Measurement device and measurement method

Abstract

A measurement device according to the present invention measures the mode-dependent loss of a measurement target that is an optical device that includes a coupled multicore optical fiber. The measurement device comprises: a light source that emits measurement light; a frequency control unit that continuously varies the optical frequency of the light source; an excitation optical fiber that outputs measurement light at an input end of the measurement target and is configured from a coupled multicore optical fiber that includes a plurality of mode-coupled cores that extend along a fiber axis and a cladding that surrounds the plurality of cores; and a light detector that detects fluctuations in the optical power outputted from the measurement target. When Δτ (ps) is the spatial mode dispersion from the light source to an output end of the excitation optical fiber and Δν (THz) is the range of the optical frequency varied by the frequency control unit, Δν×Δτ>3.

Description

Measuring device and measuring method

[0001] This application claims priority to Japanese Application No. 2024-020557, filed February 14, 2024, and incorporates by reference all of the contents of said Japanese application.

[0002] A multi-core optical fiber (hereinafter also referred to as "MCF") is an optical fiber having multiple cores made of glass, a cladding also made of glass surrounding the multiple cores, and a resin layer coating the cladding. Among MCFs, a coupled multi-core optical fiber (hereinafter also referred to as "CMCF") in which guided modes are coupled between multiple cores has the advantage of enabling large-capacity transmission and a thinner optical fiber cable because of its high core spatial density and high spatial density of information transmission. The degree of coupling of guided modes between cores is expressed by the mode coupling coefficient or power coupling coefficient, and in the present disclosure, an MCF having an inter-core mode coupling coefficient of 1 [1 / m] or more or an inter-core power coupling coefficient of 10 [1 / km] or more is defined as a CMCF.

[0003] At the input end of the CMCF, optical signals input to each of the multiple cores are partially coupled to other cores due to mode coupling while propagating. As a result, at the output end of the CMCF, multiple coupled optical signals are output from the respective cores. At the receiving end, each of the multiple coupled optical signals is converted into a digital signal by a digital coherent receiver. These digital signals are then subjected to multiple-input multiple-output (MIMO) processing to restore a digital signal equivalent to the original input signal.

[0004] In this case, the greater the mode-dependent loss (hereinafter also referred to as "MDL"), which is the difference in loss between the propagation modes of the CMCF, the less effective the MIMO processing becomes, and the more difficult it becomes to restore the signal. Therefore, when constructing a transmission path using CMCF, it is necessary to measure the MDL of the CMCF that constitutes the transmission path and manage the MDL of the entire transmission path to be low.

[0005] Patent Document 1 discloses an apparatus and method for measuring MDL by injecting measurement light into a CMCF under test via a pump CMCF, changing the mode state of the pump CMCF using a mode coupling state changing means, and analyzing fluctuations in the output light power from the fiber under test. This measurement method is called the scrambling method because the mode state is scrambled. Regarding the scrambling method, Non-Patent Document 1 discloses that, while the lower limit of MDL measurable by other MDL measurement methods is 0.1 dB, the scrambling method can measure MDL down to 0.04 dB. To maintain a low MDL for the entire transmission line, it is desirable to maintain the MDL of each component, such as an optical fiber or optical component, below 0.1 dB. The scrambling method is an effective method for measuring the MDL of such components.

[0006] The scrambling method requires a comprehensive change in mode state. To achieve this, Non-Patent Document 1 discloses that measurement light output from a wavelength-tunable light source is split by an optical splitter, and some of the split branches are phase-modulated before being input to a CMCF via a fan-out. A fan-out is a device that optically couples each of the multiple cores of a CMCF to each of the cores of multiple single-mode fibers (hereinafter also referred to as "SMF").

[0007] International Publication No. 2020 / 171187

[0008] T. Hasegawa and T. Hayashi, “Measurement of Mode Dependent Loss of Randomly-Coupled Multi-Core Fiber using Scrambling Method”, Proceedings of 26th Optoelectronics and Communications Conference (OECC), T2C. 2 (2021)

[0009] A measurement apparatus according to one aspect of the present disclosure is an optical device including a coupled multi-core optical fiber as a measurement object, and measures a mode-dependent loss of the measurement object, the measurement apparatus comprising: a light source that outputs measurement light; a frequency control unit that continuously changes the optical frequency of the light source; an excitation optical fiber that is constituted by a coupled multi-core optical fiber having a plurality of cores that extend along the fiber axial direction and are mode-coupled to each other and a cladding that surrounds the plurality of cores; and a photodetector that detects fluctuations in optical power output from the measurement object, wherein Δν × Δτ > 3 is satisfied, where Δτ [ps] is the spatial mode dispersion from the light source to the output end of the excitation optical fiber, and Δν [THz] is the range of the optical frequency that is changed by the frequency control unit.

[0010] Fig. 1 is a configuration diagram of a measurement device according to a first embodiment. Fig. 2 is a diagram showing the structure of a CMCF. Fig. 3 is a flowchart showing a measurement method according to the first embodiment. Fig. 4 is a configuration diagram of a measurement device according to a second embodiment.

[0011] [Problem to be Solved by the Present Disclosure] The method of splitting measurement light and applying phase modulation to a portion of it, as disclosed in Non-Patent Document 1, has the problem that the cost of the device is high and it is difficult to apply to CMCFs with a large number of cores.

[0012] An object of the present disclosure is to provide a measurement device and a measurement method that can measure the MDL of a CMCF with a simple configuration.

[0013] [Effects of the Present Disclosure] According to the present disclosure, a measurement device and a measurement method capable of measuring the MDL of a CMCF with a simple configuration are provided.

[0014] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be listed and described. (1) A measurement apparatus according to one aspect of the present disclosure is an optical device including a coupled multi-core optical fiber as a measurement object, and measures the mode-dependent loss of the measurement object, the measurement apparatus comprising: a light source that outputs measurement light; a frequency control unit that continuously changes the optical frequency of the light source; a pump optical fiber that is composed of a coupled multi-core optical fiber extending along the fiber axial direction and having multiple cores that are mode-coupled to each other and a cladding that surrounds the multiple cores, and outputs the measurement light to an input end of the measurement object; and a photodetector that detects fluctuations in the optical power output from the measurement object, wherein Δν × Δτ > 3 is satisfied, where Δτ [ps] is the spatial mode dispersion from the light source to the output end of the pump optical fiber and Δν [THz] is the range of the optical frequency that is changed by the frequency control unit. In this measurement apparatus, the mode state changes widely enough at the output end of the pump optical fiber, so that the MDL of the measurement object can be measured with a simple configuration.

[0015] (2) In the above (1), Δν×Δτ may be greater than 30. In this case, the MDL of the measurement object can be measured with high accuracy.

[0016] (3) In the above (1) or (2), the pump optical fiber may be bent with a diameter of 280 mm or less, have a twist of 10 or more turns over its entire length, and when the spatial mode dispersion of the pump optical fiber is Δτa [ps], Δν×Δτa > 3. In this case, the pump optical fiber has a sufficiently large spatial mode dispersion, which can improve measurement accuracy.

[0017] (4) In the above (3), Δν×Δτ may be greater than 30. In this case, the measurement accuracy can be further improved.

[0018] (5) In the above (1) or (2), the optical fiber may further include an optical branching unit, a plurality of optical paths connected to an output end of the optical branching unit, and a fan-out for coupling the output ends of the plurality of optical paths to a plurality of cores of the excitation optical fiber, wherein at least two of the plurality of optical paths have a differential group delay, and when the differential group delay is Δτg [ps], Δν × Δτg > 3. In this case, the differential group delay is sufficiently large, thereby enabling improvement in measurement accuracy.

[0019] (6) In the above (5), Δν×Δτg may be greater than 30. In this case, the measurement accuracy can be further improved.

[0020] (7) A measurement method according to an aspect of the present disclosure is a method for measuring a mode-dependent loss of an optical device including a coupled multi-core optical fiber as a measurement target, the method comprising the steps of: outputting measurement light whose optical frequency is continuously changed from a light source; outputting the measurement light to an input end of the measurement target from a pump optical fiber configured by the coupled multi-core optical fiber having multiple cores extending along a fiber axial direction and mode-coupled to each other and a cladding surrounding the multiple cores; and detecting fluctuations in optical power output from the measurement target, wherein Δν × Δτ > 3 is satisfied, where Δτ [ps] is the spatial mode dispersion from the light source to the output end of the pump optical fiber and Δν [THz] is the range over which the optical frequency of the light source varies. In this measurement method, the mode state varies widely enough at the output end of the pump optical fiber, so that the MDL of the measurement target can be measured with a simple configuration.

[0021] [Details of the Embodiments of the Present Disclosure] Specific examples of optical fiber devices according to the present disclosure will be described with reference to the drawings as necessary. The present disclosure 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 designated by the same reference numerals, and duplicate explanations will be omitted.

[0022] 1 is a configuration diagram of a measurement apparatus according to the first embodiment. The measurement apparatus 1a according to the first embodiment includes a light source 2, a frequency control unit 3, an optical branching unit 4, a plurality of optical paths 51, a fan-out 6, an excitation CMCF 7a (excitation optical fiber), and a photodetector 9. The measurement apparatus 1a is an apparatus for measuring the MDL of an object to be measured 8. The object to be measured 8 is an optical device including a CMCF 7, and is connected between an output end 7o of the excitation CMCF 7a and the photodetector 9.

[0023] The light source 2 generates measurement light and outputs the generated measurement light toward the input end of the optical branching device 4. A semiconductor laser, for example, can be used as the light source 2. The frequency control unit 3 continuously changes the optical frequency of the light source 2. For example, a means for controlling the phase of the resonator of the semiconductor laser, the reflection wavelength of the distributed diffraction grating, and the external resonator length can be used as the frequency control unit 3. This makes it possible to reduce fluctuations in optical power due to changes in optical frequency. Instead of the light source 2 and the frequency control unit 3, a wavelength-tunable light source that combines the functions of the light source 2 and the frequency control unit 3 may be used.

[0024] The optical splitter 4 splits the measurement light output from the light source 2 into multiple optical paths 51. The multiple optical paths 51 are optically connected to the output end of the optical splitter 4. At least two of the multiple optical paths 51 have an optical path length difference 52 between them. The fan-out 6 connects the multiple optical paths 51 to multiple cores 71 of the pump CMCF 7a. The pump CMCF 7a is optically connected to an input end 7i of the CMCF 7 in the measurement object 8 and outputs the measurement light to the input end 7i of the CMCF 7. The optical splitter 4, the multiple optical paths 51, and part of the fan-out 6 may be composed of optical fiber or a spatial optical system. The pump CMCF 7a may be part of the fan-out 6.

[0025] The photodetector 9 is optically coupled to the output terminal of the object to be measured 8 and detects the measurement light output from the output terminal of the object to be measured 8. The photodetector 9 detects fluctuations in the optical power output from the object to be measured 8. For example, an optical power meter can be used as the photodetector 9. An optical power meter that generates a clock signal at a constant optical frequency or at constant wavelength intervals in the light source 2 or frequency control unit 3 and records the optical power in synchronization with the clock can also be used as the photodetector 9. This makes it possible to record optical power corresponding to a uniform sampling density in the mode space even if the rate of change of the optical frequency is non-uniform. As a result, the MDL can be measured with high reproducibility.

[0026] FIG. 2 is a diagram showing the structure of a CMCF in a measurement target 8. As shown in FIG. 2, the CMCF 7 in the measurement target 8 has multiple cores 71 and a cladding 72. The multiple cores 71 extend along the fiber axis direction AX of the CMCF 7 and are mode-coupled to one another. The cladding 72 is a common cladding that surrounds the multiple cores 71. The fiber axis direction AX is parallel to the fiber axis, which is the central axis of the CMCF 7. The CMCF 7 may further have a resin layer (not shown) that covers the cladding 72 for protection. The core 71 is not necessarily located at the center of the cladding 72. In this embodiment, the CMCF 7 has four cores. The cores 71 and the cladding 72 are primarily composed of, for example, silica glass and contain an additive element for refractive index adjustment. The measurement device and measurement method disclosed herein are not limited to the number of cores being four. They may also be applied to CMCFs with known numbers of cores, such as 7, 8, 12, or 19.

[0027] The pump CMCF 7a has the same structure as the CMCF 7. That is, the pump CMCF 7a has multiple cores 71 extending along the fiber axis direction AX of the pump CMCF 7a and mode-coupled with each other, and a common cladding 72 surrounding the multiple cores 71. The pump CMCF 7a may further have a resin layer (not shown) that covers the cladding 72 for protection. The number of cores in the pump CMCF 7a is equal to the number of cores in the CMCF 7. The arrangement and shape of the multiple cores 71 and cladding 72 in the pump CMCF 7a are equal to the arrangement and shape of the multiple cores 71 and cladding 72 in the CMCF 7. The modal dispersion of the pump CMCF 7a may be different from the modal dispersion of the CMCF 7. By making the modal dispersion of the pump CMCF 7a larger than the modal dispersion of the CMCF 7, the change in mode state at the output end of the pump CMCF 7a can be increased. This makes it possible to improve the accuracy of measuring the MDL.

[0028] The measurement light output from the light source 2 is branched into multiple optical paths 51 by the optical branching device 4. The measurement light that has passed through the optical path length difference 52 is input to the excitation CMCF 7a via the fan-out 6. The measurement light output from the excitation CMCF 7a is input to the measurement object 8. The measurement light output from the measurement object 8 is detected by the photodetector 9. When the optical frequency of the measurement light is continuously changed by the frequency control unit 3, the mode state input from the excitation CMCF 7a to the measurement object 8 varies. If the measurement object 8 has an MDL, the optical power detected by the photodetector 9 varies as the mode state varies, and the MDL can be quantified from the range of this variation.

[0029] In order to measure the MDL using the measurement device 1a, it is necessary that the mode state at the output end 7o of the pump CMCF 7a change significantly when the optical frequency is changed. If the number of modes is M (hence the number of cores is M / 2), the mode state at the output end of the light source 2 is an M-dimensional vector x0, and the mode state at the output end 7o of the pump CMCF 7a is an M-dimensional vector x, the following equation holds: where Q is a transfer matrix.

[0030] Assuming that there is no MDL in the section from the output end of the light source 2 to the output end 7o of the excitation CMCF 7a, Q is expressed by the following equation using unitary matrices U and V. S is a diagonal matrix composed of eigenvalues ​​and is expressed by the following formula. When the angular frequency ω of the measurement light is changed minutely from ωa to ωb, the change in the mode state at the output end 7o of the excitation CMCF 7a is expressed as xa to xb. The change in the mode state is large when the inner product xa Ｈ This corresponds to xb being small.

[0031] The mode state x0 of the output terminal of the light source 2 can be considered not to change with a small change in the angular frequency, so the inner product xa Ｈ xb and matrix Sa Ｈ Sb is expressed by the following formula.

[0032] When the matrix of equation (5) is a constant multiple of the identity matrix, the inner product of equation (4) has a maximum value of 1, and therefore the magnitude of change in mode state u is expressed by the following equation. However, it is assumed that the absolute value of the minute change in angular frequency Δω=ωb−ωa is sufficiently small and the phase difference (θb1−θa1) does not exceed π.

[0033] If the length of the section from the output end of the light source 2 to the output end 7o of the excitation CMCF 7a is L and the propagation constant of mode m averaged over the section is βm, the magnitude of change in the mode state u is expressed by the following equation. The spatial mode dispersion Δτ [ps] from the light source 2 to the output end 7o of the excitation CMCF 7a is expressed by the following formula. The spatial mode dispersion Δτ corresponds to the group delay difference Δτg [ps] between the modes in which the group delay is maximum and minimum.

[0034] According to equation (7), by increasing the spatial mode dispersion Δτ between the light source 2 and the output end 7o of the excitation CMCF 7a, the inner product of the mode state can be reduced, enabling the large change in the mode state required for measuring the MDL. In the derivation of equation (7), it is assumed that the change in angular frequency Δω is sufficiently small. However, in actual measurements, the angular frequency may be changed over a large range without being constrained by the above assumption. Even in this case, equation (7) holds if one focuses on a narrow angular frequency range that occupies a portion of the change range. By changing the angular frequency over a sufficiently large range, the modes that give the maximum and minimum phase difference (θbm - θam) in equation (6) also change depending on the angular frequency. As a result, the change in the mode state can be made over a wider range.

[0035] 1 , the group delay difference Δτg corresponding to the optical path length difference 52 is substantially equal to the spatial mode dispersion Δτ, and the group delay difference Δτg can be considered to be the spatial mode dispersion Δτ. Therefore, when the variation range of the optical frequency of the light source 2 (i.e., the range of the optical frequency changed by the frequency control unit 3) is set to Δν [THz] = Δω / 2π, Δν × Δτ > 3 or Δν × Δτ > 30 may be satisfied. Similarly, Δν × Δτg > 3 or Δν × Δτg > 30 may be satisfied.

[0036] Specifically, when the center wavelength of the measurement light is 1550 nm, the optical frequency change width at which the wavelength-dependent loss becomes sufficiently small is 0.1 THz, and the wavelength of the measurement light is changed within a change width from 1550.4 nm (193.365 THz) to 1549.6 nm (193.465 THz), the spatial mode dispersion Δτ may be 30 ps or more, or may be 300 ps or more. Specifically, in the configuration of Figure 1, the differential group delay Δτg may be 30 ps or more, or may be 300 ps or more. In this case, the mode state changes widely enough at the output end 7o of the excitation CMCF 7a, so that the MDL of the object to be measured 8 can be measured accurately.

[0037] The MDL is measured as follows from the fluctuation of the optical power P detected by the photodetector 9. The optical power P [mW] is expressed by the following equation. Here, P0 [mW] is the power input to the excitation CMCF 7a, A [dimensionless] is the loss that occurs commonly in all modes from the input end of the excitation CMCF 7a to the output end of the object to be measured 8 (such as transmission loss due to the material of the optical fiber), m is the mode number, M is the number of modes, Tm [dimensionless] is the relative transmittance of mode m from the input end of the excitation CMCF 7a to the output end of the object to be measured 8, and cm [dimensionless] is the complex amplitude of mode m.

[0038] The basis of the mode is the eigenmode of the measurement object 8. The relative transmittance is the transmittance expressed as a ratio to the transmittance of the mode with the maximum transmittance. The complex amplitude cm of mode m is normalized as follows:

[0039] Fluctuations in the mode state cause fluctuations in the complex amplitude cm, which in turn causes fluctuations in the optical power P. The instantaneous magnitude of the fluctuations in the optical power P is quantified as the relative deviation Δp [dB] based on the average value using the following equation: Here, the coefficient (10 / ln10) is a coefficient for expressing the fluctuation of the optical power P expressed in decibel scale.

[0040] The statistical magnitude of the fluctuation of the optical power P is expressed by the following equation as the RMS relative deviation Δprms [dB] obtained by RMS averaging the relative deviation Δp.

[0041] When the modal state varies widely enough, the mean value of the modal amplitude, which is the square of the complex amplitude cm of mode m, is given by:

[0042] Since the optical power detected by the photodetector 9 is expressed by equation (9), the average value of the optical power is expressed by the following equation. where the average of T for mode m is Therefore, the relative deviation Δp is expressed by the following equation. However, ΔTm is expressed by the following formula.

[0043] Therefore, the RMS relative deviation Δprms of the detected optical power is the RMS average of the normalized deviation of the modal transmittance for the mode as follows: However, the constant term C is expressed by the following equation.

[0044] It is assumed that the MDL of the measurement object 8 is small. Therefore, if it is assumed that the deviation of Tm from the average value is sufficiently small, the following equation is obtained. When the transmittance of mode m is expressed on a logarithmic scale as in equation (22), equations (20) and (21) are obtained.

[0045] Therefore, the RMS relative deviation Δprms of the detected power expressed by equation (19) is equal to the right side of equation (25). This coincides with the definition of RMS mode-dependent loss except for the constant term C, which can be calculated numerically from equation (20). Therefore, in the configuration of Figure 1, by setting the optical frequency change range and group delay so that the mode state changes sufficiently widely as described above, the MDL of the measurement target can be calculated using the following equation.

[0046] 3 is a flowchart showing a measurement method according to the first embodiment. The measurement method according to the first embodiment is a method for measuring the MDL of a CMCF and is performed using a measurement apparatus 1a. This measurement method includes steps S1 to S6. In step S1, the measurement apparatus 1a is prepared. As described above, the measurement apparatus 1a is configured to impart spatial mode dispersion Δτ to the path from the light source 2 to the output end 7o of the pump CMCF 7a by using a group delay difference Δτg corresponding to the optical path length difference 52. In step S2, the object to be measured 8 is connected between the output end 7o of the pump CMCF 7a and the photodetector 9.

[0047] In step S3, the light source 2 outputs measurement light whose optical frequency is continuously changed. The frequency control unit 3 changes the optical frequency of the measurement light so that Δν×Δτ>3 holds. The measurement light is input to the measurement object 8 via the optical branching device 4, multiple optical paths 51, the fan-out 6, and the excitation CMCF 7a. In step S4, the optical power output from the measurement object 8 is continuously recorded by the photodetector 9, and fluctuations in the optical power are detected. In step S5, the detected optical power is substituted into equations (11) and (12) to determine the RMS relative deviation Δprms. In step S6, the RMS relative deviation Δprms is substituted into equation (26) to determine the MDL. In this manner, the MDL of the measurement object 8 can be measured.

[0048] 4 is a configuration diagram of a measurement device according to the second embodiment. The measurement device 1b according to the second embodiment differs from the measurement device 1a according to the first embodiment in that the measurement light from the light source 2 is incident on one core 71 (see FIG. 2) of an excitation CMCF 7b (excitation optical fiber). The measurement device 1b does not include the optical branching device 4, the multiple optical paths 51, and the fan-out 6.

[0049] In the measurement apparatus 1a, the group delay difference Δτg corresponding to the optical path length difference 52 can be regarded as the spatial mode dispersion Δτ, but in the measurement apparatus 1b, the pump CMCF 7b has a sufficiently large spatial mode dispersion Δτa [ps], so the spatial mode dispersion Δτa can be regarded as the spatial mode dispersion Δτ. Since the spatial mode dispersion Δτa is substantially equal to the spatial mode dispersion Δτ, Δν × Δτa may be greater than 3 or greater than 30. When the center wavelength of the measurement light is 1550 nm, the change width of the optical frequency at which the wavelength-dependent loss becomes sufficiently small is 1.0 THz, and the wavelength of the measurement light is changed within a change width from 1550.4 nm (193.365 THz) to 1549.6 nm (193.465 THz), the spatial mode dispersion Δτa may be greater than 3 ps or greater than 30 ps. At this time, the mode state changes widely enough at the output end 7o of the excitation CMCF 7b, so that the MDL of the measurement object can be measured with high accuracy.

[0050] By winding the pump CMCF 7b with a small bending diameter, spatial mode dispersion Δτa can be generated. The pump CMCF 7b may be bent with a diameter of 280 mm or less, or may be bent with a diameter of 140 mm or less. The spatial mode dispersion Δτa can change randomly due to mode coupling in the pump CMCF 7b. Therefore, by applying bending to the core 71 of the pump CMCF 7a from various directions and randomly accumulating the effects of bending, a large spatial mode dispersion Δτa can be obtained with high reproducibility. To achieve this, the pump CMCF 7b may have at least one of a connection point 711b and a twist 712b. In particular, the pump CMCF 7b may have a twist 712b of 10 turns or more, or a twist 712b of 100 turns or more over the entire length of the pump CMCF 7b. The twist may be an elastic twist accompanied by stress, or an inelastic twist.

[0051] The measurement method according to the second embodiment is performed in the same manner as the measurement method according to the first embodiment, except that it is performed using the measurement device 1b.

[0052] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments and modifications, and various modifications are possible without departing from the spirit of the present disclosure.

[0053] DESCRIPTION OF SYMBOLS 1a, 1b... Measuring device 2... Light source 3... Frequency control section 4... Optical branching device 51... Optical path 52... Optical path length difference 6... Fan-out 7a, 7b... Excitation CMCF 7... CMCF 7i... Input end of CMCF 7o... Output end of excitation CMCF 8... Measurement object 9... Photodetector 51... Optical path 52... Optical path length difference 71... Core 72... Cladding 711b... Connection point 712b... Twist AX... Fiber axial direction

Claims

1. A measuring apparatus for measuring a mode-dependent loss of an optical device including a coupled multi-core optical fiber as a measurement object, comprising: a light source that outputs measurement light; a frequency control unit that continuously changes the optical frequency of the light source; an excitation optical fiber that outputs measurement light to an input end of the measurement object, the excitation optical fiber being composed of a coupled multi-core optical fiber having a plurality of cores that extend along the fiber axial direction and are mode-coupled to each other and a cladding that surrounds the plurality of cores; and a photodetector that detects fluctuations in the optical power output from the measurement object, wherein Δν × Δτ > 3 is satisfied, where Δτ [ps] is the spatial mode dispersion from the light source to the output end of the excitation optical fiber, and Δν [THz] is the range of the optical frequency that is changed by the frequency control unit.

2. The measuring device according to claim 1, wherein Δν×Δτ>30.

3. The measurement device according to claim 1 or 2, wherein the excitation optical fiber is bent with a diameter of 280 mm or less and has a twist of 10 or more turns over its entire length, and when the spatial mode dispersion of the excitation optical fiber is Δτa [ps], Δν × Δτa > 3.

4. The measuring device according to claim 3, wherein Δν×Δτa>30.

5. A measuring device according to claim 1 or claim 2, further comprising: an optical branching device; a plurality of optical paths connected to the output end of the optical branching device; and a fan-out for coupling the output ends of the plurality of optical paths to the plurality of cores of the excitation optical fiber, respectively; wherein at least two of the plurality of optical paths have a differential group delay, and when the differential group delay is Δτg [ps], Δν×Δτg>3.

6. The measuring device according to claim 5, wherein Δν×Δτg>30.

7. A measurement method for measuring a mode-dependent loss of an optical device including a coupled multi-core optical fiber as a measurement object, comprising the steps of: outputting measurement light whose optical frequency is continuously changed from a light source; outputting the measurement light to an input end of the measurement object from an excitation optical fiber composed of a coupled multi-core optical fiber having a plurality of cores extending along a fiber axis and mode-coupled to each other and a cladding surrounding the plurality of cores; and detecting fluctuations in the optical power output from the measurement object, wherein Δν × Δτ > 3 is satisfied, where Δτ [ps] is the spatial mode dispersion from the light source to the output end of the excitation optical fiber, and Δν [THz] is the range over which the optical frequency of the light source changes.

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