Inter-spatial channel crosstalk measuring method, inter-core crosstalk measuring method for multicore optical fiber, inter-spatial channel crosstalk measuring device, and inter-core crosstalk measuring device for multicore optical fiber

Abstract

This inter-spatial channel crosstalk measuring method involves measuring inter-spatial channel crosstalk in a space division multiplexed optical system which has a first input / output unit and a second input / output unit on the side opposite to the first input / output unit, and also has a first spatial channel and a second spatial channel. This method comprises: a first step for forming or providing a light reflecting surface at the second input / output unit; a second step for inputting testing light into the first spatial channel at the first input / output unit; a third step for causing at least partial light of the testing light to be reflected at the light reflecting surface; a fourth step for detecting first light power which is the power of light that is a portion of said partial light outputted from the first spatial channel at the first input / output unit and second light power which is the power of light that is a portion of said partial light outputted from the second spatial channel at the first input / output unit; and a fifth step for calculating the magnitude of crosstalk between the first spatial channel and the second spatial channel on the basis of the first light power and the second light power.

Description

Method for measuring crosstalk between spatial channels, method for measuring crosstalk between cores of a multi-core optical fiber, device for measuring crosstalk between spatial channels, and device for measuring crosstalk between cores of a multi-core optical fiber

[0001] The present disclosure relates to a method for measuring crosstalk between spatial channels, a method for measuring crosstalk between cores in a multi-core optical fiber, an apparatus for measuring crosstalk between spatial channels, and an apparatus for measuring crosstalk between cores in a multi-core optical fiber. This application claims priority to Japanese Application No. 2023-021827 filed on February 15, 2023, and incorporates by reference all the contents of said Japanese application.

[0002] Patent Document 1 and Non-Patent Document 1 disclose methods for measuring inter-core crosstalk in a multi-core optical fiber. In the methods described in Patent Document 1 and Non-Patent Document 1, measurement light is incident on a certain core at one end of the multi-core optical fiber, and the power of measurement light emitted from the core and another core at the other end of the multi-core optical fiber is detected, thereby measuring inter-core crosstalk.

[0003] Non-Patent Document 2 discloses a method for measuring inter-core crosstalk in a multi-core optical fiber using an OTDR (Optical Time Domain Reflectometer) measurement technique. In the method described in Non-Patent Document 2, pulsed light is incident on a certain core at one end of the multi-core optical fiber, and the inter-core crosstalk is measured by detecting a time change in the power of back-scattered Rayleigh scattered light (returned light caused by Rayleigh scattering. Returned light is light that is part of incident light returned in the direction opposite to the traveling direction of the incident light due to reflection, back-scattering, etc.) emitted from the certain core and another core at the one end of the multi-core optical fiber.

[0004] International Publication No. 2012 / 115162

[0005] Tetsuya Hayashi et al., "Characterization of Crosstalk in Ultra-Low-Crosstalk Multi-Core Fiber", Journal of Lightwave Technology, Vol. 30, No. 4, (2012)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)

[0006] A method for measuring inter-core crosstalk in a multi-core optical fiber according to one aspect of the present disclosure is a method for measuring inter-core crosstalk in a multi-core optical fiber having a first end and a second end opposite to the first end, and having first and second cores, the method including: a first step of forming or providing a light reflecting surface at the second end, a second step of directing test light into a first core at the first end, a third step of reflecting at least a portion of the test light on the light reflecting surface, a fourth step of detecting a first optical power that is the power of the light emitted from the first core among the at least a portion of the light, and a second optical power that is the power of the light emitted from the second core among the at least a portion of the light, and a fifth step of calculating a magnitude of crosstalk between the first core and the second core based on the first optical power and the second optical power.

[0007] FIG. 1 is a diagram illustrating a configuration of a measurement apparatus according to an embodiment of the present disclosure. FIG. 2 is a diagram illustrating a cross section perpendicular to the central axis of a multi-core optical fiber. FIG. 3 is a diagram illustrating a configuration of a three-port optical coupler (optical circulator). FIG. 4 is a diagram illustrating a configuration of a modified example of the measurement apparatus. FIG. 5 is a diagram illustrating a configuration of another modified example of the measurement apparatus. FIG. 6 is a cross-sectional view showing an example of a light reflecting surface of a second end portion. FIG. 7 is a cross-sectional view showing an example of a light reflecting surface of a second end portion. FIG. 8 is a cross-sectional view showing an example of a light reflecting surface of a second end portion. FIG. 9 is a cross-sectional view showing an example of a light reflecting surface of a second end portion. FIG. 10 is a cross-sectional view showing an example of a light reflecting surface of a second end portion. FIG. 11 is a cross-sectional view showing an example of a light reflecting surface of a second end portion. FIG. 12 is a cross-sectional view showing an example of a light reflecting surface of a second end portion. FIG. 13 is a graph schematically illustrating the time waveform of the optical power of continuous light. FIG. 14 is a graph schematically illustrating the time waveform of the optical power of chopped light. FIG. 15 is a graph schematically illustrating the time waveform of the optical power of pulsed light. 16 is a flowchart showing an inter-core crosstalk measurement method of an embodiment. FIG. 17 is a flowchart showing a modified example of the inter-core crosstalk measurement method. FIG. 18 is a flowchart showing another modified example of the inter-core crosstalk measurement method. FIG. 19 is a flowchart showing yet another modified example of the inter-core crosstalk measurement method. FIG. 20 is a graph showing the relationship between the measurement error of inter-core crosstalk caused by backward Rayleigh scattering and the transmission loss when the angle θ is changed from 0 to 6 in 1 degree increments. FIG. 21 is a graph showing the relationship between the maximum allowable transmission loss and the angle of the input / output facet of the multi-core optical fiber when the allowable measurement error is 2 dB, 1 dB, 0.5 dB, 0.2 dB, or 0.1 dB. FIG. 22 is a graph schematically showing the time waveforms of the power of light output from a first core into which test light is incident and the power of light output from a second core when the test light is pulsed light. FIG. 23 is a diagram showing the appearance of a holder included in a measurement device according to a first modified example. Part (a) is a plan view of the holder. 24 is a diagram showing the appearance of a holding part capable of holding a ribbon optical fiber including a plurality of MCFs, with (a) being a plan view of the holding part and (b) being a side view of the holding part.Fig. 25 is a schematic cross-sectional view of a dustproof cover that houses the holder. Fig. 26 is a diagram showing a state in which the lid of the dustproof cover is open. Fig. 27 is a diagram showing a schematic configuration of a measurement device according to a reference example.

[0008] [Problem to be Solved by the Present Disclosure] In conventional crosstalk measurement of a multi-core optical fiber, test light is incident on a certain core at one end of the multi-core optical fiber, and the power of the test light emitted from the certain core and another core is detected at the other end of the multi-core optical fiber, as described in Patent Document 1 and Non-Patent Document 1. However, it may not be easy to connect a light source that outputs test light to one end of the multi-core optical fiber and a photodetector to the other end of the multi-core optical fiber.

[0009] In contrast to this, by using an OTDR measurement technique that uses backward Rayleigh scattered light, it is possible to input and output test light at only one end of a multi-core optical fiber, as described in Non-Patent Document 2. However, because the power of backward Rayleigh scattered light is minute, when the crosstalk is small, the detection signal of the backward Rayleigh scattered light is buried in noise, which causes a problem that the crosstalk cannot be measured.

[0010] An object of the present disclosure is to provide a method for measuring crosstalk between spatial channels, a method for measuring crosstalk between cores in a multi-core optical fiber, an apparatus for measuring crosstalk between spatial channels, and an apparatus for measuring crosstalk between cores in a multi-core optical fiber, which are capable of connecting a light source that outputs test light and a photodetector to one end of a multi-core optical fiber and measuring the magnitude of crosstalk even when the crosstalk is small.

[0011] [Effects of the Present Disclosure] According to the present disclosure, it is possible to provide a method for measuring crosstalk between spatial channels, a method for measuring crosstalk between cores in a multi-core optical fiber, an apparatus for measuring crosstalk between spatial channels, and an apparatus for measuring crosstalk between cores in a multi-core optical fiber, which are capable of connecting a light source that outputs test light and a photodetector to one end of a multi-core optical fiber and measuring the magnitude of crosstalk even when the crosstalk is small.

[0012] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0013] [1] According to one aspect of the present disclosure, there is provided a method for measuring crosstalk between spatial channels in a space-division multiplexing optical system having a first input / output unit and a second input / output unit opposite the first input / output unit, and having a first spatial channel and a second spatial channel. The method includes a first step, a second step, a third step, a fourth step, and a fifth step. In the first step, an optical reflecting surface is formed or provided on the second input / output unit. In the second step, test light is incident on the first spatial channel at the first input / output unit. In the third step, at least a portion of the test light is reflected by the optical reflecting surface. In the fourth step, a first optical power is detected, which is the power of the at least a portion of the light emitted from the first spatial channel at the first input / output unit, and a second optical power is detected, which is the power of the at least a portion of the light emitted from the second spatial channel at the first input / output unit. In the fifth step, the magnitude of crosstalk between the first spatial channel and the second spatial channel is calculated based on the first optical power and the second optical power.

[0014] In the measurement method [1] above, it is sufficient to connect the light source outputting the test light and the photodetector to the first input / output port of the space-division multiplexing optical system; there is no need to connect either of them to the second input / output port. Therefore, even when simultaneous access to both input / output ports of the space-division multiplexing optical system is difficult, the light source and the photodetector can be easily connected. Furthermore, by halving the number of connections required for measurement, crosstalk measurement of the space-division multiplexing optical system can be performed efficiently. Additionally, in the measurement method [1] above, the magnitude of crosstalk is calculated based on the power of light reflected by an optical reflecting surface provided (or formed) in the second input / output port. The power of light reflected by the optical reflecting surface is significantly greater than the power of backscattered Rayleigh light in the optical device in OTDR measurement. Therefore, even when crosstalk is small, the detection signal is not buried in noise, and crosstalk can be measured effectively.

[0015] [2] In the measurement method of [1] above, the first optical power may be a sum of optical power components emitted from first spatial channels among return optical power components of the test light including backscattered light at each position in the longitudinal direction of the space division multiplexing optical system. Furthermore, the second optical power may be a sum of optical power components emitted from second spatial channels among return optical power components of the test light including backscattered light at each position in the longitudinal direction of the space division multiplexing optical system.

[0016] [3] In the measurement method of [2] above, the space division multiplexing optical system may be a space division multiplexing optical fiber, and the light reflecting surface may be an end face of the space division multiplexing optical fiber. The wavelength of the test light is λ (μm), the constant of the circumference of a circle is π, the refractive index of the first spatial channel and the second spatial channel of the space division multiplexing optical fiber is n, and the transmission loss coefficient of the first spatial channel and the second spatial channel is α (km －１), 0.5 times the mode field diameter of the first spatial channel and the second spatial channel is w (μm), the angle formed by the plane tangent to the center of the first spatial channel and the second spatial channel at the end face and the plane perpendicular to the central axis of the space division multiplexing optical fiber is φ (rad), the length of the space division multiplexing optical fiber is L (km), and the effective cross-sectional area of ​​the first spatial channel and the second spatial channel is A (μm). ｅｆｆ (μm ２ ), the Rayleigh scattering loss coefficients of the first spatial channel and the second spatial channel are denoted by α Ｒ (km －１ ), the value of formula (A) is 10 ０．２ Furthermore, when the first optical power is PW1, the second optical power is PW2, and the magnitude of crosstalk between the first spatial channel and the second spatial channel is XT, XT may be calculated using formula (B). This reduces the crosstalk measurement error due to backward Rayleigh scattered light to 10 dB or less. Alternatively, when evaluating crosstalk using Equation (1) described below, the crosstalk measurement error due to backward Rayleigh scattered light can be reduced to 2 dB or less. Therefore, crosstalk can be measured with higher accuracy.

[0017] [4] In the measurement method of [2] above, the space division multiplexing optical system may be a space division multiplexing optical fiber, and the light reflecting surface may be an end face of the space division multiplexing optical fiber. The angle formed by the plane tangent to the centers of the first spatial channel and the second spatial channel at the end face and the plane perpendicular to the central axis of the space division multiplexing optical fiber is defined as θ (degrees), and the loss coefficients of the first spatial channel and the second spatial channel are defined as α. ｄＢ (dB·km －１ ), the length of the space division multiplexing optical fiber is L (km), the angle θ and the transmission loss α ｄＢ L may satisfy formula (C). Furthermore, the wavelength of the test light is 1.625 μm or less, and the effective cross-sectional areas of the first and second spatial channels at that wavelength are 60 μm ２Furthermore, when the first optical power is PW1, the second optical power is PW2, and the magnitude of crosstalk between the first spatial channel and the second spatial channel is XT, XT may be calculated using formula (D). This reduces the crosstalk measurement error caused by backward Rayleigh scattered light to 10 dB or less, enabling crosstalk to be measured with higher accuracy.

[0018] [5] In the measurement method of [2] above, the space division multiplexing optical system may be a space division multiplexing optical fiber, and the light reflecting surface may be an end face of the space division multiplexing optical fiber. The wavelength of the test light is λ (μm), the constant of the circumference of a circle is π, the refractive index of the first spatial channel and the second spatial channel of the space division multiplexing optical fiber is n, and the transmission loss coefficient of the first spatial channel and the second spatial channel is α (km －１ ), 0.5 times the mode field diameter of the first spatial channel and the second spatial channel is w (μm), the angle formed by the plane tangent to the center of the first spatial channel and the second spatial channel at the end face and the plane perpendicular to the central axis of the space division multiplexing optical fiber is φ (rad), the length of the space division multiplexing optical fiber is L (km), and the effective cross-sectional area of ​​the first spatial channel and the second spatial channel is A (μm). ｅｆｆ (μm ２ ), the Rayleigh scattering loss coefficients of the first spatial channel and the second spatial channel are denoted by α Ｒ (km －１ ), where the first optical power is PW10, the second optical power is PW20, and the magnitude of crosstalk between the first spatial channel and the second spatial channel is XT, then XT may be calculated using equation (E).

[0019] [6] In the measurement method of [2] above, the space division multiplexing optical system is a space division multiplexing optical fiber, the light reflecting surface is an end face of the space division multiplexing optical fiber, the wavelength of the test light is λ (μm), pi is the ratio of the circumference of a circle to its circumference, n is the refractive index of the first spatial channel and the second spatial channel of the space division multiplexing optical fiber, and α (km －１ ), the length of the space division multiplexing optical fiber is L (km), the effective area of ​​the first spatial channel and the second spatial channel is Aｅｆｆ (μm ２ ), the Rayleigh scattering loss coefficients of the first spatial channel and the second spatial channel are denoted by α Ｒ (km －１ ), where the first optical power is PW10 and the second optical power is PW20, XT may be calculated using formula (F).

[0020] [7] In the measurement method of [1] above, in the fourth step, of the test light incident on the first spatial channel at the first incident / exit unit, a time-intensity waveform of light reflected in the first spatial channel, in the second spatial channel, and at the optical reflecting surface and then output from the first spatial channel at the first incident / exit unit may be analyzed, and the power of the light reflected at a longitudinal position corresponding to the optical reflecting surface and output from the first spatial channel may be determined as the first optical power. Furthermore, in the fourth step, of the test light incident on the first spatial channel at the first incident / exit unit, a time-intensity waveform of light reflected in the first spatial channel, in the second spatial channel, and at the optical reflecting surface and then output from the second spatial channel at the first incident / exit unit may be analyzed, and the power of the light reflected at a longitudinal position corresponding to the optical reflecting surface and output from the second spatial channel may be determined as the second optical power.

[0021] [8] In the measurement methods [1] to [7] above, the test light may be pulsed light. Furthermore, the first optical power and the second optical power may be the power of light emitted from the first spatial channel and the second spatial channel, respectively, at a timing when a delay time corresponding to twice the distance between the first input / output unit and the reflecting surface has elapsed since the test light was incident on the first spatial channel in the second step. This eliminates the influence of backward Rayleigh scattering light from the first optical power and the second optical power, enabling more accurate measurement of crosstalk.

[0022] [9] In the measurement method of [7] or [8] above, the space division multiplexing optical system may be a space division multiplexing optical fiber, and the light reflecting surface may be an end face of the space division multiplexing optical fiber. The wavelength of the test light is λ (μm), pi is the ratio of the circumference of a circle to its circumference, n is the refractive index of the first spatial channel and the second spatial channel of the space division multiplexing optical fiber, w (μm) is 0.5 times the mode field diameter of the first spatial channel and the second spatial channel, φ (rad) is the angle formed by a plane tangent to the centers of the first spatial channel and the second spatial channel at the end face and a plane perpendicular to the central axis of the space division multiplexing optical fiber, and A is the effective cross-sectional area of ​​the first spatial channel and the second spatial channel. ｅｆｆ (μm ２ ), the Rayleigh scattering loss coefficients of the first spatial channel and the second spatial channel are denoted by α Ｒ (km －１ ), when the pulse width of the test light propagating through the first spatial channel and the second spatial channel in the space division multiplexing optical fiber is T (km), the value of formula (G) may be 10 or more. This makes it possible to make the ratio (A1 / A2, gain of signal light due to reflection) of the power A1 of the pulsed light (first optical power and second optical power) reflected by the optical reflecting surface to the power A2 of the pulsed light of backward Rayleigh scattering light that does not include reflected light generated immediately before reflection 10 dB or more, thereby enabling crosstalk to be measured with higher accuracy.

[0023]

[10] In the measurement methods of [1] to [9] above, the space division multiplexing optical system may be a space division multiplexing optical fiber or a space division multiplexing optical device, the light reflecting surface is an incident / exit surface of the space division multiplexing optical fiber or the space division multiplexing optical device, and the first step may include a step of forming the incident / exit surface by breaking or cleaving the incident / exit portion of the space division multiplexing optical fiber or the space division multiplexing optical device. This makes it possible to easily form the light reflecting surface of the second incident / exit portion.

[0024]

[11] In the measurement methods [1] to

[10] above, the optical reflecting surface may be an incident / exit surface of a space division multiplexing optical system. The first step may include providing a reflective film on the incident / exit surface or immersing the incident / exit surface in liquid metal. This increases the reflectivity of the optical reflecting surface, thereby increasing the first optical power and the second optical power, allowing for more accurate measurement of crosstalk.

[0025]

[12] In the measurement methods [1] to

[11] above, the optical reflecting surface may be an incident / exit surface of a space division multiplexing optical system. The first step may include a step of arranging the incident / exit surface so that it is in contact with only the gas. This increases the reflectivity of the optical reflecting surface, thereby increasing the first optical power and the second optical power, and enabling more accurate measurement of crosstalk.

[0026]

[13] In the measurement method of [1] above, the first step may include connecting a second incident / exit surface of a second space-division multiplexing optical system different from the space-division multiplexing optical system, the second incident / exit surface having a reflective film as a light-reflecting surface provided on the first incident / exit surface, to the incident / exit surface of the space-division multiplexing optical system at the second incident / exit section. This increases the reflectivity of the light-reflecting surface, increases the first optical power and the second optical power, and enables more accurate measurement of crosstalk.

[0027]

[14] In the measurement method of [1] above, the first step may include a step of optically coupling second input / output surfaces of the first single-core optical fiber and the second single-core optical fiber, each having a reflective film as an optical reflecting surface provided on the first input / output surface, to the first spatial channel and the second spatial channel, respectively, at the second input / output section. This increases the reflectivity at the optical reflecting surface, further increasing the first optical power and the second optical power, and enabling more accurate measurement of crosstalk.

[0028]

[15] In the measurement methods of the above [3] to [6], [9] and

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

[0029]

[16] In the measurement method of the above [1] or [2], the space division multiplexing optical system may be a multi-core optical device, a multi-core optical system, a multi-mode optical device, or a multi-mode optical system.

[0030]

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

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

[0031]

[18] A method for measuring inter-core crosstalk in a multi-core optical fiber according to one aspect of the present disclosure is a method for measuring inter-core crosstalk in a multi-core optical fiber having a first end and a second end opposite to the first end, and having first and second cores. The method includes a first step, a second step, a third step, a fourth step, and a fifth step. In the first step, a light reflecting surface is formed or provided at the second end. In the second step, test light is incident on the first core at the first end. In the third step, at least a portion of the test light is reflected by the light reflecting surface. In the fourth step, a first optical power is detected, which is the power of at least a portion of the light output from the first core at the first end, and a second optical power is detected, which is the power of at least a portion of the light output from the second core at the first end. In the fifth step, a magnitude of crosstalk between the first core and the second core is calculated based on the first optical power and the second optical power.

[0032] In the measurement method

[18] , it is sufficient to connect a light source that outputs test light and a photodetector to the first end of the multi-core optical fiber, and there is no need to connect either of them to the second end. Therefore, even when simultaneous access to both ends of the multi-core optical fiber is not easy, the light source and the photodetector can be easily connected. Furthermore, by halving the number of connections in the measurement, crosstalk measurement of the multi-core optical fiber can be performed efficiently. In addition, in the measurement method

[18] , the magnitude of crosstalk is calculated based on the power of light reflected by an optical reflecting surface provided (or formed) at the second end. The power of light reflected by the optical reflecting surface is much greater than the power of backward Rayleigh scattered light in the optical fiber in OTDR measurement. Therefore, even when the crosstalk is small, the detection signal is not buried in noise, and crosstalk can be suitably measured.

[0033]

[19] In the measurement method of

[18] above, the light-reflecting surface may be an end face of a multi-core optical fiber. The first step may include a step of forming a substantially flat end face by forming a minute scratch on the outer periphery of the multi-core optical fiber near the second end and then applying tensile stress or bending stress in the longitudinal axis direction of the fiber to cleave the multi-core optical fiber, thereby forming the substantially flat end face as the light-reflecting surface. This makes it easy to form the light-reflecting surface of the second end. The cleaving method may be a method other than the above, as long as it can form a flat end face on the cut surface of the optical fiber. For example, laser cleaving may be used. Using laser cleaving can form a more uniform end face and make the reflectivity more uniform among the multiple cores. Alternatively, when the second end is mounted on a ferrule, capillary, V-groove, or the like, the light-reflecting surface may be formed by polishing. A more uniform end face can be formed and the reflectivity can be more uniform among the multiple cores.

[0034]

[20] In the measurement method of

[18] or

[19] , the light-reflecting surface may be an end face of a multi-core optical fiber. The first step may include a step of providing a reflective film on the end face or a step of immersing the end face in liquid metal. This increases the reflectivity of the light-reflecting surface, thereby increasing the first optical power and the second optical power, and enabling more accurate measurement of crosstalk.

[0035]

[21] In the measurement method of

[18] or

[19] , the light-reflecting surface may be an end face of a multi-core optical fiber. The first step may include a step of arranging the end face so that it is in contact with only the gas. This increases the reflectivity of the light-reflecting surface, thereby increasing the first optical power and the second optical power, and enabling more accurate measurement of crosstalk.

[0036]

[22] In the measurement methods

[18] to

[21] above, the test light may be pulsed light. The first optical power and the second optical power may be the power of light output from the first core and the second core, respectively, at a timing when a delay time corresponding to twice the optical distance between the first end and the reflecting surface has elapsed since the test light was input into the first core in the second step. This eliminates the influence of backward Rayleigh scattered light from the first optical power and the second optical power, allowing crosstalk to be measured more accurately.

[0037]

[23] In the measurement method of

[22] above, the light reflecting surface may be an end face of a multi-core optical fiber, where λ (μm) is the wavelength of the test light, π is the constant of the circumference of a circle, n is the refractive index of the core of the multi-core optical fiber, w (μm) is 0.5 times the mode field diameter of the first core and the second core, φ (rad) is the angle formed by a plane tangent to the end face at the center of the first core and the second core and a plane perpendicular to the central axis of the multi-core optical fiber, and A is the effective cross-sectional area of ​​the first core and the second core. ｅｆｆ (μm ２ ), the Rayleigh scattering loss coefficients of the first core and the second core are defined as α Ｒ (km －１), when the pulse width of the test light propagating through the cores in the multi-core optical fiber is T (km), the value of formula (H) may be 10 or more. This makes it possible to make the ratio (A1 / A2, gain of signal light due to reflection) of the power A1 of the pulsed light (first optical power and second optical power) reflected by the optical reflecting surface to the power A2 of the pulsed light of backward Rayleigh scattering light that does not include reflected light generated immediately before reflection 10 dB or more, thereby enabling crosstalk to be measured with higher accuracy.

[0038]

[24] In the measurement method of

[18] above, the first step may include a step of connecting a second end face of a second multi-core optical fiber different from the multi-core optical fiber, the second end face having a reflective film as a light-reflecting surface provided on the first end face, to the end face of the multi-core optical fiber at the second end. This increases the reflectivity at the light-reflecting surface, thereby increasing the first optical power and the second optical power, and enabling more accurate measurement of crosstalk.

[0039]

[25] In the measurement method of

[18] above, the first step may include a step of optically coupling second end faces of the first single-core optical fiber and the second single-core optical fiber, each having a reflective film as a light-reflecting surface provided on the first end face, to the first core and the second core, respectively, at the second end. This increases the reflectivity at the light-reflecting surface, further increasing the first optical power and the second optical power, and enabling more accurate measurement of crosstalk.

[0040]

[26] A spatial channel crosstalk measurement device according to one aspect of the present disclosure is a device for measuring spatial channel crosstalk in a space division multiplexing optical system having a first input / output unit and a second input / output unit opposite the first input / output unit, and having N spatial channels (N is an integer equal to or greater than 2). The measurement device includes a light source unit, an optical detector unit, and a calculation unit. The light source unit inputs test light into each of the N spatial channels at the first input / output unit. The optical detector detects first optical power and second optical power. The first optical power is the power of light emitted from the first spatial channel into which the test light was input, among at least a portion of the test light reflected by an optical reflecting surface formed or provided on the second input / output unit. The second optical power is the power of light emitted from a second spatial channel different from the first spatial channel. 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.

[0041] In the measurement device of

[26] above, it is sufficient to connect the light source unit and the photodetector unit to the first input / output port of the space-division multiplexing optical system; there is no need to connect either of them to the second input / output port. Therefore, even when simultaneous access to both input / output ports of the space-division multiplexing optical system is difficult, the light source and the photodetector can be easily connected. Furthermore, by halving the number of connections required for measurement, crosstalk measurement of the space-division multiplexing optical system can be performed efficiently. Additionally, in the measurement device of

[26] above, the magnitude of crosstalk is calculated based on the power of light reflected by an optical reflecting surface provided (or formed) at the second end. The power of light reflected by the optical reflecting surface is significantly greater than the power of backscattered Rayleigh light in OTDR measurement. Therefore, even when crosstalk is small, the detection signal is not buried in noise, and crosstalk can be measured effectively.

[0042]

[27] The spatial channel crosstalk measuring device of

[26] may further include N three-port optical couplers and an optical component. The N three-port optical couplers have a first port, a second port, and a third port, and output light input to the first port from the second port, and output light input to the second port from the third port. The optical component optically couples each of the N spatial channels at the first input / output unit of the space division multiplexing optical system to the second port of each of the N three-port optical couplers. The light source unit is optically coupled to the first port of the N three-port optical couplers. The optical detector unit is optically coupled to the third port of the N three-port optical couplers. This makes it possible to easily realize a configuration in which test light from the light source unit is input to each of the N cores, and the light emitted from the first core and the light emitted from the second core are detected by the optical detector unit.

[0043]

[28] In the spatial inter-channel crosstalk measuring device of

[27] above, the light source unit may have N light sources, and each of the N light sources may be optically coupled to a first port of each of the N three-port optical couplers. This makes it possible to input test light to the first port of each of the N three-port optical couplers with a simple configuration.

[0044]

[29] In the spatial inter-channel crosstalk measuring device according to

[27] , the light source unit may include a single light source and a first optical switch. The first optical switch selectively optically couples the single light source to a first port of any one of the N three-port optical couplers. This allows the number of light sources to be reduced.

[0045]

[30] In the spatial inter-channel crosstalk measuring device according to any one of

[27] to

[29] above, the optical detection unit may include N optical receivers. Each of the N optical receivers may be optically coupled to the third port of each of the N three-port optical couplers. This makes it possible to detect light output from the third port of each of the N three-port optical couplers with a simple configuration.

[0046]

[31] In the spatial inter-channel crosstalk measurement device according to any one of

[27] to

[29] above, the optical detection unit may include a single optical receiver and a second optical switch. The second optical switch selectively optically couples the single optical receiver to any one of the third ports of the N three-port optical couplers. This allows the number of optical receivers to be reduced.

[0047]

[32] In the inter-spatial channel crosstalk measuring device according to any one of

[26] to

[31] above, the first optical power may be determined by analyzing a time-intensity waveform of light, among test light incident on the first spatial channel at the first input / output unit, reflected within the first spatial channel, within the second spatial channel, and at the optical reflecting surface, and then output from the first spatial channel at the first input / output unit, and determining the power of light reflected at a longitudinal position corresponding to the optical reflecting surface and output from the first spatial channel. The second optical power may be determined by analyzing a time-intensity waveform of light, among test light incident on the first spatial channel at the first input / output unit, reflected within the first spatial channel, within the second spatial channel, and at the optical reflecting surface, and then output from the second spatial channel at the first input / output unit, and determining the power of light reflected at a longitudinal position corresponding to the optical reflecting surface and output from the second spatial channel.

[0048]

[33] In the spatial channel crosstalk measurement device according to any one of

[26] to

[31] above, the test light may be pulsed light. The first optical power and the second optical power may be the power of light emitted from the first spatial channel and the second spatial channel, respectively, at a timing when a delay time corresponding to twice the distance between the first input / output unit and the reflecting surface has elapsed since the test light is incident on the first spatial channel. This eliminates the influence of backward Rayleigh scattering light from the first optical power and the second optical power, enabling more accurate measurement of crosstalk.

[0049]

[34] In the spatial channel crosstalk measurement device according to any one of

[26] to

[33] above, the light reflecting surface may be an incident / exit surface of the space division multiplexing optical system. The spatial channel crosstalk measurement device may further include a holder that holds the space division multiplexing optical system so that the incident / exit surface is in contact with only the gas.

[0050]

[35] The spatial inter-channel crosstalk measurement device according to any one of

[26] to

[33] may further include a second space division multiplexing optical system having a reflective film as a light reflecting surface provided on the first incident / exit surface. The second incident / exit surface of the second space division multiplexing optical system may be connected to the incident / exit surface of the space division multiplexing optical system at the second incident / exit section. This increases the reflectivity of the light reflecting surface, thereby increasing the first optical power and the second optical power, enabling more accurate measurement of crosstalk.

[0051]

[36] The spatial channel crosstalk measurement device according to any one of

[26] to

[33] may further include N single spatial channel optical fibers each having a first incident / exit surface provided with a reflective film as an optical reflecting surface. The second incident / exit surface of each of the N single spatial channel optical fibers may be optically coupled to each of the N spatial channels at the second incident / exit section. This increases the reflectivity at the optical reflecting surface, thereby increasing the first optical power and the second optical power, enabling more accurate measurement of crosstalk.

[0052]

[37] In the spatial channel crosstalk measuring device according to

[26] to

[36] above, the space division multiplexing optical system may be a space division multiplexing optical fiber.

[0053]

[38] In the spatial channel crosstalk measuring device of

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

[0054]

[39] In the spatial channel crosstalk measuring device according to any one of

[26] to

[36] above, the space division multiplexing optical system may be a multi-core optical device, a multi-core optical fiber, a multi-mode optical device, a multi-mode optical fiber, or two or more of them optically coupled together.

[0055]

[40] In the inter-spatial channel crosstalk measuring device according to

[26] to

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

[0056]

[41] An apparatus for measuring inter-core crosstalk of a multi-core optical fiber according to one aspect of the present disclosure is an apparatus for measuring inter-core crosstalk of a multi-core optical fiber having a first end and a second end opposite to the first end and having N cores (N is an integer equal to or greater than 2). The apparatus includes a light source unit, a light detection unit, and a calculation unit. The light source unit incidents test light onto each of the N cores at the first end. The light detection unit detects, of at least a portion of the test light reflected by a light reflecting surface formed or provided at the second end, a first optical power which is the power of light emitted from the first core onto which the test light was incident, and a second optical power which is the power of light emitted from a second core different from the first core. The calculation unit calculates the magnitude of crosstalk between the first core and the second core based on the first optical power and the second optical power.

[0057] In the measurement device of

[41] above, it is sufficient to connect the light source unit and the photodetector unit to the first end of the multi-core optical fiber, and there is no need to connect either of them to the second end. Therefore, even when simultaneous access to both ends of the multi-core optical fiber is not easy, the light source and the photodetector can be easily connected. Furthermore, by halving the number of connections in measurement, crosstalk measurement of the multi-core optical fiber can be efficiently performed. In addition, in the measurement device of

[41] above, the magnitude of crosstalk is calculated based on the power of light reflected by a light reflecting surface provided (or formed) at the second end. The power of light reflected by the light reflecting surface is much greater than the power of backscattered Rayleigh scattered light in OTDR measurement. Therefore, even when crosstalk is small, the detection signal is not buried in noise, and crosstalk can be suitably measured.

[0058]

[42] The measurement device of

[41] above may further include N optical circulators and an optical component. The N optical circulators have a first port, a second port, and a third port, and output light input to the first port from the second port, and output light input to the second port from the third port. The optical component optically couples each of the N cores at the first end of the multi-core optical fiber to the second port of each of the N optical circulators. The light source unit is optically coupled to the first port of the N optical circulators. The optical detection unit is optically coupled to the third port of the N optical circulators. This makes it possible to easily realize a configuration in which test light from the light source unit is incident on each of the N cores, and the light emitted from the first core and the light emitted from the second core are detected by the optical detection unit.

[0059]

[43] In the measurement device of

[42] above, the light source unit may have N light sources, and each of the N light sources may be optically coupled to the first port of each of the N optical circulators. This makes it possible to input test light to the first port of each of the N optical circulators with a simple configuration.

[0060]

[44] In the measurement device of

[42] above, the light source unit may include a single light source and a first optical switch. The first optical switch selectively optically couples the single light source to a first port of any one of the N optical circulators. This makes it possible to reduce the number of light sources.

[0061]

[45] In the measurement device of any one of

[42] to

[44] above, the optical detection unit may have N optical receivers. Each of the N optical receivers may be optically coupled to the third port of each of the N optical circulators. This makes it possible to detect light output from the third port of each of the N optical circulators with a simple configuration.

[0062]

[46] In the measurement device of

[43] or

[44] above, the optical detection unit may include a single optical receiver and a second optical switch. The second optical switch selectively optically couples the single optical receiver to any third port of the N optical circulators. This makes it possible to reduce the number of optical receivers.

[0063]

[47] In the measurement device of any one of

[41] to

[46] above, the test light may be pulsed light. The first optical power and the second optical power may be the power of light output from the first core and the second core, respectively, at a timing when a delay time corresponding to twice the optical distance between the first end and the reflecting surface has elapsed since the test light was input into the core. This makes it possible to eliminate the influence of backward Rayleigh scattered light from the first optical power and the second optical power, thereby enabling more accurate measurement of crosstalk.

[0064]

[48] ​​In the measurement device of any one of

[41] to

[47] above, the light-reflecting surface may be an end face of a multi-core optical fiber. The measurement device may further include a holding unit that holds the multi-core optical fiber so that the end face is in contact with only the gas. This increases the reflectivity of the light-reflecting surface, thereby increasing the first optical power and the second optical power, and enabling more accurate measurement of crosstalk.

[0065]

[49] The measurement device according to any one of

[41] to

[47] may further include a second multi-core optical fiber having a first end face provided with a reflective film as a light-reflecting surface. The second end face of the second multi-core optical fiber is connected to the end face of the multi-core optical fiber at a second end. This increases the reflectivity at the light-reflecting surface, thereby increasing the first optical power and the second optical power, and enabling more accurate measurement of crosstalk.

[0066]

[50] The measurement device according to any one of

[41] to

[47] may further include N single-core optical fibers each having a first end face provided with a reflective film as a light-reflecting surface. The second end face of each of the N single-core optical fibers is optically coupled to each of the N cores at the second end. This increases the reflectivity at the light-reflecting surface, thereby increasing the first optical power and the second optical power, and enabling more accurate measurement of crosstalk.

[0067] [Details of the embodiments of the present disclosure] Specific examples of the method for measuring crosstalk between spatial channels, the method for measuring crosstalk between cores in a multi-core optical fiber, the device for measuring crosstalk between spatial channels, and the device for measuring crosstalk between cores in a multi-core optical fiber according to the present embodiments will be described with reference to the drawings as necessary. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the following description, the same elements in the drawings will be denoted by the same reference numerals, and duplicated description will be omitted.

[0068] FIG. 1 is a diagram illustrating the configuration of a measurement apparatus 1A according to an embodiment of the present disclosure. The measurement apparatus 1A is an apparatus for measuring crosstalk between spatial channels in a space division multiplexing (SDM) optical system. The concept of an SDM optical system includes an SDM optical fiber, an SDM optical device, and a system in which one or more SDM optical fibers and one or more SDM optical devices are optically coupled. An SDM optical fiber is, for example, a multi-core optical fiber (hereinafter referred to as MCF) having multiple cores within a single fiber or a multi-mode optical fiber having multiple spatial modes (also referred to as propagation modes or guided modes) within a single core. Each core of an MCF has one or more spatial modes. Therefore, there are optical fibers that are both MCF and MMF. An SDM optical fiber includes all optical fibers that have multiple spatial modes within a single fiber. A spatial channel in an SDM optical fiber corresponds to a spatial mode. The concept of an SDM optical device includes optical devices other than SDM optical fibers that have multiple spatial channels, such as optical waveguide chips and SDM optical fiber fan-outs (such as free-space optical fan-outs, fiber bundle fan-outs, and optical waveguide fan-outs). For example, in optical devices with cores, such as optical waveguide chips, fiber bundle fan-outs, or optical waveguide fan-outs, spatial modes correspond to spatial channels. In free-space optical SDM optical devices, spatial channels do not necessarily correspond to spatial modes in the portion where light propagates through free space. However, free-space optical SDM optical devices are optically coupled to other optical fibers or optical devices at their input and output ports. Therefore, optical paths in the free-space optical SDM optical device that correspond to the spatial modes of other optical fibers or optical devices correspond to spatial channels. The concept of an SDM optical system includes, for example, SDM optical fibers and SDM optical devices optically coupled or connected to each other. An SDM optical system has multiple spatial channels. The following description explains an apparatus for measuring inter-core crosstalk in an MCF 10, an example of an SDM optical system.

[0069] First, the configuration of the MCF 10 will be described. The MCF 10 has a first end (first input / output portion) 10a and a second end (second input / output portion) 10b opposite the first end 10a. FIG. 2 is a diagram showing a cross section perpendicular to the central axis of the MCF 10. As shown in FIG. 2, the MCF 10 has a glass fiber 11 and a coating resin 12 that coats the outer surface of the glass fiber 11. The glass fiber 11 has N cores as N spatial channels (N is an integer of 2 or greater). In the illustrated example, the glass fiber 11 has four cores (i.e., N=4): 13a, 13b, 13c, and 13d. Furthermore, the glass fiber 11 has a cladding 14. Furthermore, the glass fiber 11 may have a marker 15. In the illustrated example, the cores 13 a, 13 b, 13 c, and 13 d are arranged at equal intervals on concentric circles centered on the central axis of the MCF 10 in a cross section perpendicular to the central axis of the MCF 10. The cladding 14 is a common cladding that surrounds the cores 13 a, 13 b, 13 c, and 13 d and the marker 15. The marker 15 has a refractive index different from that of the cladding 14.

[0070] 1 again, the measurement device 1A includes a light source unit 20A, a light detection unit 30A, a circulator unit (optical coupler unit) 40, a fan-in / fan-out (FIFO) 50, and a calculation unit 60.

[0071] The light source unit 20A emits test light at the first end 10a, which is incident on one of the cores 13a, 13b, 13c, and 13d. 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 cores 13a, 13b, 13c, and 13d. The first optical switch 22 selectively optically couples the input port 22a to one of the output ports 22b, 22c, 22d, and 22e. The input port 22a is optically coupled to the light source 21.

[0072] The optical detection unit 30A detects light emitted from each of the cores 13a, 13b, 13c, and 13d at the first end 10a. The optical detection unit 30A of this embodiment has 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 cores 13a, 13b, 13c, and 13d. The second optical switch 32 selectively optically couples the output port 32a to one of the input ports 32b, 32c, 32d, and 32e. The output port 32a is optically coupled to the optical receiver 31.

[0073] The circulator unit 40 has optical circulators 41, 42, 43, and 44, which are three-port optical couplers, the same number as the cores 13a, 13b, 13c, and 13d. FIG. 3 is a diagram showing the configuration of the optical circulator 41. The configurations of the optical circulators 42, 43, and 44 are the same as the configuration of the optical circulator 41. As shown in FIG. 3, the optical circulator 41 has a first port P1, a second port P2, and a third port P3. The optical circulator 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. Almost no light L1 input to the first port P1 is output from the third port P3. Almost no light L2 input to the second port P2 is output from the first port P1. Almost no light input to the third port P3 is output from either the first port P1 or the second port P2.

[0074] 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.

[0075] Referring again to FIG. 1 , the first ports P1 of the optical circulators 41, 42, 43, and 44 are optically coupled to the output ports 22b, 22c, 22d, and 22e, respectively, of the first optical switch 22. This allows the first optical switch 22 to selectively optically couple the light source 21 to one of the first ports P1 of the optical circulators 41, 42, 43, and 44. The third ports P3 of the optical circulators 41, 42, 43, and 44 are optically coupled to the input ports 32b, 32c, 32d, and 32e, respectively, of the second optical switch 32. This allows the second optical switch 32 to selectively optically couple the optical receiver 31 to one of the third ports P3 of the optical circulators 41, 42, 43, and 44. In the above description, three-port optical circulators 41, 42, 43, and 44 are used as an example of a three-port optical coupler, but the three-port optical coupler is not limited to this and may be a 1x2 optical fiber coupler or a 2x2 optical fiber coupler in which one port is terminated for reflection suppression. By using a fused optical fiber coupler as the optical fiber coupler, reflection within the optical fiber coupler can be suppressed. The optical fiber coupler may also be a waveguide-type optical fiber coupler.

[0076] When the three-port optical coupler is a 1x2 optical fiber coupler or a 2x2 optical fiber coupler with one port terminated for reflection suppression, 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 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 output almost exclusively from the third port P3. Light L2 input to the second port P2 is output with low loss from the first port P1, and light input to the third port P3 is also output with low loss from both the second port P2 and the third port P3, but this does not significantly affect the measurement. Light input to the third port P3 is output almost exclusively from the first port P1. However, in a 1x2 optical fiber coupler or a 2x2 optical fiber coupler in which one port is subjected to reflection suppression termination processing, the insertion loss between port P1 and port P2 and the insertion loss between port P2 and port P3, which have low insertion losses, are higher than those in an optical circulator. This is because, for example, if a 1x2 optical fiber coupler or a 2x2 optical fiber coupler is an optical power splitter with a branching ratio of 50:50, a theoretical loss of about 3 dB occurs.

[0077] 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.

[0078] The FIFO 50 is an optical component that optically couples each of the cores 13a, 13b, 13c, and 13d at the first end 10a of the MCF 10 to the second ports P2 of the optical circulators 41, 42, 43, and 44. The FIFO 50 has an input / output port 50a connected to the MCF 10 and input / output ports 50b, 50c, 50d, and 50e connected to the second ports P2 of the optical circulators 41, 42, 43, and 44, respectively. The input / output port 50a of the FIFO 50 is formed, for example, by thinning the tip ends of the same number of single-core fibers as the cores 13a, 13b, 13c, and 13d of the MCF 10 by etching and bundling these tip ends. In the bundle at the tip end, the spacing between the cores of the single-core fibers is the same as the spacing between the cores 13a, 13b, 13c, and 13d of the MCF 10. When the bundle at the tip end abuts against the first end 10a of the MCF 10, the first end of each single-core fiber is optically coupled to the cores 13a, 13b, 13c, and 13d, respectively. The second end of each single-core fiber constitutes the input / output ports 50b, 50c, 50d, and 50e, respectively, and is optically coupled to the second port P2 of the optical circulators 41, 42, 43, and 44, respectively.

[0079] The calculation unit 60 calculates the magnitude of inter-core crosstalk based on the detection result of the optical detection unit 30A. The calculation unit 60 is configured by a computer including, for example, a CPU, a memory, and a storage device. 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 communicably) connected to the optical receiver 31.

[0080] FIG. 4 shows the configuration of a measurement apparatus 1B, which is a variation of the measurement apparatus 1A. The measurement apparatus 1B includes an optical detector 30B instead of the optical detector 30A shown in FIG. The optical detector 30B has optical receivers (power meters) 33, 34, 35, and 36, the same number 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 ports P3 of the optical circulators 41, 42, 43, and 44, respectively. The calculation unit 60 is electrically (or communicably) connected to the optical receivers 33, 34, 35, and 36.

[0081] 5 is a diagram showing the configuration of a measurement apparatus 1C as another modified example of the measurement apparatus 1A. The measurement apparatus 1C includes a light source unit 20B instead of the light source unit 20A shown in FIG. 1. The light source unit 20B has light sources 23, 24, 25, and 26, the same number 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 optical circulators 41, 42, 43, and 44, respectively. The measurement apparatus may include a light detector unit 30B instead of the light detector unit 30A, and a light source unit 20B instead of the light source unit 20A.

[0082] Here, an example of the configuration of the second end 10b of the MCF 10 will be described. The second end 10b is formed or provided with a light-reflecting surface that reflects at least a portion of the test light propagating through each of the cores 13a, 13b, 13c, and 13d from the first end 10a to the second end 10b. The light-reflecting surface is configured to reflect the test light with a reflectance of 0.001 or more, 0.01 or more, 0.1 or more, or 0.5 or more. FIGS. 6 to 12 are cross-sectional views showing examples of the light-reflecting surface of the second end 10b, taken along the central axis AX of the glass fiber 11 of the MCF 10. The coating resin 12 is omitted from these figures. The second end 10b includes an end face (incident / exit face) 11a of the glass fiber 11. Here, reflectance can be defined as Pr / Pi, where Pi is the power of light incident on the light-reflecting surface and Pr is the power of light reflected by the light-reflecting surface. IEC-61300-3-6 defines the return loss as the value obtained by multiplying the decibel value of the reflectance (Pr / Pi) by −1, and also provides an example of a method for measuring return loss, so the reflectance of a light-reflecting surface can also be measured by the method described in IEC-61300-3-6.

[0083] In the example shown in FIG. 6 , the end face 11a of the glass fiber 11 is a light-reflecting surface that reflects at least a portion of the test light due to the difference in refractive index between the glass fiber 11 and an external medium (e.g., air). In this example, the end face 11a is a cleaved or polished flat surface. In this example, the end faces 131 of the cores 13a, 13b, 13c, and 13d are flush with the end face 11a without any step (the end face 131 may be concave or convex with respect to the end face 11a by a distance of significantly less than 1 μm). When an imaginary plane H2 parallel to the end face 11a is defined, the centers of the end faces 131 of the cores 13a, 13b, 13c, and 13d at the end face 11a are tangent to the imaginary plane H2. The imaginary plane H2 forms an angle φ with an imaginary plane H1 perpendicular to the central axis AX of the MCF 10. The angle φ is an angle (unit: rad) defined by the radian system. In one example, the angle φ is equal to or greater than 0 rad and equal to or less than 2π / 180 rad.

[0084] In the example shown in FIG. 7 , the end face 11 a of the glass fiber 11 is also a light-reflecting surface that reflects at least a portion of the test light due to the difference in refractive index between the glass fiber 11 and an external medium (e.g., air). However, in this example, the end face 11 a is not flat but is a curved surface that is convex outward, i.e., toward the external medium. In this example, the end faces 131 of the cores 13 a, 13 b, 13 c, and 13 d are flush with the end face 11 a without any step (the end face 131 may be concave or convex relative to the end face 11 a by a distance sufficiently smaller than 1 μm). An imaginary plane H2 a is defined on the end face 11 a, tangent to the center of the end face 131 of the cores 13 a, 13 b, 13 c, and 13 d. The imaginary plane H2 a forms an angle φ with the imaginary plane H1 that is perpendicular to the central axis AX of the MCF 10. The range of the angle φ is the same as that of the embodiment shown in FIG. 6 .

[0085] 8, a reflective film 71 is provided on the end face 11a of the glass fiber 11. The reflective film 71 is, for example, a dielectric multilayer film or a metal film. The interface between the reflective film 71 and the glass fiber 11 (i.e., the end face 11a) is a light-reflecting surface that reflects at least a portion of the test light.

[0086] 9, a reflecting member 72 is provided on the end face 11a of the glass fiber 11. The reflecting member 72 is provided from the end face 11a to the side face of the glass fiber 11 and surrounds the second end 10b. The reflecting member 72 is, for example, a dielectric multilayer film, a metal film, or a liquid metal. The interface between the reflecting member 72 and the glass fiber 11 (i.e., the end face 11a) is a light-reflecting surface that reflects at least a portion of the test light.

[0087] 10 , the end face 11 a of the glass fiber 11 is immersed in liquid metal 74. The liquid metal 74 is contained in a container 73 that is open at the top, and the end face 11 a of the glass fiber 11 is immersed from above in the liquid metal 74. When the end face 11 a comes into contact with the liquid metal 74, the interface between the liquid metal 74 and the glass fiber 11 (i.e., the end face 11 a) becomes a light-reflecting surface that reflects at least a portion of the test light.

[0088] In the example shown in FIG. 11 , the measurement apparatus 1A, 1B, or 1C further includes a reflection enhancement device 80 at the second end 10b of the MCF 10. The reflection enhancement device 80 includes an MCF 81 separate from the MCF 10 and a reflective film 82 provided on a first end face 81a of the MCF 81. The reflective film 82 is, for example, a dielectric multilayer film or a metal film. The second end face of the MCF 81 is connected to the end face 11a of the MCF 10 at the second end 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 may be by fusion splicing or by using an optical connector. In this example, the reflective film 82 constitutes a light-reflecting surface that reflects at least a portion of the test light.

[0089] In the example shown in FIG. 12 , the measurement apparatus 1A, 1B, or 1C further includes a reflection enhancement device 90 at the second end 10 b of the MCF 10. The reflection enhancement device 90 includes single-core optical fibers (SCFs) 92, the number of which is equal to the number of cores 13 a, 13 b, 13 c, and 13 d of the MCF 10, and a FIFO 91. A first end face 92 a of each SCF 92 is provided with a reflective film 93 as a light-reflecting surface that reflects at least a portion of the test light. The reflective film 93 is, for example, a dielectric multilayer film or a metal film. The second end face of each SCF 92 is optically coupled to the cores 13 a, 13 b, 13 c, and 13 d at the second end 10 b via the FIFO 91.

[0090] Referring back to FIG. 1 , the test light output from the light source unit 20A may be continuous light, chopped light, or pulsed light. FIG. 13 is a graph schematically illustrating the time waveform of the optical power of continuous light. When the test light is continuous light, its optical power is constant regardless of time. Furthermore, when the test light is continuous light, a general optical power meter can be used as the optical receivers 31, 33, 34, and 35. FIG. 14 is a graph schematically illustrating the time waveform of the optical power of chopped light. When the test light is chopped light, its time waveform is a rectangular waveform. The duty ratio (the proportion of the time during which the optical power is at its peak relative to the total time) is, for example, 0.5 or 0.25. When the test light is chopped light, using an optical power meter compatible with chopping detection (synchronous detection / phase detection) as the optical receivers 31, 33, 34, and 35 can suppress measurement noise caused by ambient light and improve detection sensitivity. 15 is a graph showing a schematic diagram of the time waveform of the optical power of pulsed light. When the test light is pulsed light, multiple pulse waveforms are repeated at a predetermined period, and measurement noise can be reduced by repeating measurements and averaging them. By making the pulse width sufficiently smaller than the pulse repetition period, the time waveform of the optical power can be measured with sufficiently high spatial resolution over the entire length of the optical fiber.

[0091] Next, the operation of the measurement apparatus 1A, 1B, or 1C according to this embodiment will be described, along with a method for measuring inter-core crosstalk according to this embodiment using the measurement apparatus 1A, 1B, or 1C. Fig. 16 is a flowchart showing the method for measuring inter-core crosstalk according to this embodiment.

[0092] First, in step ST1, a light-reflecting surface is formed or provided on the second end 10b. Examples of the light-reflecting surface are as described in Figures 8 to 10. That is, step ST1 may include step ST11 of providing a reflective film 71 (see Figure 8) or a reflective member 72 (see Figure 9) on the end face 11a of the glass fiber 11, or immersing the end face 11a in liquid metal 74 (see Figure 10).

[0093] Next, in step ST2, test light is incident from light source unit 20A or 20B onto one of cores 13a, 13b, 13c, and 13d (first spatial channel or first core; here, core 13a is used as an example) at first end 10a. At this time, light source unit 20A (see FIGS. 1 and 4) switches first optical switch 22 to cause test light to be incident onto core 13a. First optical switch 22 may be switched by a control signal from calculation unit 60 or may be switched manually. Furthermore, light source unit 20B (see FIG. 5) causes test light to be incident from light source 23, among light sources 23, 24, 25, and 26, which corresponds to core 13a.

[0094] Subsequently, in step ST3, at least a part of the test light is reflected by the light reflecting surface of the second end portion 10b.

[0095] Thereafter, a portion of the light reflected by the light reflecting surface is emitted from the core 13a, and another portion is emitted from cores other than the core 13a (second spatial channels or second cores; in this case, cores 13b, 13c, and 13d) due to inter-core crosstalk. In step ST4, a first optical power, which is the power of the light emitted from the core 13a, and a second optical power, which is the power of the light emitted from each of the cores 13b, 13c, and 13d, are detected. At this time, the optical detection unit 30A (see FIGS. 1 and 5 ) switches the second optical switch 32, thereby causing the light from the cores 13a, 13b, 13c, and 13d to be sequentially incident on the optical receiver 31. The second optical switch 32 may be switched by a control signal from the calculation unit 60 or may be switched manually. In the light detection section 30B (see FIG. 4), the light from the cores 13a, 13b, 13c, and 13d is incident on the light receivers 33, 34, 35, and 36 corresponding to the cores 13a, 13b, 13c, and 13d, respectively.

[0096] The first optical power is the sum of the optical power components emitted from the core 13 a (first spatial channel) among the return optical power components of the test light including backscattered light at each position in the longitudinal direction of the MCF 10. The second optical power is the sum of the optical power components emitted from the cores 13 b, 13 c, and 13 d (second spatial channels) among the return optical power components of the test light including backscattered light at each position in the longitudinal direction of the MCF 10.

[0097] Subsequently, in step ST5, based on the first optical power and the second optical power, the magnitude of crosstalk between the core 13a and the cores 13b, 13c, and 13d is calculated in the calculation unit 60. At this time, the magnitude XT of crosstalk between the core 13a and the cores 13b, 13c, and 13d is calculated by the following formula (1) based on the first optical power PW1 and the second optical power PW2.

[0098] When measuring the first optical power PW1, the power of the light emitted from the core 13a at the first end 10a is measured. At this time, the light emitted from the core 13a includes not only light reflected by the light reflecting surface at the second end 10b but also backscattered light (noise light) generated when the test light incident on one of the cores 13a, 13b, 13c, and 13d at the first end 10a propagates longitudinally through the MCF 10, and unintended reflected light generated at places other than the light reflecting surface. Similarly, when measuring the second optical power PW2, the power of the light emitted from each of the cores 13b, 13c, and 13d at the first end 10a is measured. At this time, the light emitted from each of the cores 13b, 13c, and 13d includes not only light reflected by the light-reflecting surface of the second end 10b, but also backscattered light generated when the test light incident on one of the cores 13a, 13b, 13c, and 13d at the first end 10a propagates longitudinally through the MCF 10, and returned light (noise light) such as unintentional reflected light generated at places other than the light-reflecting surface.

[0099] The above-described steps ST2 to ST5 are repeated in sequence with the cores 13a, 13b, 13c, and 13d as the first cores (first spatial channels) onto which the test light is incident.

[0100] 17 is a flowchart showing a modified example of the inter-core crosstalk measurement method. In this modified example, step ST1 of forming or providing a light-reflecting surface at second end 10b includes step ST12 instead of step ST11. In step ST12, as shown in FIG. 11 , the second end face of MCF 81, which has a reflective film 82 as a light-reflecting surface provided on its first end face 81a, is connected to end face 11a of MCF 10 at second end 10b.

[0101] 18 is a flowchart showing another variation of the inter-core crosstalk measuring method. In this variation, step ST1 of forming or providing a light-reflecting surface at second end 10b includes step ST13 instead of step ST11. In step ST13, as shown in FIG. 12 , the second end face of each of multiple SCFs 92, each of which has a reflective film 93 as a light-reflecting surface provided on its first end face 92a, is optically coupled to cores 13a, 13b, 13c, and 13d at second end 10b.

[0102] 19 is a flowchart showing yet another modified example of the inter-core crosstalk measuring method. In this modified example, step ST1 of forming or providing a light-reflecting surface at the second end 10b includes step ST14 instead of step ST11 described above. In step ST14, as shown in FIG. 6 or 7, the end face 11a of the glass fiber 11 is polished or cleaved to form a flat surface or a curved surface, which is used as the light-reflecting surface. Alternatively, the end face 11a may be simply cleaved to form a substantially flat surface, and used as the light-reflecting surface.

[0103] The effects obtained by the measurement apparatuses 1A, 1B, and 1C and the inter-core crosstalk measurement method according to the present embodiment described above will now be described. FIG. 27 is a diagram schematically illustrating the configuration of a measurement apparatus 200 according to a reference example. The measurement apparatus 200 shown in FIG. 27 is an apparatus for measuring inter-core crosstalk of an MCF 10, and 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, the first optical switch 202, and the FIFO 203 are connected to a first end 10a of the MCF 10. The FIFO 204, the second optical switch 205, and the optical receiver 206 are connected to a second end 10b of the MCF 10. Test light output from the light source 201 is incident at the first end 10a on a first core (first spatial channel) selected by the first optical switch 202 from among the multiple cores of the MCF 10. At the second end 10b, the second optical switch 205 sequentially selects the first core to which the test light is incident and a second core (second spatial channel) different from the first core. The optical power of the test light propagated through the first core and the optical power of the crosstalk light propagated through the second core are detected by the optical receiver 206. Based on these optical powers, inter-core crosstalk is calculated.

[0104] 27 , test light is input to a first core at a first end 10 a of an MCF 10, and the power of the test light output from the first core and the second core is detected at a second end 10 b of the MCF 10. However, when the MCF 10 to be measured is installed, it may not be easy to connect a unit consisting of a light source 201, a first optical switch 202, and a FIFO 203 to the first end 10 a of the MCF 10 and to connect a unit consisting of a FIFO 204, a second optical switch 205, and a photodetector 206 to the second end 10 b of the MCF 10, which is far away from the first end 10 a.

[0105] In the measurement apparatuses 1A, 1B, and 1C and the measurement method of this embodiment, it is sufficient to connect the light source unit 20A or 20B and the optical detection unit 30A or 30B to the first end 10a of the MCF 10; there is no need to connect either of these units to the second end 10b. Therefore, even in an MCF 10 in which the first end 10a and the second end 10b are separated by several kilometers or more, the light source unit 20A or 20B and the optical detection unit 30A or 30B can be easily connected to the MCF 10. Furthermore, in the measurement apparatuses 1A, 1B, and 1C and the measurement method of this embodiment, the magnitude of crosstalk is calculated based on the power of light reflected by a light-reflecting surface provided (or formed) at the second end 10b. The power of light reflected by the light-reflecting surface is significantly greater than the power of backward Rayleigh scattered light. Therefore, even when the crosstalk is small, the detection signal is not buried in noise, and inter-core crosstalk can be suitably measured.

[0106] As described above, the optical reflecting surface may be configured to reflect at least a portion of the test light with a reflectance of 0.001 or greater, in which case the first optical power PW1 and the second optical power PW2 are set to be sufficiently large to reduce errors caused by backscattering in the measurement results and improve the accuracy of inter-core crosstalk measurement.

[0107] As described above, the light-reflecting surface is the end face 11 a of the MCF 10, and step ST1 may include step ST11 of providing a reflective film 71 or a reflective member 72 on the end face 11 a, or immersing the end face 11 a in a liquid metal 74. This increases the reflectivity of the light-reflecting surface, making the first optical power PW1 and the second optical power PW2 larger, and enabling more accurate measurement of crosstalk.

[0108] As described above, the measurement apparatuses 1A, 1B, and 1C may include an MCF 81 having a first end face 81a provided with a reflective film 82 as a light-reflecting surface. The second end face of the MCF 81 is connected to the end face 11a of the MCF 10 at the second end 10b. Similarly, in the measurement method, step ST1 may include step ST12 of connecting the second end face of the MCF 81 having a first end face 81a provided with a reflective film 82 as a light-reflecting surface to the end face 11a of the MCF 10 at the second end 10b. These measurement apparatuses 1A, 1B, and 1C and measurement methods increase the reflectivity of the light-reflecting surface, thereby increasing the first optical power PW1 and the second optical power PW2, thereby enabling more accurate measurement of crosstalk. Furthermore, the measurement can be performed simply by connecting the MCF 81 to the end face 11a of the MCF 10 without providing a light-reflecting film on the end face 11a of the MCF 10, thereby simplifying the measurement process.

[0109] As described above, the measurement apparatuses 1A, 1B, and 1C may include SCFs 92, each having a first end face 92a provided with a reflective film 93 as a light-reflecting surface, and the number of SCFs 92 is the same 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 end 10b. Similarly, in the measurement method, step ST1 may include step ST13, in which the second end face of each SCF 92, each having a first end face 92a provided with a reflective film 93 as a light-reflecting surface, and the number of SCFs 92 is the same 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 end 10b. These measurement devices 1A, 1B, and 1C and measurement methods can increase the reflectivity of the optical reflecting surface, thereby increasing the first optical power PW1 and the second optical power PW2, and thereby measuring crosstalk with higher accuracy. Furthermore, since it is not necessary to provide an optical reflecting film on the end face 11a of the MCF 10, it is possible to simply connect an optical component such as a FIFO 91 to the end face 11a of the MCF 10, thereby simplifying the measurement process.

[0110] As described above, in the measurement apparatuses 1A, 1B, and 1C, the end face 11a of the MCF 10 may be used as a light reflecting surface, which eliminates the need for special devices such as the reflection enhancing devices 80 and 90, thereby simplifying the configuration of the measurement apparatus.

[0111] As in the present embodiment, the measurement apparatuses 1A, 1B, and 1C may include optical circulators 41, 42, 43, and 44 and a FIFO 50. The optical circulators 41, 42, 43, and 44 have a first port P1, a second port P2, and a third port P3, and output light input to the first port P1 from the second port P2, and output light input to the second port P2 from the third port P3. The FIFO 50 optically couples each of the cores 13a, 13b, 13c, and 13d at the first end 10a of the MCF 10 to the second port P2 of each of the optical circulators 41, 42, 43, and 44. The light source unit 20A or 20B is optically coupled to the first port P1 of the optical circulators 41, 42, 43, and 44. The optical detection unit 30A or 30B is optically coupled to the third port P3 of the optical circulators 41, 42, 43, and 44. This makes it possible to easily realize a configuration 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 light emitted from the core to which the test light is incident and light emitted from another core are detected by the optical detection unit 30A or 30B.

[0112] 5, the light source unit 20B may have the same number of light sources 23, 24, 25, and 26 as the cores 13a, 13b, 13c, and 13d, and the light sources 23, 24, 25, and 26 may be optically coupled to the first port P1 of each of the optical circulators 41, 42, 43, and 44. This allows test light to be input to the first port P1 of each of the optical circulators 41, 42, 43, and 44 with a simple configuration.

[0113] 1, 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 a first port P1 of any one of the optical circulators 41, 42, 43, and 44. This makes it possible to reduce the number of light sources.

[0114] 4, the optical detection unit 30B may have the same number of optical receivers 33, 34, 35, and 36 as the number of cores 13a, 13b, 13c, and 13d, and the optical receivers 33, 34, 35, and 36 may be optically coupled to the third port P3 of each of the optical circulators 41, 42, 43, and 44. This makes it possible to detect light output from the third port P3 of each of the optical circulators 41, 42, 43, and 44 with a simple configuration.

[0115] 1, the optical detection unit 30A may include a single optical receiver 31 and a second optical switch 32 that selectively optically couples the single optical receiver 31 to a third port P3 of any one of the optical circulators 41, 42, 43, and 44. This makes it possible to reduce the number of optical receivers.

[0116] The inter-core crosstalk of the 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 the inter-core crosstalk of the MCF 10. This makes it possible to reduce the inter-core crosstalk measurement error of the MCF 10 caused by the inter-core crosstalk of the FIFO 50 to 1 dB or less, 0.5 dB or less, 0.2 dB or less, or 0.1 dB or less, respectively, and enables the inter-core crosstalk of the MCF 10 to be measured with higher accuracy.

[0117] Here, we will describe crosstalk measurement errors in the measurement apparatuses 1A, 1B, and 1C and the measurement method of this embodiment. In addition to light reflected from the light-reflecting surface, backward Rayleigh scattered light is also incident on the optical detection unit 30A (30B). If the first optical power PW1 and the second optical power PW2 are too small due to reasons such as low reflectivity of the light-reflecting surface or high transmission loss caused by an excessively long MCF 10, the difference between the first optical power PW1 and the second optical power PW2 and the optical power of the backward Rayleigh scattered light becomes small, and the measurement accuracy of inter-core crosstalk decreases.

[0118] The present inventors have found that when the optical reflecting surface is the end face 11a of the MCF 10 (see FIGS. 6 and 7), the measurement error of the inter-core crosstalk due to backward Rayleigh scattering in measurements using continuous light or chopped light is expressed in decibels as ΔXT ｄＢ It has been found that [dB] can be expressed by the following formula (2). where λ is the wavelength of the test light (μm), π is the ratio of the circumference of a circle to its diameter, n is the refractive index of the cores 13 a, 13 b, 13 c, and 13 d of the MCF 10, and α R is the Rayleigh scattering loss coefficient (km －１ ), α is the transmission loss coefficient (km －１ ), w is 0.5 times (μm) the mode field diameter of the cores 13a, 13b, 13c, and 13d, φ is the angle (rad) formed by an imaginary plane H2 (or H2a) that is in contact with the centers of the cores 13a, 13b, 13c, and 13d at the end face 11a and an imaginary plane H1 that is perpendicular to the central axis AX of the MCF 10, L is the length (km) of the MCF 10, A ｅｆｆ is the effective cross-sectional area (μm ２ ) where, however, when the refractive indexes of two or more of the cores 13a, 13b, 13c, and 13d are different from each other, n is the average value of the refractive indexes of the cores 13a, 13b, 13c, and 13d. Similarly, when the Rayleigh scattering loss coefficients of two or more of the cores 13a, 13b, 13c, and 13d are different from each other, α Ris the average value of the Rayleigh scattering loss coefficients of the cores 13a, 13b, 13c, and 13d. When the transmission loss coefficients of two or more of the cores 13a, 13b, 13c, and 13d are different from each other, α is the average value of the transmission loss coefficients of the cores 13a, 13b, 13c, and 13d. When the mode field diameters of two or more of the cores 13a, 13b, 13c, and 13d are different from each other, w is 0.5 times the average value of the mode field diameters of the cores 13a, 13b, 13c, and 13d. When the angles formed between the imaginary plane H2 and the imaginary plane H1 are different from each other in two or more of the cores 13a, 13b, 13c, and 13d, φ is the average value of the angles of the cores 13a, 13b, 13c, and 13d. When the effective cross-sectional areas of two or more of the cores 13a, 13b, 13c, and 13d are different from each other, A ｅｆｆ is the average value of the effective cross-sectional areas of the cores 13a, 13b, 13c, and 13d. R , α, w are values ​​at wavelength λ.

[0119] The average value of the transmission loss of the cores 13a, 13b, 13c, and 13d is α ｄＢ It is expressed as L (dB). ｄＢ (dB·km －１ ) is the average value of the transmission loss coefficient α (km －１ ) by 10 / ln(10) (≈4.34). Also, φ (rad) expressed in radians can be converted to θ (degrees) (θ = 180φ / π) in degrees. The measurement error of inter-core crosstalk due to backward Rayleigh scattering is ｅｆｆ The average value of 110 μm ２ , the average value of w is 0.5 (A ｅｆｆ / π), and the average value of n is 1.444, the relationship between the angle θ (degrees) is shown in Figure 20. Figure 20 shows the relationship between the measurement error of the inter-core crosstalk due to backward Rayleigh scattering and the transmission loss α ｄＢ 20 is a graph showing the relationship between the measurement error and the transmission loss α ｄＢ Curves G51 to G57 show the cases where the angle θ is 0 degrees, 1 degree, 2 degrees, 3 degrees, 4 degrees, 5 degrees, and 6 degrees, respectively.

[0120] 20, it can be seen that the measurement error is significantly reduced when the angle θ is 2 degrees or less. When the angle θ is 2 degrees or less, the relationship between the angle θ and the transmission loss α ｄＢ The relationship between L and the measurement error of the inter-core crosstalk due to backward Rayleigh scattering is shown as the maximum allowable transmission loss α when the allowable measurement error is set to 10 dB, 5 dB, 2 dB, 1 dB, 0.5 dB, 0.2 dB, and 0.1 dB. ｄＢ Rearranging the relationship between L (dB) and angle θ (degrees) gives Figure 21. Figure 21 shows the maximum allowable transmission loss α when the allowable measurement error is 2 dB, 1 dB, 0.5 dB, 0.2 dB, or 0.1 dB. ｄＢ 21 is a graph showing the relationship between L (dB) and angle θ (degrees). In FIG. 21, the vertical axis represents the maximum allowable transmission loss α ｄＢ The horizontal axis represents the angle θ (degrees), and the horizontal axis represents the transmission loss α. The curves G61 to G65 represent the cases where the allowable measurement error is 2 dB, 1 dB, 0.5 dB, 0.2 dB, and 0.1 dB, respectively. ｄＢ L must be within a range below the corresponding curve, depending on the allowable measurement error. ０．２ By setting the value to 0.1 or less, the crosstalk measurement error caused by backward Rayleigh scattered light can be reduced to 2 dB or less.

[0121] If PW1 measured in a state including a large amount of noise due to backward Rayleigh scattered light is denoted as PW10, and PW2 measured in a state including a large amount of noise due to backward Rayleigh scattered light is denoted as PW20, the magnitude XT of the crosstalk between core 13a and cores 13b, 13c, and 13d can be calculated by the following equation (3). In this case, crosstalk measurement errors due to backward Rayleigh scattered light can be suppressed, and crosstalk can be measured with higher accuracy.

[0122] Furthermore, when a light reflecting surface is formed or provided with a target of φ=0 (rad), the magnitude XT of crosstalk between core 13a and cores 13b, 13c, and 13d is calculated based on PW10 and PW20 using the following formula (4):

[0123] where the wavelength λ and the effective cross-sectional area A ｅｆｆ When the wavelength λ is longer, and the effective cross section A ｅｆｆ The smaller the angle θ and the transmission loss α ｄＢ It was found that the allowable range between the effective cross-sectional area A and L narrows. ｅｆｆ is A ｅｆｆ = kπw ２ where k is a constant, and 0.95≦k≦1.1. According to the research of the present inventor, the smaller the constant k is, the smaller the angle θ and the transmission loss α ｄＢ The range of values ​​allowed between L and .

[0124] The wavelength λ is 1.625 μm, and the effective cross section A ｅｆｆ is 60 μm ２ , the result of examining the case where the constant k is 0.95 shows that the wavelength λ is 1.625 μm or less and the effective cross-sectional area A ｅｆｆ is 60 μm ２ If the angle θ and the transmission loss α ｄＢ It is preferable that L satisfies the following mathematical formula (5) or (6), which represents the range below the curve G41 in Fig. 20. This reduces the measurement error of inter-core crosstalk caused by backward Rayleigh scattered light to 10 dB or less, enabling more accurate measurement of inter-core crosstalk.

[0125] The wavelength λ is 1.625 μm or less, and the effective cross-sectional area A ｅｆｆ is 60 μm ２ If the angle θ and the transmission loss α ｄＢL may satisfy the following mathematical formula (7) that represents the range below the curve G42, may satisfy the following mathematical formula (8) that represents the range below the curve G43, may satisfy the following mathematical formula (9) that represents the range below the curve G44, may satisfy the following mathematical formula (10) that represents the range below the curve G45, may satisfy the following mathematical formula (11) that represents the range below the curve G46, or may satisfy the following mathematical formula (12) that represents the range below the curve G47. This reduces the measurement errors of inter-core crosstalk due to backward Rayleigh scattered light to 5 dB or less, 2 dB or less, 1 dB or less, 0.5 dB or less, 0.2 dB or less, and 0.1 dB or less, respectively, enabling inter-core crosstalk to be measured with even greater accuracy.

[0126] Due to measurement errors in inter-core crosstalk caused by backward Rayleigh scattering, the measured value of inter-core crosstalk is larger than the true value of inter-core crosstalk. Therefore, if the measured value of inter-core crosstalk is smaller than the upper specification limit of inter-core crosstalk, it is guaranteed that the true value of inter-core crosstalk is smaller than the upper specification limit of inter-core crosstalk. However, as described above, the measurement error in inter-core crosstalk caused by backward Rayleigh scattering varies depending on the reflectivity of the light-reflecting surface of the second end 10b. Furthermore, in reality, the reflectivity of the light-reflecting surface depends on the state of the light-reflecting surface for each measurement. Therefore, the measurement error in inter-core crosstalk caused by backward Rayleigh scattering varies within a predetermined range (e.g., within the above-mentioned range of 10 dB or less, 5 dB or less, 2 dB or less, 1 dB or less, 0.5 dB or less, 0.2 dB or less, or 0.1 dB or less) for each measurement. Therefore, the angle θ and the transmission loss α, which can keep the upper limit of the measurement error in inter-core crosstalk caused by backward Rayleigh scattering low, are used. ｄＢ It is advisable to perform the measurement within the range of L.

[0127] Next, a case where the test light is pulsed light as shown in Fig. 15 will be described. Fig. 22 is a graph schematically showing the time waveforms of the power of light (curve G21) emitted from a first core (here, assumed to be core 13a) into which the test light is incident and the power of light (curve G22) emitted from each of the second cores (here, cores 13b, 13c, and 13d) when the test light is pulsed light. In Fig. 22, the vertical axis represents the optical power (logarithmic scale), and the horizontal axis represents the delay time after the test light is incident. The delay time is proportional to the distance from the first end 10a, and specifically, corresponds to the value obtained by dividing twice the distance from the first end 10a by the group velocity of the pulsed light in the MCF 10.

[0128] When pulsed test light is incident on the core 13a at the first end 10a of the MCF 10, backward Rayleigh scattered light is emitted from the cores 13a, 13b, 13c, and 13d at the first end 10a during a period T1 until at least a portion of the test light reflected by the light-reflecting surface of the second end 10b returns to the first end 10a. The optical power of the backward Rayleigh scattered light gradually decreases over time (in other words, as the pulsed light travels through the core 13a). After a certain amount of time has passed, the power of the light emitted from each of the cores 13b, 13c, and 13d becomes weak and falls below the detection limit of the optical receiver (indicated by the dashed line D in the figure) and becomes undetectable.

[0129] At least a portion of the test light reflected by the light-reflecting surface of the second end 10b returns to the first end 10a after the time period T1 has elapsed (in other words, after a delay time corresponding to twice the distance between the first end 10a and the light-reflecting surface has elapsed since the test light was incident on the core 13a). At this time, as shown by curve G21, the power of the light emitted from the core 13a increases instantaneously (waveform portion E1 in the figure). As shown by curve G22, the power of the light emitted from each of the cores 13b, 13c, and 13d also increases instantaneously (waveform portion E2 in the figure). The peak values ​​of these waveform portions E1 and E2 are used as the first optical power PW1 and the second optical power PW2, respectively, in calculating the inter-core crosstalk.

[0130] In this way, the first optical power PW1 and the second optical power PW2 may be the power of the light emitted from the first core and the second core, respectively, at a timing when a delay time equivalent to twice the distance between the first end 10 a and the light reflecting surface has elapsed since the test light was incident on the first core. This makes it possible to eliminate the influence of backward Rayleigh scattered light from the first optical power PW1 and the second optical power PW2, and to measure inter-core crosstalk with higher accuracy.

[0131] In the above measurement method using pulsed light as test light, when the light reflecting surface is the end face 11a of the MCF 10, the ratio A1 of the power of the pulsed light reflected at the light reflecting surface (i.e., the first light power PW1 and the second light power PW2) to the power A2 of the pulsed light of backward Rayleigh scattered light that does not include reflected light generated immediately before the reflection (gain of signal light due to reflection G=A1 / A2) is calculated by the following formula (13): where T is the pulse width (km) of the test light propagating through the core in the MCF 10. By making this value G 10 or more, the gain G of the signal light due to the reflection can be made 10 dB or more, and the inter-core crosstalk can be measured with higher accuracy. ２ That's it, 10 ３ Above or equal to 10 ４ As a result, the gain G of the signal light due to the reflection can be set to 20 dB or more, 30 dB or more, or 40 dB or more, respectively, and inter-core crosstalk can be measured with even higher accuracy.

[0132] [First Modification] A first modification of the above embodiment will be described. FIG. 23 is a diagram showing the appearance of a holder 100A included in a measurement device according to the first modification. Part (a) of FIG. 23 is a plan view of the holder 100A. Part (b) of FIG. 23 is a side view of the holder 100A. Part (c) of FIG. 23 is a front view of the holder 100A. The holder 100A holds the MCF 10 so that the end face 11a of the MCF 10 is in contact only with gas (e.g., air) or vacuum. In other words, the holder 100A holds the MCF 10 so that the end face 11a of the MCF 10 does not come into contact with anything other than gas or vacuum. At the end portion of the MCF 10, including the end face 11a, the coating resin 12 is removed, exposing the glass fiber 11. No reflective film or reflection-enhancing device is provided on the end face 11a.

[0133] The holding unit 100A has a flat base 101A and a lid 102. The flat base 101A is a plate-shaped member that can be made of, for example, metal or resin. A V-groove 103 extending in one direction is formed on the main surface of the flat base 101A. The MCF 10 is accommodated in the V-groove 103 in the portion having the coating resin 12. The lid 102, which is disposed opposite the main surface of the flat base 101A, is then placed over the MCF 10. As a result, the MCF 10 is held within the area defined by the V-groove 103 and the lid 102, with the end face 11a suspended in mid-air.

[0134] FIG. 24 is a diagram showing the appearance of a holding unit 100B capable of holding a ribbon fiber 10A including multiple MCFs 10. Part (a) of FIG. 24 is a plan view of the holding unit 100B. Part (b) of FIG. 24 is a side view of the holding unit 100B. Part (c) of FIG. 24 is a front view of the holding unit 100B. The ribbon fiber 10A has a coating resin 16. The coating resin 16 covers the multiple MCFs 10 arranged in a row and secures the multiple MCFs 10 to one another. The holding unit 100B holds the ribbon fiber 10A so that the end faces 11a of the multiple MCFs 10 are in contact only with gas (e.g., air) or vacuum. In other words, the holding unit 100B simultaneously holds multiple MCFs 10 so that the end faces 11a of the multiple MCFs 10 do not come into contact with anything other than gas or vacuum.

[0135] The holding unit 100B has a flat base 101B and a lid 102. Similar to the flat base 101A, the flat base 101B is a plate-shaped member and may be made of, for example, metal or resin. A plurality of V-grooves 103 extending in one direction are formed on the main surface of the flat base 101B. Each of the multiple MCFs 10 of the optical fiber ribbon 10A is accommodated in the V-groove 103 at the portion having the coating resins 12 and 16. The lid 102, which is disposed opposite the main surface of the flat base 101B, covers the optical fiber ribbon 10A. As a result, the multiple MCFs 10 are held within an area defined by the multiple V-grooves 103 and the lid 102, with their end faces 11a suspended in midair.

[0136] As in this modification, the measurement apparatuses 1A, 1B, and 1C may further include a holding unit 100A (or a holding unit 100B when a plurality of MCFs 10 constitute the ribbon fiber 10A) for holding the MCF 10 so that the end face 11a is in contact with only gas or vacuum. Similarly, step ST1 of the measurement method shown in FIG. 16 may include, instead of step ST11, a step of positioning the end face 11a so that it is in contact with only gas or vacuum. This increases the reflectivity at the optical reflecting surface, increases the first optical power PW1 and the second optical power PW2, and enables more accurate measurement of inter-core crosstalk.

[0137] FIG. 25 is a diagram schematically illustrating a cross section of a dustproof cover 105 that houses the holder 100A (or 100B). The dustproof cover 105 includes a mounting base 104 and a lid 106 that can be opened and closed. The holder 100A (or 100B) is housed within the dustproof cover 105, placed on the mounting base 104, and supported at a higher position by the mounting base 104. FIG. 26 is a diagram illustrating a state in which the lid 106 is open. The MCF 10 is placed in the holder 100A (or 100B) with the lid 106 open. After the MCF 10 is placed, the lid 106 is closed.

[0138] In this way, by covering the end face 11 a of the MCF 10 held by the holder 100A (or 100B) with the dustproof cover 105, the risk of dust in the gas adhering to the end face 11 a can be reduced, and the light reflectance at the end face 11 a can be stably maintained. Furthermore, by raising the position of the end face 11 a of the MCF 10 by the mounting table 104, the risk of dust on the bottom surface of the dustproof cover 105 adhering to the end face 11 a can be reduced, and the light reflectance at the end face 11 a can be more stably maintained.

[0139] [Second Modification] In the above embodiment, the reflectances at the light-reflecting surfaces of the cores 13a, 13b, 13c, and 13d of the MCF 10 are described as being equal. However, the reflectances at the light-reflecting surfaces of the cores 13a, 13b, 13c, and 13d may differ. Such differences in reflectance are typically due to differences in the angle φ between the cores 13a, 13b, 13c, and 13d. For example, if the reflectances at the light-reflecting surfaces of the cores 13a and 13b differ, the inter-core crosstalk is measured by irradiating the test light onto the core 13a (i.e., the core 13a is the first core), and then the inter-core crosstalk is measured by irradiating the test light onto the core 13b (i.e., the core 13b is the first core). The geometric mean of the two measured inter-core crosstalk values ​​(linear values) is then used as the final inter-core crosstalk measurement value. This reduces measurement errors due to differences in reflectance.

[0140] In this case, if the absolute value of the decibel value of the reflectivity ratio between the cores 13a, 13b, 13c, and 13d is 15.8 dB or less, 9.0 dB or less, 6.1 dB or less, 4.2 dB or less, 2.7 dB or less, or 1.9 dB or less, the measurement error caused by the difference in reflectivity can be suppressed to 5 dB or less, 2 dB or less, 1 dB or less, 0.5 dB or less, 0.2 dB or less, or 0.1 dB or less, respectively. The absolute value of the decibel value of the reflectivity ratio is calculated, for example, by the following equation (14). １ is 0.5 times the mode field diameter of the core into which the test light is incident, w ２ is 0.5 times the mode field diameter of a core other than the core into which the test light is incident, φ １ is the angle φ of the core at which the test light is incident, φ２ is the angle φ of a core other than the core on which the test light is incident.

[0141] The spatial channel crosstalk measuring method, the inter-core crosstalk measuring method of a multi-core optical fiber, the spatial channel crosstalk measuring device, and the inter-core crosstalk measuring device of a multi-core optical fiber according to the present disclosure are not limited to the above-described embodiments and modifications, and various other modifications are possible. For example, the above-described modifications may be combined with each other depending on the required purpose and effect. Furthermore, the shape of the light reflecting surface at the second end 10b of the MCF 10 is not limited to the above-described embodiments. The number of cores in the MCF 10 is also not limited to the above-described embodiments.

[0142] DESCRIPTION OF SYMBOLS 1A, 1B, 1C... Measuring device 10... Multi-core optical fiber (MCF) 10a... First end portion 10A... Ribbon fiber 10b... Second end portion 11... Glass fiber 11a... End face 12... Coating resin 13a, 13b, 13c, 13d... Core 14... Cladding 15... Marker 16... Coating resin 20A, 20B... Light source section 21, 23, 24, 25, 26... Light source 22... First optical switch 22a... Input port 22b, 22c, 22d, 22e... Output port 30A, 30B... Light detection section 31, 33, 34, 35, 36... Photoreceiver 32... Second optical switch 32a... Output port 32b, 32c, 32d, 32e... Input port 40... Circulator section DESCRIPTION OF SYMBOLS 41, 42, 43, 44...Optical circulator 50...Fan-in / fan-out (FIFO) 50a, 50b, 50c, 50d, 50e...Input / output port 60...Calculation unit 71...Reflective film 72...Reflective member 73...Container 74...Liquid metal 80...Reflection enhancing device 81...MCF 82...Reflective film 90...Reflection enhancing device 91...FIFO 92...Single-core optical fiber (SCF) 93...Reflective film 100A, 100B...Holding unit 101A, 101B...Flat base 102...Cover 103...V-groove 104...Placement base 105...Dustproof cover 106...Cover 131...End face 200...Measuring device 201...Light source 202...First optical switch 203, 204...FIFO 205...Second optical switch 206...Optical receiver AX...Central axis line E1, E2...Waveform portions G21, G22, G51 to G57, G61 to G65...Curves H1, H2, H2a...Imaginary plane L1, L2...Light P1...First port P2...Second port P3...Third port PW1...First optical power PW2...Second optical power ST1, ST2, ST3, ST4, ST5, ST11, ST12, ST13...Step T1...Period φ...Angle

Claims

1. 1. A method for measuring crosstalk between spatial channels of a space division multiplexed optical system having a first input / output section and a second input / output section opposite to the first input / output section, and having first spatial channels and second spatial channels, the method comprising: a first step of forming or providing a light reflecting surface at the second input / output portion; a second step of injecting test light into the first spatial channel at the first input / output unit; a third step of reflecting at least a portion of the test light on the light reflecting surface; a fourth step of detecting a first optical power, which is the power of light emitted from the first spatial channel at the first input / output unit among the at least part of the light, and a second optical power, which is the power of light emitted from the second spatial channel at the first input / output unit among the at least part of the light; a fifth step of calculating 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; A method for measuring spatial inter-channel crosstalk, comprising:

2. the first optical power is a sum of optical power components emitted from the first spatial channel among return optical power components of the test light including backscattered light at each position in the longitudinal direction of the space division multiplexing optical system, 2. The method for measuring crosstalk between spatial channels according to claim 1, wherein the second optical power is a sum of optical power components emitted from the second spatial channels among return optical power components of the test light including backscattered light at each position in the longitudinal direction of the space division multiplexing optical system.

3. the space division multiplexing optical system is a space division multiplexing optical fiber; the light reflecting surface is an end face of the space division multiplexing optical fiber, The wavelength of the test light is λ (μm), the circular constant is π, the refractive index of the first spatial channel and the second spatial channel of the space division multiplexing optical fiber is n, and the transmission loss coefficient of the first spatial channel and the second spatial channel is α (km －１ ), 0.5 times the mode field diameter of the first spatial channel and the second spatial channel is w (μm), the angle formed by a plane tangent to the centers of the first spatial channel and the second spatial channel at the end face and a plane perpendicular to the central axis of the space division multiplexing optical fiber is φ (rad), the length of the space division multiplexing optical fiber is L (km), and the effective cross-sectional area of ​​the first spatial channel and the second spatial channel is A ｅｆｆ (μm ２ ), the Rayleigh scattering loss coefficients of the first spatial channel and the second spatial channel are defined as α Ｒ (km －１ ), the value of formula (A) is 10 ０．２ is as follows:

3. The method for measuring crosstalk between spatial channels according to claim 2, wherein XT is calculated using formula (B), where PW1 is the first optical power, PW2 is the second optical power, and XT is a magnitude of crosstalk between the first spatial channel and the second spatial channel. [Equation 1] [Equation 2]

4. the space division multiplexing optical system is a space division multiplexing optical fiber; the light reflecting surface is an end face of the space division multiplexing optical fiber, The angle formed by the plane tangent to the centers of the first spatial channel and the second spatial channel on the end face and the plane perpendicular to the central axis of the space division multiplexing optical fiber is defined as θ (degrees), and the loss coefficients of the first spatial channel and the second spatial channel are defined as α ｄＢ (dB·km －１ ), where the length of the space division multiplexing optical fiber is L (km), the angle θ and the transmission loss α ｄＢ L satisfies formula (C), the wavelength of the test light is 1.625 μm or less, and the effective cross-sectional areas of the first spatial channel and the second spatial channel at the wavelength are 60 μm or less. ２ That's all, 3. The method for measuring crosstalk between spatial channels according to claim 2, wherein XT is calculated using formula (D), where PW1 is the first optical power, PW2 is the second optical power, and XT is a magnitude of crosstalk between the first spatial channel and the second spatial channel. [Equation 3] [Equation 4]

5. the space division multiplexing optical system is a space division multiplexing optical fiber; the light reflecting surface is an end face of the space division multiplexing optical fiber, The wavelength of the test light is λ (μm), the circular constant is π, the refractive index of the first spatial channel and the second spatial channel of the space division multiplexing optical fiber is n, and the transmission loss coefficient of the first spatial channel and the second spatial channel is α (km －１ ), 0.5 times the mode field diameter of the first spatial channel and the second spatial channel is w (μm), the angle formed by a plane tangent to the centers of the first spatial channel and the second spatial channel at the end face and a plane perpendicular to the central axis of the space division multiplexing optical fiber is φ (rad), the length of the space division multiplexing optical fiber is L (km), and the effective cross-sectional area of ​​the first spatial channel and the second spatial channel is A ｅｆｆ (μm ２ ), the Rayleigh scattering loss coefficients of the first spatial channel and the second spatial channel are defined as α Ｒ (km －１ 3. The method for measuring crosstalk between spatial channels according to claim 2, wherein XT is calculated using formula (E) where XT is the magnitude of crosstalk between the first spatial channel and the second spatial channel, and XT is the first optical power, PW10, and PW20, respectively. [Equation 5]

6. the space division multiplexing optical system is a space division multiplexing optical fiber; the light reflecting surface is an end face of the space division multiplexing optical fiber, The wavelength of the test light is λ (μm), the circular constant is π, the refractive index of the first spatial channel and the second spatial channel of the space division multiplexing optical fiber is n, and the transmission loss coefficient of the first spatial channel and the second spatial channel is α (km －１ ), the length of the space division multiplexing optical fiber is L (km), the effective area of ​​the first spatial channel and the second spatial channel is A ｅｆｆ (μm ２ ), the Rayleigh scattering loss coefficients of the first spatial channel and the second spatial channel are defined as α Ｒ (km －１ 3. The method for measuring spatial channel crosstalk according to claim 2, wherein XT is calculated using formula (F) where the first optical power is PW10 and the second optical power is PW20. [Equation 6]

7. In the fourth step, analyze a time-intensity waveform of light reflected in the first spatial channel, in the second spatial channel, and at the optical reflecting surface and then emitted from the first spatial channel at the first input / output unit, of the test light incident on the first spatial channel at the first input / output unit; and determine the power of the light reflected at a longitudinal position corresponding to the optical reflecting surface and emitted from the first spatial channel as the first optical power; 2. The method for measuring crosstalk between spatial channels according to claim 1, further comprising: analyzing a time-intensity waveform of light, of the test light incident on the first spatial channel at the first incident / exit unit, reflected within the first spatial channel, within the second spatial channel, and at the optical reflecting surface and then emitted from the second spatial channel at the first incident / exit unit; and determining the power of the light reflected at a longitudinal position corresponding to the optical reflecting surface and emitted from the second spatial channel as the second optical power.

8. the test light is pulsed light, 2. The method for measuring crosstalk between spatial channels according to claim 1, wherein the first optical power and the second optical power are powers of light emitted from the first spatial channel and the second spatial channel, respectively, at a timing when a delay time equivalent to twice the optical distance between the first incident / exit unit and the optical reflecting surface has elapsed since the test light is incident on the first spatial channel in the second step.

9. the space division multiplexing optical system is a space division multiplexing optical fiber; the light reflecting surface is an end face of the space division multiplexing optical fiber, The wavelength of the test light is λ (μm), pi is the ratio of the circumference of a circle to its circumference, n is the refractive index of the first spatial channel and the second spatial channel of the space division multiplexing optical fiber, w (μm) is 0.5 times the mode field diameter of the first spatial channel and the second spatial channel, φ (rad) is the angle formed by a plane tangent to the centers of the first spatial channel and the second spatial channel on the end face and a plane perpendicular to the central axis of the space division multiplexing optical fiber, and A is the effective cross-sectional area of ​​the first spatial channel and the second spatial channel. ｅｆｆ (μm ２ ), the Rayleigh scattering loss coefficients of the first spatial channel and the second spatial channel are defined as α Ｒ (km －１ 8. The method for measuring crosstalk between spatial channels according to claim 7, wherein, when a pulse width of the test light propagating through the first spatial channel and the second spatial channel in the space division multiplexing optical fiber is T (km), the value of formula (G) is 10 or more. [Equation 7]

10. the space division multiplexing optical system is a space division multiplexing optical fiber or a space division multiplexing optical device; the light reflecting surface is an input / output surface of the space division multiplexing optical fiber or the space division multiplexing optical device, 10. The spatial channel crosstalk measuring method according to claim 1, wherein the first step includes a step of forming the input / output surfaces by breaking or cleaving the input / output portion of the space division multiplexing optical fiber or the space division multiplexing optical device.

11. the light reflecting surface is an input / output surface of the space division multiplexing optical system; 10. The method for measuring spatial inter-channel crosstalk according to claim 1, wherein the first step includes providing a reflective film on the incident / exit surface or immersing the incident / exit surface in a liquid metal.

12. the light reflecting surface is an input / output surface of the space division multiplexing optical system; 10. The method for measuring spatial channel crosstalk according to claim 1, wherein the first step includes a step of arranging the entrance / exit surface so as to be in contact with only a gas.

13. 2. The method for measuring spatial channel crosstalk according to claim 1, wherein the first step includes a step of connecting a second incident / exit surface of a second space division multiplexing optical system different from the space division multiplexing optical system, the second incident / exit surface having a reflective film as the light reflecting surface provided on its first incident / exit surface, to the incident / exit surface of the space division multiplexing optical system at the second incident / exit section.

14. 2. The method for measuring crosstalk between spatial channels according to claim 1, wherein the first step includes a step of optically coupling second incident / exit surfaces of a first single-core optical fiber and a second single-core optical fiber, each having a reflective film as the light reflecting surface provided on its first incident / exit surface, to the first spatial channel and the second spatial channel, respectively, at the second incident / exit portion.

15. 10. The method for measuring spatial channel crosstalk according to claim 3, wherein the space division multiplexing optical fiber is a multi-core optical fiber or a multi-mode optical fiber.

16. The method for measuring spatial channel crosstalk according to claim 1 or 2, wherein the space division multiplexing optical system is a multi-core optical device, a multi-core optical system, a multi-mode optical device, or a multi-mode optical system.

17. 10. The method for measuring crosstalk between spatial channels according to claim 1, wherein the first spatial channel and the second spatial channel are a first core and a second core, respectively, or a first mode and a second mode, respectively.

18. A method for measuring inter-core crosstalk of a multi-core optical fiber having a first end and a second end opposite to the first end, and having a first core and a second core, comprising: a first step of forming or providing a light reflecting surface on the second end; a second step of injecting test light into the first core at the first end; a third step of reflecting at least a portion of the test light on the light reflecting surface; a fourth step of detecting a first optical power, which is the power of light emitted from the first core at the first end, among the at least a portion of the light, and a second optical power, which is the power of light emitted from the second core at the first end, among the at least a portion of the light; a fifth step of calculating a magnitude of crosstalk between the first core and the second core based on the first optical power and the second optical power; A method for measuring inter-core crosstalk in a multi-core optical fiber, comprising:

19. the light reflecting surface is an end face of the multi-core optical fiber, 19. The method for measuring inter-core crosstalk of a multi-core optical fiber according to claim 18, wherein the first step includes a step of forming the end face by cleaving the multi-core optical fiber.

20. the light reflecting surface is an end face of the multi-core optical fiber, 20. The method for measuring inter-core crosstalk of a multi-core optical fiber according to claim 18 or 19, wherein the first step includes a step of providing a reflective film on the end face or a step of immersing the end face in a liquid metal.

21. the light reflecting surface is an end face of the multi-core optical fiber, 20. The method for measuring inter-core crosstalk of a multi-core optical fiber according to claim 18 or 19, wherein the first step includes a step of arranging the end face so that it is in contact with only a gas.

22. the test light is pulsed light, 20. The method for measuring inter-core crosstalk of a multi-core optical fiber according to claim 18 or 19, wherein the first optical power and the second optical power are powers of light emitted from the first core and the second core, respectively, at a timing when a delay time equivalent to twice the optical distance between the first end and the light reflecting surface has elapsed since the test light is incident on the first core in the second step.

23. the light reflecting surface is an end face of the multi-core optical fiber, The wavelength of the test light is λ (μm), pi is the ratio of the circumference of a circle to its diameter, n is the refractive index of the core of the multi-core optical fiber, w (μm) is 0.5 times the mode field diameter of the first core and the second core, φ (rad) is the angle formed by the plane tangent to the centers of the first core and the second core on the end face and the plane perpendicular to the central axis of the multi-core optical fiber, and A is the effective cross-sectional area of ​​the first core and the second core. ｅｆｆ (μm ２ ), the Rayleigh scattering loss coefficient of the first core and the second core is α Ｒ (km －１ 23. The method for measuring inter-core crosstalk of a multi-core optical fiber according to claim 22, wherein a value of formula (H) is 10 or more, when a pulse width of the test light propagating through cores in the multi-core optical fiber is T (km). [Equation 8]

24. 19. The method for measuring inter-core crosstalk of a multi-core optical fiber according to claim 18, wherein the first step includes a step of connecting a second end face of a second multi-core optical fiber different from the multi-core optical fiber, the second end face having a reflective film as the light reflecting surface provided on a first end face thereof, to the end face of the multi-core optical fiber at the second end.

25. 19. The method for measuring inter-core crosstalk of a multi-core optical fiber according to claim 18, wherein the first step includes a step of optically coupling second end faces of a first single-core optical fiber and a second single-core optical fiber, each having a reflective film as the light reflecting surface provided on its first end face, to the first core and the second core, respectively, at the second end.

26. 1. An apparatus for measuring crosstalk between spatial channels of a space division multiplexing optical system having a first input / output section and a second input / output section opposite to the first input / output section, and having N spatial channels (N is an integer of 2 or more), comprising: a light source unit that inputs test light into each of the N spatial channels at the first input / output unit; an optical detection unit that detects, from at least a portion of the test light reflected by a light reflecting surface formed or provided in the second input / output unit, a first optical power that is the power of light emitted from a first spatial channel into which the test light is incident, and a second optical power that is the power of light emitted from a second spatial channel different from the first spatial channel; a calculation unit that 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; 1. A spatial channel crosstalk measuring apparatus comprising:

27. N three-port optical couplers each having a first port, a second port, and a third port, each configured to output light input to the first port from the second port and output light input to the second port from the third port; an optical component that optically couples each of the N spatial channels at the first input / output portion of the space division multiplexing optical system to the second port of each of the N three-port optical couplers; Further provided with the light source unit is optically coupled to the first ports of the N three-port optical couplers; 27. The spatial inter-channel crosstalk measuring apparatus according to claim 26, wherein the optical detector is optically coupled to the third ports of the N three-port optical couplers.

28. the light source unit has N light sources, 28. The apparatus for measuring spatial inter-channel crosstalk of claim 27, wherein each of the N light sources is optically coupled to the first port of each of the N three-port optical couplers.

29. The light source unit is A single light source, a first optical switch that selectively optically couples the single light source to the first port of any of the N three-port optical couplers; 28. The apparatus for measuring spatial inter-channel crosstalk according to claim 27, comprising:

30. the light detection unit has N light receivers, 30. The apparatus for measuring spatial inter-channel crosstalk according to claim 27, wherein each of the N optical receivers is optically coupled to the third port of each of the N three-port optical couplers.

31. The light detection unit a single optical receiver; a second optical switch that selectively optically couples the single optical receiver to the third port of any of the N three-port optical couplers; 30. An apparatus for measuring spatial channel crosstalk according to any one of claims 27 to 29, comprising:

32. the first optical power is the power of light reflected at a longitudinal position corresponding to the optical reflecting surface and emitted from the first spatial channel by analyzing a time-intensity waveform of the test light incident on the first spatial channel at the first input / output unit, the time-intensity waveform of the light reflected in the first spatial channel, the second spatial channel, and the optical reflecting surface and then emitted from the first spatial channel at the first input / output unit; 30. The inter-spatial channel crosstalk measuring device according to claim 26, wherein the second optical power is determined by analyzing a time-intensity waveform of light, of the test light incident on the first spatial channel at the first incident / exit unit, that is reflected within the first spatial channel, within the second spatial channel, and at the optical reflecting surface and then emitted from the second spatial channel at the first incident / exit unit, and determining that the second optical power is the power of light that is reflected at a longitudinal position corresponding to the optical reflecting surface and emitted from the second spatial channel.

33. the test light is pulsed light, 30. The inter-spatial channel crosstalk measuring device according to claim 26, wherein the first optical power and the second optical power are powers of light emitted from the first spatial channel and the second spatial channel, respectively, at a timing when a delay time equivalent to twice the optical distance between the first incident / exit unit and the optical reflecting surface has elapsed since the test light is incident on the first spatial channel.

34. the light reflecting surface is an input / output surface of the space division multiplexing optical system; 30. The spatial channel crosstalk measuring apparatus according to claim 26, further comprising a holder that holds the space division multiplexing optical system so that the entrance and exit surface is in contact only with gas.

35. a second space division multiplexing optical system in which a reflective film as the light reflecting surface is provided on a first incident and exit surface; 30. The spatial channel crosstalk measuring device according to claim 26, wherein a second input / output surface of the second space division multiplexing optical system is connected to an input / output surface of the space division multiplexing optical system at the second input / output section.

36. N single spatial channel optical fibers each having a reflective film as the light reflecting surface provided on a first incident / exit surface, 30. The apparatus for measuring crosstalk between spatial channels according to claim 26, wherein a second input / output surface of each of the N single-spatial-channel optical fibers is optically coupled to each of the N spatial channels at the second input / output portion.

37. 30. The apparatus for measuring spatial channel crosstalk according to any one of claims 26 to 29, wherein the space division multiplexing optical system is a space division multiplexing optical fiber.

38. 38. The apparatus for measuring spatial inter-channel crosstalk according to claim 37, wherein the space division multiplexing optical fiber is a multi-core optical fiber or a multi-mode optical fiber.

39. 30. The spatial channel crosstalk measuring apparatus according to claim 26, wherein the space division multiplexing optical system is a multi-core optical device, a multi-core optical fiber, a multi-mode optical device, a multi-mode optical fiber, or two or more of them optically coupled together.

40. 30. The inter-spatial-channel crosstalk measuring apparatus according to claim 26, wherein the first spatial channel and the second spatial channel are a first core and a second core, respectively, or a first mode and a second mode, respectively.

41. An apparatus for measuring inter-core crosstalk of a multi-core optical fiber having a first end and a second end opposite to the first end, and having N cores (N is an integer of 2 or more), comprising: a light source unit that irradiates test light onto each of the N cores at the first end; an optical detection unit that detects a first optical power, which is the power of light emitted from a first core into which the test light is incident, and a second optical power, which is the power of light emitted from a second core different from the first core, of at least a portion of the test light reflected by a light reflecting surface formed or provided on the second end portion; a calculation unit that calculates a magnitude of crosstalk between the first core and the second core based on the first optical power and the second optical power; An apparatus for measuring inter-core crosstalk in a multi-core optical fiber, comprising:

42. N optical circulators each having a first port, a second port, and a third port, each outputting light input to the first port from the second port and outputting light input to the second port from the third port; an optical component that optically couples each of the N cores at the first end of the multi-core optical fiber to the second port of each of the N optical circulators; Further provided with the light source unit is optically coupled to the first ports of the N optical circulators; 42. The apparatus for measuring inter-core crosstalk of a multi-core optical fiber according to claim 41, wherein the optical detection unit is optically coupled to the third ports of the N optical circulators.

43. the light source unit has N light sources, 43. The apparatus for measuring inter-core crosstalk of a multi-core optical fiber according to claim 42, wherein each of the N light sources is optically coupled to the first port of each of the N optical circulators.

44. The light source unit is A single light source, a first optical switch that selectively optically couples the single light source to the first port of any of the N optical circulators; The inter-core crosstalk measuring device of a multi-core optical fiber according to claim 42, comprising:

45. the light detection unit has N light receivers, 45. The apparatus for measuring inter-core crosstalk of a multi-core optical fiber according to claim 42, wherein each of the N optical receivers is optically coupled to the third port of each of the N optical circulators.

46. The light detection unit a single optical receiver; a second optical switch that selectively optically couples the single optical receiver to the third port of any one of the N optical circulators; The inter-core crosstalk measuring device of a multi-core optical fiber according to claim 43, comprising:

47. the test light is pulsed light, 45. The device for measuring core-to-core crosstalk of a multi-core optical fiber according to claim 41, wherein the first optical power and the second optical power are powers of light emitted from the first core and the second core, respectively, at a timing when a delay time equivalent to twice the optical distance between the first end and the light reflecting surface has elapsed since the test light is incident on the first core.

48. the light reflecting surface is an end face of the multi-core optical fiber, 45. The device for measuring inter-core crosstalk of a multi-core optical fiber according to claim 41, further comprising: a holding part that holds the multi-core optical fiber so that the end face is in contact with only gas.

49. a second multi-core optical fiber having a reflective film as the light reflecting surface provided on a first end face thereof; 45. The device for measuring inter-core crosstalk of a multi-core optical fiber according to claim 41, wherein a second end face of the second multi-core optical fiber is connected to an end face of the multi-core optical fiber at the second end portion.

50. N single-core optical fibers each having a reflective film as the light-reflecting surface provided on a first end face thereof, 45. The device for measuring core-to-core crosstalk of a multi-core optical fiber according to claim 41, wherein a second end face of each of the N single-core optical fibers is optically coupled to each of the N cores at the second end portion.

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