalk. The calculation unit 60 is electrically (or communicatively) connected to the light receiver 31.

[0099] Figure 4 This is a diagram showing the structure of a measuring device 1B as a modified example of the measuring device 1A. The measuring device 1B includes a light detection unit 30B instead of Figure 1 The optical detection unit 30A is shown. The optical detection unit 30B includes the same number of optical receivers (power meters) 33, 34, 35, and 36 as the cores 13a, 13b, 13c, and 13d of the MCF 10. The optical receivers 33, 34, 35, and 36 are optically coupled to the third port P3 of each of the three-port optical couplers 41, 42, 43, and 44. The computing unit 60 is electrically (or communicatively) connected to the optical receivers 33, 34, 35, and 36.

[0100] Figure 5 This is a diagram showing the structure of a measuring device 1C as another modified example of the measuring device 1A. The measuring device 1C includes a light source unit 20B instead of Figure 1The light source unit 20A is shown. The light source unit 20B includes the same number of light sources 23, 24, 25, and 26 as the cores 13a, 13b, 13c, and 13d of the MCF 10. The light sources 23, 24, 25, and 26 are optically coupled to the first ports P1 of the three-port optical couplers 41, 42, 43, and 44, respectively. The measurement device may also include a light detection unit 30B instead of the light detection unit 30A, and a light source unit 20B instead of the light source unit 20A.

[0101] Here, an example of the configuration of the second incident surface 10b of the MCF 10 is described. A light reflection suppression portion is formed or provided on the second incident surface 10b. This light reflection suppression portion suppresses reflection of the test light propagating from the first incident surface 10a to the second incident surface 10b in each of the cores 13a, 13b, 13c, and 13d. Figure 6 、 Figure 7 and Figures 10 to 16 The figures are cross-sectional views showing an example of a light reflection suppression portion formed or provided on the second incident and output surface 10b, showing a cross section of the MCF 10 along the central axis AX of the glass fiber 11. In these figures, the coating resin 12 is omitted. The second incident and output surface 10b includes the end face of the glass fiber 11. Here, if the power of light incident on the light reflecting surface is Pi and the power of light reflected by the light reflecting surface is Pr, the reflectivity can be defined by Pr / Pi. In IEC-61300-3-6, the value obtained by multiplying the decibel value of the reflectivity (Pr / Pi) by -1 is defined as the return loss (Retern Loss). In addition, the method for measuring the return loss (Retern Loss) is illustrated, so the reflectivity of the light reflecting surface can also be measured by the method described in IEC-61300-3-6. The reflectivity of the light reflection suppression portion can be measured by treating the light reflection suppression portion as a light reflecting surface.

[0102] Assuming transmission loss α dB L is 0.01 dB or greater, the refractive index n is 1.3 to 2, the wavelength λ is 1.31 μm to 1.625 μm, and w is 2 μm to 8 μm. In this case, if the return loss is RL (dB) when the crosstalk measurement error caused by the reflected light at the second incident / exit surface 10 b is ε (dB), then RL can be approximated by the following mathematical formula (1).

[0103]

[0104] Therefore, when RL (dB) satisfies the following mathematical formula (2) or mathematical formula (3), the measurement error ε can be reduced to 1 dB or less.

[0105]

[0106] Furthermore, by making the angle θ (degrees) satisfy the following mathematical formula (4), the measurement error ε can be reduced to 0.5 dB or less.

[0107]

[0108] Furthermore, when the angle θ (degrees) satisfies the following mathematical formula (5), the measurement error ε can be reduced to 0.1 dB or less.

[0109]

[0110] exist Figure 6 In the example shown, the second incident surface 10b is tilted relative to an imaginary plane H1 perpendicular to the central axis AX of the MCF 10, thereby reflecting the test light and emitting it from the side of the glass fiber 11 to the outside, thereby suppressing reflection of the test light into the glass fiber 11. In this example, the second incident surface 10b is a flat surface obtained by cutting or polishing. The end faces 131 of the cores 13a, 13b, 13c, and 13d are aligned with the second incident surface 10b without any step difference. When an imaginary plane H2 parallel to the second incident surface 10b is defined, the centers of the end faces 131 of the cores 13a, 13b, 13c, and 13d contact the imaginary plane H2 at the second incident surface 10b. The imaginary plane H2 forms an angle θ with the imaginary plane H1 perpendicular to the central axis AX of the MCF 10. Angle θ is defined using the radian method. In one example, angle θ is greater than 4 degrees and less than 90 degrees.

[0111] exist Figure 7 In the example shown, the second incident surface 10b also reflects the test light and emits it to the outside from the side of the glass fiber 11, thereby suppressing the test light from being reflected into the glass fiber 11. However, the second incident surface 10b in this example is not flat, but a curved surface that is convex toward the outside, that is, toward the external medium. The second incident surface 10b is formed, for example, by grinding. The end faces 131 of the cores 13a, 13b, 13c, and 13d are surfaces that are aligned with the second incident surface 10b without a step difference. An imaginary plane H2a is defined as being connected to the center of the end faces 131 of the cores 13a, 13b, 13c, and 13d at the second incident surface 10b. At this time, the imaginary plane H2a forms an angle θ with the imaginary plane H1 that is orthogonal to the center axis AX of the MCF10. The range of the angle θ is the same as Figure 6 The morphology shown is the same.

[0112] Here, the wavelength of the test light is λ (μm), the average refractive index of the cores 13a, 13b, 13c, and 13d of the MCF 10 is n, and 0.5 times the average mode field diameter of the cores 13a, 13b, 13c, and 13d is w (μm). Figure 8It shows that n=1.444, λ=1.55(μm), w=5(μm 2 ) is a graph showing the relationship between the crosstalk measurement error caused by the reflected light at the second incident and exit surface 10b and the angle θ. Figure 8 In FIG, the horizontal axis represents the angle θ (degrees), and the vertical axis represents the measurement error (dB). dB L = 0.01dB, α dB L = 0.1dB, α dB L = 0.2dB, α dB L = 0.5dB, α dB L = 1dB, α dB L = 2dB, α dB L = 5dB and α dB Curves for each case of L=10 dB It should be noted that it is assumed that the second incident and exit surface 10 b is not in contact with liquid or solid, but is in contact with a medium such as gas or vacuum whose refractive index can be approximately 1.

[0113] Reference Figure 8 It can be seen that as the angle θ increases, the reflected light returning to the first incident surface 10a at the second incident surface 10b is gradually suppressed. When the angle θ is greater than a predetermined angle, the measurement error is sufficiently suppressed. dB The smaller L (dB) is, the larger the angle θ required to suppress the measurement error is.

[0114] Figure 9 It shows that n = 1.444, λ = 1.55 (μm), α dB · A graph showing the relationship between the crosstalk measurement error caused by the reflected light at the second incident and exit surface 10b and the angle θ when L=0.01(dB). Figure 9 In the figure, the horizontal axis represents the angle θ (degrees) and the vertical axis represents the measurement error (dB). 2 , w=0.1μm 2 , w=0.2μm 2 , w=0.5μm 2 , w=1μm 2 , w=2μm 2 , w=5μm 2 and w = 10 μm 2 The curves in each case. It should be noted that Figure 8 Likewise, it is assumed that the second incident and exit surface 10 b is not in contact with liquid or solid, but is in contact with a medium such as gas or vacuum whose refractive index can be approximately 1.

[0115] Reference Figure 9It can be seen that as the angle θ increases, the reflected light returning to the first incident surface 10a at the second incident surface 10b is gradually suppressed. 2 ) is smaller, the larger the angle θ required to suppress the measurement error.

[0116] The transmission loss α dB When L is greater than 0.01 dB, the refractive index n is greater than 1.3 and less than 3, the wavelength λ is greater than 1.31 μm and less than 1.625 μm, and w is greater than 2 μm and less than 8 μm, the angle θ (degrees) at which the crosstalk measurement error caused by the reflected light at the second incident and exit surface 10b is ε (dB) can be approximated by the following mathematical formula (6).

[0117]

[0118] Therefore, when the angle θ (degrees) satisfies the following mathematical formula (7) or mathematical formula (8), the measurement error ε can be reduced to 1 dB or less.

[0119]

[0120] Furthermore, by making the angle θ (degrees) satisfy the following mathematical formula (9), the measurement error ε can be reduced to 0.5 dB or less.

[0121]

[0122] Furthermore, by making the angle θ (degrees) satisfy the following mathematical formula (10), the measurement error ε can be reduced to 0.1 dB or less.

[0123]

[0124] exist Figure 10 In the example shown, an antireflection film 71 (refractive index matching film) is provided on the second incident and exit surface 10b. The antireflection film 71 is a solid film containing a material having a refractive index that matches that of the glass fiber 11 and is in contact with the second incident and exit surface 10b. The material constituting the antireflection film 71 is, for example, a dielectric. The antireflection film 71 constitutes a light reflection suppression portion that suppresses reflection of the test light into the glass fiber 11.

[0125] exist Figure 11In the example shown, an anti-reflection material 72 is provided on the second incident surface 10b. The anti-reflection material 72 contains a substance having a refractive index that matches the refractive index of the glass fiber 11 and is in contact with the second incident surface 10b. The anti-reflection material 72 is provided from the second incident surface 10b to the side of the glass fiber 11 and surrounds the second incident surface 10b. The substance constituting the anti-reflection material 72 is, for example, an alicyclic compound or an aliphatic compound. The anti-reflection material 72 can be a solid, a liquid, or even a gel. The anti-reflection material 72 constitutes a light reflection suppression portion that suppresses reflection of the test light into the glass fiber 11.

[0126] exist Figure 12 In the example shown, the second incident and exit surface 10b is immersed in liquid 74. Liquid 74 is contained in a container 73 with an upper opening, and the second incident and exit surface 10b is immersed in liquid 74 from above. Liquid 74 contains a substance having a refractive index that matches that of the glass fiber 11 and is in contact with the second incident and exit surface 10b. The substance constituting liquid 74 is, for example, an alicyclic compound or an aliphatic compound. Liquid 74 constitutes a light reflection suppressing portion that suppresses reflection of the test light into the glass fiber 11.

[0127] exist Figure 10 、 Figure 11 and Figure 12 In the example shown, the average refractive index of the cores 13a, 13b, 13c, and 13d of the MCF 10 is set to n, and the refractive index of the material having a refractive index matching that of the glass fiber 11 is set to n0. Furthermore, the relative refractive index difference Δ0 is defined as Δ0 = |n-n0| / n0. In this case, in order to minimize the measurement error ε (dB), it is best to reduce the relative refractive index difference Δ0. Furthermore, in this case, the transmission loss α dB The smaller L is, the more the relative refractive index difference Δ0 needs to be minimized. The closer the angle θ is to 0 degrees, the more the relative refractive index difference Δ0 needs to be minimized. The larger the refractive index n is, the more the relative refractive index difference Δ0 needs to be minimized. The shorter the wavelength λ is, the more the relative refractive index difference Δ0 needs to be minimized. The larger the w is, the more the relative refractive index difference Δ0 needs to be minimized.

[0128] The relative refractive index difference Δ0 and α dB The relationship of ·L (dB) satisfies the following mathematical formula (11), and the measurement error ε can be reduced to 1 dB or less.

[0129] Δ0≤1.14×10 -2 (α dB L) 0.473 …(11)

[0130] Furthermore, by the relative refractive index difference Δ0 and α dBThe relationship of ·L (dB) satisfies the following mathematical formula (12), and the measurement error ε can be reduced to 0.5 dB or less.

[0131] Δ0≤8.10×10 -2 (α dB ) 0.482 …(12)

[0132] Furthermore, by the relative refractive index difference Δ0 and α dB The relationship of ·L (dB) satisfies the following mathematical formula (13), and the measurement error ε can be reduced to 0.1 dB or less.

[0133] Δ0≤3.63×10 -3 (α dB ) 0.502 …(13)

[0134] exist Figure 13 In the example shown, the measuring device 1A, 1B, or 1C further includes an anti-reflection device 80 disposed on the second incident surface 10b of the MCF 10. The anti-reflection device 80 includes an MCF 81 different from the MCF 10 and an anti-reflection film 82 disposed on the first end face 81a of the MCF 81. The anti-reflection film 82 is, for example, a dielectric multilayer film. The second end face of the MCF 81 is connected to the second incident surface 10b. That is, the MCF 81 has the same number of cores as the cores 13a, 13b, 13c, and 13d of the MCF 10 (only two cores 83a and 83b are shown in the figure), and these cores are optically coupled to the cores 13a, 13b, 13c, and 13d of the MCF 10, respectively. The connection between the MCF 81 and the MCF 10 can be by fusion splicing or by using an optical connector. In this example, the anti-reflection film 82 constitutes a light reflection suppression portion that suppresses reflection of the test light into the glass fiber 11.

[0135] exist Figure 14 In the example shown, the measuring apparatus 1A, 1B, or 1C further includes an anti-reflection device 90 disposed on the second incident surface 10b of the MCF 10. The anti-reflection device 90 includes the same number of single-core fibers (SCFs) 92 and FIFOs 91 as the number of cores 13a, 13b, 13c, and 13d of the MCF 10. An anti-reflection film 93 is provided on the first end face 92a of each SCF 92, serving as a light reflection suppression portion for suppressing reflection of the test light into the SCF 92. The anti-reflection film 93 is, for example, a dielectric multilayer film. The second end face of each SCF 92 is optically coupled to the cores 13a, 13b, 13c, and 13d at the second incident surface 10b via the FIFO 91.

[0136] exist Figure 15In the example shown, the measuring device 1A, 1B, or 1C further includes another optical fiber 100, serving as a light reflection suppression unit, disposed on the second incident surface 10b of the MCF 10. The optical fiber 100 includes a cladding 104 primarily composed of the same material as the cladding 14 of the MCF 10, and a coating resin 102 coating the cladding 104. The first end face of the optical fiber 100 is fused to the second incident surface 10b of the MCF 10. The second end face of the optical fiber 100 is open. The optical fiber 100 has no core at all. In other words, the glass fiber of the optical fiber 100 consists solely of the cladding 104. Therefore, the optical fiber 100 has no spatial channels (cores) that match the cores 13a, 13b, 13c, or 13d of the MCF 10. The cores 13a, 13b, 13c, and 13d of the MCF 10 are in contact only with the cladding 104 of the optical fiber 100.

[0137] exist Figure 16 In the example shown, the measurement device 1A, 1B, or 1C further includes another optical fiber 110, which serves as a light reflection suppression unit and is disposed on the second incident surface 10b of the MCF 10. The optical fiber 110 includes a core 113, a cladding 114 covering the core 113 and primarily composed of the same material as the cladding 14 of the MCF 10, and a coating resin 112 covering the cladding 114. The first end face of the optical fiber 110 is fused to the second incident surface 10b of the MCF 10. The second end face of the optical fiber 110 is open. The core 113 is not matched to any of the cores 13a, 13b, 13c, or 13d of the MCF 10. In other words, the optical fiber 110 does not have any spatial channels that are matched to the cores 13a, 13b, 13c, or 13d of the MCF 10. The cores 13a, 13b, 13c, and 13d of the MCF 10 are only in contact with the cladding 114 of the optical fiber 110.

[0138] Refer again Figure 1 The test light output from the light source unit 20A may be either continuous light or chopped light. Figure 17 This graph schematically illustrates the time waveform of the optical power of continuous light. When the test light is continuous light, its optical power remains constant regardless of time. Furthermore, when the test light is continuous light, a typical optical power meter can be used as the optical receivers 31, 33, 34, and 35. Figure 18 This is a graph schematically showing the temporal waveform of the optical power of chopped light. When the test light is chopped light, the temporal waveform of its optical power is a rectangular wave. The duty cycle (the proportion of the total time during which the optical power reaches its peak) is, for example, 0.5 or 0.25. When the test light is chopped light, using an optical power meter compatible with chopped detection (synchronous detection, phase detection) as optical receivers 31, 33, 34, and 35 allows for detection with reduced measurement noise caused by ambient light, etc., thereby improving detection sensitivity.

[0139] Next, the operation of the measurement apparatus 1A, 1B, or 1C according to the present embodiment will be described, and the spatial inter-channel crosstalk measurement method according to the present embodiment using the measurement apparatus 1A, 1B, or 1C will be described. Figure 19 3 is a flowchart illustrating a method for measuring spatial inter-channel crosstalk according to this embodiment.

[0140] First, as step ST1, a light reflection suppression portion is formed or provided on the second incident and exit surfaces 10b. An example of a light reflection suppression portion is shown in FIG. Figures 6 to 16 That is, step ST1 may also include providing an anti-reflection film 71 (see Figure 10 ) or anti-reflection material 72 (refer to Figure 11 ), or immersing the second incident and exit surface 10b in the liquid 74 (refer to Figure 12 ) step ST11.

[0141] Next, as step ST2, the test light is incident on the first spatial channel at the first incident surface 10a. The test light is incident on any one of the cores 13a, 13b, 13c, and 13d (the first spatial channel, here, the core 13a is used as an example) from the light source unit 20A or 20B. At this time, the light source unit 20A (refer to Figure 1 、 Figure 4 ), the test light is made incident on the core 13a by switching the first optical switch 22. The first optical switch 22 can be switched according to a control signal from the operation unit 60 or manually. Figure 5 ), the test light is made incident from the light source 23 corresponding to the core 13a among the light sources 23, 24, 25, and 26.

[0142] Next, as step ST3 , at least a portion of the test light is backscattered by Rayleigh scattering inside the MCF 10 .

[0143] Then, part of the light that undergoes backscattering Rayleigh inside the MCF 10 is emitted from the core 13a, while the other part is emitted from a core different from the core 13a (the second spatial channel or the second core, here, each of the cores 13b, 13c, and 13d) due to inter-core crosstalk. In step ST4, the first optical power is detected, and the second optical power is detected. The first optical power is the power of the light emitted from the core 13a, and the second optical power is the power of the light emitted from each of the cores 13b, 13c, and 13d. At this time, in the light detection unit 30A (refer to Figure 1 、 Figure 5), the light from the cores 13a, 13b, 13c, and 13d is sequentially incident on the optical receiver 31 by switching the second optical switch 32. The second optical switch 32 can be switched according to a control signal from the operation unit 60 or manually. Figure 4 ), the light from the cores 13a, 13b, 13c, and 13d is respectively incident on the optical receivers 33, 34, 35, and 36 corresponding to the cores 13a, 13b, 13c, and 13d.

[0144] The first optical power is the sum of the optical power components emitted from core 13a (first spatial channel) of the return optical power components of the test light, including backscattered light at each position in the longitudinal direction of MCF 10. The second optical power is the sum of the optical power components emitted from each of cores 13b, 13c, and 13d (second spatial channel) of the return optical power components of the test light, including backscattered light at each position in the longitudinal direction of MCF 10.

[0145] Next, as step ST5, the magnitude of the crosstalk between the core 13a and the cores 13b, 13c, and 13d is calculated in the calculation unit 60 based on the first optical power and the second optical power. At this time, the magnitude of the crosstalk XT between the core 13a and the cores 13b, 13c, and 13d is calculated based on the first optical power PW1 and the second optical power PW2, for example, according to the following mathematical formula (14).

[0146]

[0147] Among them, α(km -1 ) is the average value of the transmission loss coefficients of the cores 13a, 13b, 13c, and 13d. In addition, L (km) is the length of the MCF 10.

[0148] When measuring the first optical power PW1, the power of the light emitted from the fiber core 13a at the first incident surface 10a is measured. In this case, the light emitted from the fiber core 13a includes almost no light reflected from the second incident surface 10b and mainly includes light that is backscattered by Rayleigh within the MCF 10. Similarly, when measuring the second optical power PW2, the power of the light emitted from each of the fiber cores 13b, 13c, and 13d at the first incident surface 10a is measured. In this case, the light emitted from each of the fiber cores 13b, 13c, and 13d includes almost no light reflected from the second incident surface 10b and mainly includes light that is backscattered by Rayleigh within the MCF 10.

[0149] The first cores (first spatial channels) into which the test light is incident are set as cores 13a, 13b, 13c, and 13d, and the above-mentioned steps ST2 to ST5 are repeated in sequence.

[0150] Figure 20 1 is a flowchart showing a modified example of the method for measuring spatial crosstalk between channels. In this modified example, the step ST1 of forming or setting the light reflection suppression portion on the second incident and exit surface 10b includes a step ST12 instead of the above-mentioned step ST11. In step ST12, as shown in FIG. Figure 13 As shown, the second end surface of the MCF 81 in which the anti-reflection film 82 as the light reflection suppressing portion is provided on the first end surface 81 a is connected to the second incident and exit surface 10 b .

[0151] Figure 21 FIG. 1 is a flowchart showing another modified example of the method for measuring spatial crosstalk between channels. In this modified example, the step ST1 of forming or setting the light reflection suppression portion on the second incident and exit surface 10b includes a step ST13 instead of the above-mentioned step ST11. In step ST13, as shown in FIG. Figure 14 As shown, the second end faces of the plurality of SCFs 92 each having an anti-reflection film 93 as a light reflection suppressing portion provided on the first end face 92a are optically coupled to the cores 13a, 13b, 13c, and 13d at the second incident and output face 10b.

[0152] Figure 22 FIG. 1 is a flowchart showing another modified example of the method for measuring spatial crosstalk between channels. In this modified example, the step ST1 of forming or setting the light reflection suppression portion on the second incident and exit surface 10b includes a step ST14 instead of the above-mentioned step ST11. In step ST14, as shown in FIG. Figure 6 or Figure 7 As shown, the second incident and exit surface 10b is made flat or curved by grinding or cutting, and these are used as the light reflection suppressing portion. Alternatively, the substantially flat second incident and exit surface 10b that has been simply cut may be used as the light reflection suppressing portion.

[0153] Figure 23 FIG. 1 is a flowchart showing another modified example of the method for measuring spatial crosstalk between channels. In this modified example, the step ST1 of forming or setting the light reflection suppression portion on the second incident and exit surface 10b includes a step ST15 instead of the above-mentioned step ST11. In step ST15, as shown in FIG. Figure 15 or Figure 16 As shown, the optical fiber 100 or 110 serving as the light reflection suppressing portion is fused to the second incident and output surface 10b.

[0154] The effects obtained by the above-described measurement devices 1A, 1B, and 1C and the spatial channel crosstalk measurement method according to the present embodiment will be described. Figure 24 It is a diagram schematically showing the configuration of a measuring device 200 according to a reference example. Figure 24The illustrated measurement device 200 is used to measure inter-core crosstalk in an MCF 10. It includes a single light source 201, a first optical switch 202, a FIFO 203, a FIFO 204, a second optical switch 205, and a single optical receiver 206. The light source 201, first optical switch 202, and FIFO 203 are connected to the first incident surface 10a of the MCF 10. The FIFO 204, second optical switch 205, and optical receiver 206 are connected to the second incident surface 10b of the MCF 10. Test light output from the light source 201 enters the first core (first spatial channel) selected by the first optical switch 202 among the multiple cores of the MCF 10 at the first incident surface 10a. At the second incident surface 10b, the second optical switch 205 sequentially selects the first core into which the test light entered and a second core (second spatial channel) different from the first core. The optical power of the test light propagating in the first core and the optical power of the crosstalk light propagating in the second core are detected by the optical receiver 206. The inter-core crosstalk is calculated based on these optical powers.

[0155] exist Figure 24 In the illustrated measurement device 200, test light is directed into the first core at the first incident surface 10a of the MCF 10, and the power of the test light emitted from the first and second cores is detected at the second incident surface 10b of the MCF 10. However, if the MCF 10 to be measured has already been installed, it may be difficult to connect the unit consisting of the light source 201, the first optical switch 202, and the FIFO 203 to the first incident surface 10a of the MCF 10, and to connect the unit consisting of the FIFO 204, the second optical switch 205, and the optical receiver 206 to the second incident surface 10b of the MCF 10, which is farther away from the first incident surface 10a.

[0156] In the measuring devices 1A, 1B, and 1C and the measuring method of the present embodiment, it is sufficient to connect the light source unit 20A or 20B and the light detection unit 30A or 30B to the first incident surface 10a of the MCF 10, and there is no need to connect any of them to the second incident surface 10b. Therefore, even in the case of an MCF 10 where the first incident surface 10a and the second incident surface 10b are more than a few kilometers apart, the light source unit 20A or 20B and the light detection unit 30A or 30B can be easily connected to the MCF 10. Furthermore, by reducing the number of connections during measurement by half, the crosstalk measurement of the MCF 10 can be performed efficiently. In addition, by detecting the sum of the backscattered Rayleigh light, for example, rather than positionally decomposing the backscattered Rayleigh light components, as in the present embodiment, the level of the detection signal can be increased, thereby improving the accuracy of crosstalk measurement. In this case, if a light reflection suppression unit is not provided, a portion of the test light will reflect off the second incident surface 10b, and this reflected test light may be mixed with the backscattered Rayleigh light. Because the power of backscattered Rayleigh light is very low, in this case, it may be impossible to improve the accuracy of crosstalk measurement. In the measurement devices 1A, 1B, and 1C and the measurement method of this embodiment, a light reflection suppression unit that suppresses the reflection of the test light is formed or provided on the second incident surface 10b. This reduces the reflection of a portion of the test light off the second incident surface 10b, thereby reducing the mixing of reflected light with the backscattered Rayleigh light. As a result, it is possible to measure inter-channel crosstalk with high accuracy.

[0157] As described above, the crosstalk magnitude XT may be calculated using equation (14). For example, by using such an equation, the crosstalk magnitude can be calculated based only on the sum of the powers of the backscattered Rayleigh light from the first incident surface 10a to the second incident surface 10b.

[0158] As mentioned above, it can also be, α dB L (dB) is 0.01 dB or more. This allows Rayleigh backscattered light having a power level sufficient for measurement to be returned to the first incident and exit surface 10 a.

[0159] As described above, the return loss RL (dB) of the reflection at the second incident and exit surface 10b may satisfy the equation (3). This allows the measurement error caused by the reflection of the test light at the second incident and exit surface 10b to be reduced to 1 dB or less.

[0160] As described above, the light reflection suppressing unit may suppress the reflection of the test light at the second incident and exit surface 10b of the MCF 10. In this case, Figure 6The angle θ (degrees) formed between the virtual plane H2 and the virtual plane H1 satisfies the equation (16). This makes it possible to reduce the measurement error caused by the reflection of the test light at the second incident and exit surface 10b to 1 dB or less.

[0161] As described above, step ST1 of forming or providing the light reflection suppressing portion on the second incident and exit surface 10b may include step ST14 in which the light reflection suppressing portion is formed by cutting the MCF 10 to form the second incident and exit surface 10b. This facilitates forming the light reflection suppressing portion on the second incident and exit surface 10b.

[0162] As described above, step ST1 of forming or providing the light reflection suppressing portion on the second incident and exit surface 10b may include step ST14 in which the light reflection suppressing portion is formed by polishing the second incident and exit surface 10b of the MCF 10. This facilitates forming the light reflection suppressing portion on the second incident and exit surface 10b.

[0163] As described above, the light reflection suppression portion may include a substance in contact with the second incident-output surface 10b of the MCF 10, and this substance may have a refractive index that matches the refractive index of the MCF 10. Similarly, step ST1 of forming or providing the light reflection suppression portion on the second incident-output surface 10b may include step ST11, in which the light reflection suppression portion is provided by bringing a substance having a refractive index that matches the refractive index of the MCF 10 into contact with the second incident-output surface 10b of the MCF 10. This facilitates providing the light reflection suppression portion on the second incident-output surface 10b.

[0164] As described above, the substance having a refractive index matching that of the MCF 10 may be a liquid, a gel, or a solid.

[0165] As described above, the light reflection suppression unit may include another optical fiber 100 (or 110) having a cladding 104 (or 114) primarily composed of the same material as the cladding 14 of the MCF 10. Alternatively, step ST1 may include step ST15 of fusing the end face of the optical fiber 100 (or 110) to the second incident surface 10b of the MCF 10. Alternatively, the optical fiber 100 (or 110) after fusion splicing may not have a spatial channel (core) that matches the cores 13a, 13b, 13c, and 13d of the MCF 10. This makes it easier to provide the light reflection suppression unit on the second incident surface 10b.

[0166] As described above, the measuring devices 1A, 1B, and 1C may include an MCF 81 having an anti-reflection film 82 as a light reflection suppression portion provided on the first end face 81a. The second end face of the MCF 81 may be connected to the second incident / exit face 10b at the second incident / exit face 10b. Similarly, in the measuring method, step ST1 may include step ST12 of connecting the second end face of the MCF 81 having the anti-reflection film 82 as the light reflection suppression portion provided on the first end face 81a to the second incident / exit face 10b. According to these measuring devices 1A, 1B, and 1C and the measuring method, the reflectivity at the light reflection suppression portion can be reduced, and crosstalk can be measured with higher accuracy. In addition, since there is no need to provide an anti-reflection film on the second incident / exit face 10b, it is sufficient to simply connect the MCF 81 to the second incident / exit face 10b, thereby simplifying the measurement operation.

[0167] As described above, the measurement devices 1A, 1B, and 1C may include an equal number of SCFs 92, each having an anti-reflection coating 93 as a light reflection suppressing portion provided on the first end face 92a, as the number of cores 13a, 13b, 13c, and 13d. The second end face of each SCF 92 is optically coupled to each of the cores 13a, 13b, 13c, and 13d at the second incident / output face 10b. Similarly, in the measurement method, step ST1 may include step ST13, in which the second end face of each SCF 92, each having an anti-reflection coating 93 as a light reflection suppressing portion provided on the first end face 92a, as the number of cores 13a, 13b, 13c, and 13d, is optically coupled to each of the cores 13a, 13b, 13c, and 13d at the second incident / output face 10b. These measurement devices 1A, 1B, and 1C and their measurement methods reduce the reflectance at the light reflection suppressing portion, enabling more accurate crosstalk measurement. Furthermore, since there is no need to provide an antireflection film on the second incident and exit surface 10 b , it is sufficient to simply connect optical components such as the FIFO 91 to the second incident and exit surface 10 b , thereby facilitating measurement operations.

[0168] As described above, in the measuring devices 1A, 1B, and 1C, the second incident and exit surfaces 10b may be used as the light reflection suppressing portion. This eliminates the need for special devices such as the anti-reflection devices 80 and 90, and simplifies the structure of the measuring device.

[0169] As in this embodiment, the measuring devices 1A, 1B, and 1C may include three-port optical couplers 41, 42, 43, and 44 and a FIFO 50. The three-port optical couplers 41, 42, 43, and 44 have a first port P1, a second port P2, and a third port P3. Light input to the first port P1 is output from the second port P2, and light input to the second port P2 is output from the third port P3. The FIFO 50 optically couples the fiber cores 13a, 13b, 13c, and 13d at the first incident and output surface 10a of the MCF 10 to the second port P2 of each of the three-port optical couplers 41, 42, 43, and 44. The light source unit 20A or 20B is optically coupled to the first port P1 of the three-port optical coupler 41, 42, 43, and 44. The light detection unit 30A or 30B is optically coupled to the third port P3 of the three-port optical coupler 41, 42, 43, and 44. This makes it possible to easily implement a structure in which test light from the light source unit 20A or 20B is incident on each of the cores 13a, 13b, 13c, and 13d, and the light detector 30A or 30B detects light emitted from the core into which the test light is incident and light emitted from the other cores.

[0170] like Figure 5 As in the measurement device 1C shown in FIG. 1 , the light source unit 20B may include the same number of light sources 23, 24, 25, and 26 as the cores 13a, 13b, 13c, and 13d, and each of the light sources 23, 24, 25, and 26 may be optically coupled to the first port P1 of each of the three-port optical couplers 41, 42, 43, and 44. This allows the input of test light to the first port P1 of each of the three-port optical couplers 41, 42, 43, and 44 with a simple configuration.

[0171] like Figure 1 As in the measurement device 1A shown, the light source unit 20A may include a single light source 21 and a first optical switch 22 that selectively optically couples the single light source 21 to the first port P1 of any of the three-port optical couplers 41, 42, 43, and 44. This can reduce the number of light sources.

[0172] like Figure 4 As in the measurement device 1B shown in FIG. 1 , the light detection unit 30B may include the same number of optical receivers 33, 34, 35, and 36 as the number of cores 13a, 13b, 13c, and 13d, and each of the optical receivers 33, 34, 35, and 36 may be optically coupled to the third port P3 of each of the three-port optical couplers 41, 42, 43, and 44. This allows detection of light output from the third port P3 of each of the three-port optical couplers 41, 42, 43, and 44 with a simple configuration.

[0173] like Figure 1As in the measurement device 1A shown, the light detection unit 30A may include a single light receiver 31 and a second optical switch 32 that selectively optically couples the single light receiver 31 to the third port P3 of any of the three-port optical couplers 41, 42, 43, and 44. This can reduce the number of light receivers.

[0174] Alternatively, the inter-core crosstalk of FIFO 50 may be 0.259 times or less, 0.122 times or less, 0.047 times or less, or 0.023 times or less of the inter-core crosstalk of MCF 10. Consequently, the inter-core crosstalk measurement error of MCF 10 caused by the inter-core crosstalk of FIFO 50 can be reduced to 1 dB or less, 0.5 dB or less, 0.2 dB or less, or 0.1 dB or less, respectively, allowing the inter-core crosstalk of MCF 10 to be measured with higher accuracy.

[0175] The spatial crosstalk measurement method and device according to the present disclosure are not limited to the above-described embodiments and variations, and various other variations are possible. For example, the configuration of the light reflection suppression portion on the second incident / output surface 10b of the MCF 10 is not limited to the above-described embodiment. The number of cores in the MCF 10 is also not limited to the above-described embodiment.

[0176] Description of Reference Numerals

[0177] 1A, 1B, 1C measuring device; 10 multi-core optical fiber (MCF); 10a first incident and exit surface; 10b second incident and exit surface; 11 glass fiber; 12 coating resin; 13a, 13b, 13c, 13d fiber core; 14 cladding; 15 marker; 20A, 20B light source; 21, 23, 24, 25, 26 light source; 22 first optical switch; 22a input port; 22b, 22c, 22d , 22e output ports; 30A, 30B light detection units; 31 optical receiver; 32 second optical switch; 32a output port; 32b, 32c, 32d, 32e input ports; 33, 34, 35, 36 optical receivers; 40 optical coupler unit; 41, 42, 43, 44 three-port optical coupler; 50 FIFO; 50a, 50b, 50c, 50d, 50e input and output ports; 60 Operation unit; 71 anti-reflection film; 72 anti-reflection material; 73 container; 74 liquid; 80, 90 anti-reflection device; 81 multi-core optical fiber (MCF); 81a, 92a first end face; 82, 93 anti-reflection film; 91 FIFO; 92 single-core optical fiber; 100, 110 optical fiber; 102, 112 coating resin; 104, 114 cladding; 113 core; 131 end face; 200 measuring device; 201 light source; 202 first optical switch; 203, 204 FIFO; 205 second optical switch; 206 optical receiver; AX center axis; H1, H2, H2a imaginary planes; L1, L2 light; P1 first port; P2 second port; P3 third port; ST1, ST2, ST3, ST4, ST5, ST11, ST12, ST13, ST14, ST15 steps; θ angle.

Claims

1. A method for measuring spatial channel crosstalk, comprising measuring the spatial channel crosstalk of a space-division multiplexing optical fiber, wherein the space-division multiplexing optical fiber has a first input / output surface and a second input / output surface, and has a first spatial channel and a second spatial channel, the method comprising: In a first step, a light reflection suppression portion is formed or provided on the second incident and output surface, wherein the light reflection suppression portion suppresses reflection of the test light; The second step is to make the test light incident on the first spatial channel at the first incident and exit surfaces; The third step is to cause at least a portion of the test light to be Rayleigh backscattered in the spatial division multiplexing optical fiber; In a fourth step, a first optical power and a second optical power are detected, where the first optical power is the power of light emitted from the first spatial channel at the first incident surface in the at least a portion of the light, and the second optical power is the power of light emitted from the second spatial channel at the first incident surface in the at least a portion of the light. as well as In a fifth step, based on the first optical power and the second optical power, a magnitude of crosstalk between the first spatial channel and the second spatial channel is calculated.

2. The method for measuring spatial channel crosstalk according to claim 1, wherein: The test light is continuous light or chopped light, The first optical power is the sum of optical power components emitted from the first spatial channel in the return optical power components of the test light including backscattered light at each position in the longitudinal direction of the space-division multiplexing optical fiber. The second optical power is the sum of optical power components emitted from the second spatial channel in the return optical power components of the test light including backscattered light at each position in the longitudinal direction of the space-division multiplexing optical fiber.

3. The method for measuring spatial channel crosstalk according to claim 1 or 2, wherein: In the fifth step, the first optical power is set to PW1, the second optical power is set to PW2, and the average value of the transmission loss coefficient of the first spatial channel and the second spatial channel is set to α (km -1 ), when the length of the space-division multiplexing optical fiber is set to L (km), the size of the crosstalk XT is calculated using the following mathematical formula (A):

4. The method for measuring spatial channel crosstalk according to any one of claims 1 to 3, wherein: When the loss coefficients α of the first spatial channel and the second spatial channel are dB Defined as α dB (dB / km)=(10 / ln10)α, α dB L(dB) is 0.01dB or higher.

5. The method for measuring spatial channel crosstalk according to any one of claims 1 to 4, wherein: The light reflection suppressing unit suppresses reflection of the test light at the second incident and output surface of the space-division multiplexing optical fiber. When the wavelength of the test light is λ (μm), the average refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is n, 0.5 times the average mode field diameter of the first spatial channel and the second spatial channel is w (μm), and the return loss of reflection at the second incident and exit surface is RL (dB), RL satisfies the following mathematical formula (B):

6. The method for measuring spatial channel crosstalk according to any one of claims 1 to 4, wherein: The light reflection suppressing unit suppresses reflection of the test light at the second incident and output surface of the space-division multiplexing optical fiber. When the wavelength of the test light is λ (μm), the average refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is n, and 0.5 times the average mode field diameter of the first spatial channel and the second spatial channel is w (μm), the angle θ (degrees) formed by a plane contacting the centers of the first spatial channel and the second spatial channel and a plane orthogonal to the central axis of the space-division multiplexing optical fiber satisfies the following mathematical formula (C):

7. The method for measuring spatial channel crosstalk according to any one of claims 1 to 6, wherein: The first step includes forming the light reflection suppressing portion by cutting the space-division multiplexing optical fiber to form the second incident and exit surfaces.

8. The method for measuring spatial channel crosstalk according to any one of claims 1 to 6, wherein: The first step includes forming the light reflection suppressing portion by polishing the second incident and output surface of the space-division multiplexing optical fiber.

9. The method for measuring spatial channel crosstalk according to any one of claims 1 to 5, wherein: The first step includes providing the light reflection suppressing portion by bringing a substance having a refractive index matching that of the space-division multiplexing optical fiber into contact with the second incident and exit surface of the space-division multiplexing optical fiber.

10. The method for measuring spatial channel crosstalk according to claim 9, wherein: When the average refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is set to n and the refractive index of the material is set to n0, the relative refractive index difference Δ0 defined as Δ0 = |n-n0| / n0 and α dB The relationship of L (dB) satisfies the following mathematical formula (D): Δ0≤1.14×10 -2 (a dB L) 0.473 ...(D).

11. The method for measuring spatial channel crosstalk according to any one of claims 1 to 4, wherein: The light reflection suppressing portion includes another optical fiber having a cladding mainly composed of the same material as the cladding of the space-division multiplexing optical fiber, The first step includes the step of fusing the end face of the other optical fiber to the second incident and output face of the space division multiplexing optical fiber, The other optical fiber after fusion splicing does not have both a spatial channel matching the first spatial channel of the space-division multiplexing optical fiber and a spatial channel matching the second spatial channel.

12. The method for measuring spatial channel crosstalk according to any one of claims 1 to 11, wherein: The space division multiplexing optical fiber is a multi-core optical fiber or a multi-mode optical fiber.

13. The method for measuring spatial channel crosstalk according to any one of claims 1 to 12, wherein: The first spatial channel and the second spatial channel are a first fiber core and a second fiber core, respectively, or a first mode and a second mode, respectively.

14. A device for measuring spatial crosstalk between channels, for measuring spatial crosstalk between channels of a space-division multiplexing optical fiber, wherein the space-division multiplexing optical fiber has a first input / output surface and a second input / output surface, and has N spatial channels, where N is an integer greater than or equal to 2, the device comprising: a light source unit configured to cause the test light to be incident on each of the N spatial channels at the first incident and exit surfaces; a light reflection suppressing portion formed or provided on the second incident and exit surfaces to suppress reflection of the test light; an optical detection unit configured to detect a first optical power and a second optical power, wherein the first optical power is the power of light emitted from a first spatial channel into which the test light is incident, in at least a portion of the test light subjected to Rayleigh backscattering in the spatial division multiplexing optical fiber; and the second optical power is the power of light emitted from a second spatial channel different from the first spatial channel, in the at least a portion of the light. as well as A calculation unit calculates a magnitude of crosstalk between the first spatial channel and the second spatial channel based on the first optical power and the second optical power.

15. The spatial channel crosstalk measurement device according to claim 14, wherein: The test light is continuous light or chopped light, The first optical power is the sum of optical power components emitted from the first spatial channel in the return optical power components of the test light including backscattered light at each position in the longitudinal direction of the space-division multiplexing optical fiber. The second optical power is the sum of optical power components emitted from the second spatial channel in the return optical power components of the test light including backscattered light at each position in the longitudinal direction of the space-division multiplexing optical fiber.

16. The spatial channel crosstalk measurement device according to claim 14 or 15, wherein: In the calculation unit, the first optical power is set to PW1, the second optical power is set to PW2, and the average value of the transmission loss coefficients of the first spatial channel and the second spatial channel is set to α (km -1 ), when the length of the space-division multiplexing optical fiber is set to L (km), the size of the crosstalk XT is calculated using the following mathematical formula (E):

17. The spatial channel crosstalk measurement device according to any one of claims 14 to 16, wherein: When the loss coefficients α of the first spatial channel and the second spatial channel are dB Defined as α dB (dB / km)=(10 / ln10)α, α dB L(dB) is 0.01dB or higher.

18. The spatial channel crosstalk measurement device according to any one of claims 14 to 17, wherein: The light reflection suppressing unit suppresses reflection of the test light at the second incident and output surface of the space-division multiplexing optical fiber. When the wavelength of the test light is λ (μm), the average refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is n, 0.5 times the average mode field diameter of the first spatial channel and the second spatial channel is w (μm), and the return loss of reflection at the second incident and exit surface is RL (dB), RL satisfies the following mathematical formula (F):

19. The spatial channel crosstalk measurement device according to any one of claims 14 to 17, wherein: The light reflection suppressing unit suppresses reflection of the test light at the second incident and output surface of the space-division multiplexing optical fiber. When the wavelength of the test light is λ (μm), the average refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is n, and 0.5 times the average mode field diameter of the first spatial channel and the second spatial channel is w (μm), an angle θ (degrees) formed by a plane contacting the centers of the first spatial channel and the second spatial channel and a plane orthogonal to the central axis of the space-division multiplexing optical fiber satisfies the following mathematical formula (G):

20. The spatial channel crosstalk measurement device according to any one of claims 14 to 18, wherein: The light reflection suppressing portion includes a substance in contact with the second incident and output surface of the space-division multiplexing optical fiber, and the substance has a refractive index that matches the refractive index of the space-division multiplexing optical fiber.

21. The spatial channel crosstalk measurement device according to claim 20, wherein: When the average refractive index of the first spatial channel and the second spatial channel of the space-division multiplexing optical fiber is set to n and the refractive index of the material is set to n0, the relative refractive index difference Δ0 defined as Δ0 = |n-n0| / n0 and α dB The relationship between L (dB) satisfies the following mathematical formula (H): Δ0≤1.14×10 -2 (a dB L) 0.473 ...(H)。 22. The spatial channel crosstalk measurement device according to any one of claims 14 to 17, wherein: The light reflection suppressing portion includes another optical fiber having a cladding mainly made of the same material as the cladding of the space-division multiplexing optical fiber, and the end face of the other optical fiber is fused to the second incident and output face of the space-division multiplexing optical fiber. The other optical fiber does not have both a spatial channel that matches the first spatial channel and a spatial channel that matches the second spatial channel of the spatial division multiplexing optical fiber.

23. The spatial channel crosstalk measurement device according to any one of claims 14 to 22, wherein: The space division multiplexing optical fiber is a multi-core optical fiber or a multi-mode optical fiber.

24. The spatial channel crosstalk measurement device according to any one of claims 14 to 23, wherein: The first spatial channel and the second spatial channel are a first fiber core and a second fiber core, respectively, or a first mode and a second mode, respectively.

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