JP2019020276A - Spatial multiplexing optical transmission path evaluation device and method - Google Patents

Abstract

To provide a device and a method that can easily perform a spatial multiplexing optical transmission path evaluation with a simpler device configuration than the conventional device and method.SOLUTION: A reference interferometer is provided in the local optical path of a main measurement interferometer, and signals from 2 interferometers are obtained by 1 photodetectors and an A/D converter, and the reference interferometer and the beat signal of the main measurement interferometer are separated by a digital filtering processing. The delay fiber used in the reference interferometer must be τ≠τ-τ, where the reference interferometer and the main measurement interferometer beat signal are not duplicated on the frequency axis. However, since the beat signal corresponding to the τ≒τ-τis required in order to compensate the phase noise of the light source, signals of τ≒τ-τare calculated from the signals obtained from the delay fibers of τ≠τ-τby the following signal processing.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a spatial multiplexing optical transmission line evaluation apparatus and method for evaluating characteristics of a spatial multiplexing optical transmission line using a multimode optical fiber, a multi-core optical fiber or the like.

  As a technique for expanding the optical transmission capacity per optical fiber, there is a spatial multiplexing optical transmission technique (hereinafter, SDM) using a multi-mode optical fiber or a multi-core optical fiber. In SDM, transmission capacity is expanded by multiplexing signals on different spatial channels such as modes and cores. However, it is known that if there is mode coupling or a delay time difference between spatial channels, signal quality is deteriorated and signal restoration processing is complicated. Therefore, mode coupling and delay time difference are important indicators in evaluating a spatially multiplexed optical transmission line, and a measurement method that can accurately evaluate them is required.

As a spatial multiplexing optical transmission line evaluation method, there is a frequency sweep optical interference method (hereinafter, FMCW method) (see Non-Patent Document 1). FIG. 1 shows an example of an apparatus configuration of a conventionally proposed FMCW method. This measuring apparatus includes two Mach-Zehnder interferometers. One is a main measuring interferometer 102 including a transmission path to be measured, and the other is a reference interferometer 103 used for monitoring phase noise of the light source 101. A light source having a frequency sweeping unit is used as the light source 101, and continuous light that is frequency swept linearly with respect to time is emitted. At this time, the beat signal I _FUT (t) obtained by the main measurement interferometer 102 can be described as follows.

Where a _m and τ _m are the amplitude and delay time of the transmitted light of the spatial channel m, τ _Lo is the delay time of the local light, γ is the frequency sweep speed, ν ₀ is the initial frequency of the test light, and θ (t) is the light source The phase noises φ _FUT (t) and φ _Lo (t) are phase noises derived from disturbances such as vibrations in the transmission path to be measured and the local optical path, respectively. Here, the emitted light E (t) from the light source, the transmitted light E _m (t) of the spatial channel m, and the local light E _Lo (t) are respectively expressed as follows.

The beat signal I _Ref (t ′) obtained by the reference interferometer 103 is described by the following equation.

Here, τ _Ref is the delay time of the delay fiber used in the reference interferometer 103. Here, it is assumed that the reference interferometer 103 is installed in an environment that is not affected by disturbance, and phase noise derived from the fiber can be ignored. When acquiring I _Ref (t ′), if the acquisition start time is adjusted so that t ′ = t−τ _Lo , the beat signal of the reference interferometer 103 is expressed by the following equation.

Using I _Ref (t), the phase noise included in I _FUT (t) is reduced. Specifically, by detecting the I _Ref (t) using a used or, or 90-degree hybrid optical circuit a Hilbert transform of I _Ref (t), it extracts the phase information of the I _Ref (t), optionally I _FUT (t) is resampled based on the time interval t _i (i is a natural number) corresponding to the phase change δ interval of (Non-patent Document 2). The phase noise Θ (t _i ) remaining after resampling is as follows:

Here, t _i is a time sequence that satisfies the following equation.

Note that equation (7), the small delay time difference between the spatial _{channels, τ n ≒ τ m = τ} FUT (τ n is the delay time of the spatial channel n) was able to approximation. From equation (7), when a delay fiber satisfying τ _Ref = τ _FUT −τ _Lo is used for the reference interferometer, Θ (t _i ) = 0 and the phase noise of the light source is canceled. If φ _FUT (t) and φ _Lo (t) are ignored, the beat frequency γ (τ _m −τ _Lo ) in I _FUT (t) corresponds to the delay time of the spatial channel m. Therefore, the beat frequency spectrum intensity waveform obtained by Fourier transforming I _FUT (t) corresponds to a so-called impulse response waveform, and mode coupling and delay time difference between spatial channels can be evaluated.

T.-J. Ahn and D. Y. Kim, “High-resolution differential mode delay measurement for a multimode optical fiber using a modified optical frequency domain reflectometer,” Opt. Express 13 (20), 8256-8262 (2005). J. Song, W. Li, P. Lu, Y. Xu, L. Chen, and X. Bao, “Long-Range High Spatial Resolution Distributed Temperature and Strain Sensing Based on Optical Frequency-Domain Reflectometry,” IEEE Photonics Journal 6 (3), 6801408 (2014).

  However, as described above, by using the reference interferometer 103, the influence of the phase noise of the light source 101 can be compensated. However, in order to obtain the compensation effect, the main measurement interferometer 102 and the reference interferometer 103 are respectively used. Since it is necessary to prepare the light receivers 104 and 106 and the A / D converters 105 and 107, the cost of the device configuration is high, and the data acquisition timing is not synchronized between the two interferometers 102 and 103. must not.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an apparatus and a method capable of easily evaluating a spatially multiplexed optical transmission line with a simpler apparatus configuration than conventional ones. .

In order to solve the above-mentioned problems, the present invention is a spatially multiplexed optical transmission line evaluation apparatus, comprising: a light source that emits continuous light that has been swept in frequency; and a plurality of light sources that demultiplex the continuous light and provide different delays. An interferometer that includes an optical path and outputs a beat signal by combining continuous light after propagation through a measured spatially multiplexed optical fiber and continuous light after propagation through the plurality of optical paths, wherein the plurality of optical paths are The interferometer, comprising: a first optical path for providing a delay τ _Lo to the measured spatially multiplexed optical fiber; and a second optical path for providing a delay τ _Ref to the first optical path; A light detection means for detecting the beat signal; and an arithmetic processing means for evaluating a delay time difference or mode coupling between the spatial channels of the measured spatial multiplexing optical fiber using the beat signal. Obtained by the light detection means Filtering the beat signal, the first beat signal generated by the interference between the continuous light after propagation through the measured spatially multiplexed optical fiber and the continuous light after propagation through the first optical path, A second beat signal generated by interference between continuous light after propagation through the first optical path and continuous light after propagation through the second optical path is separated and extracted, and phase noise of the light source of the second beat signal is extracted. Using the resulting phase time change X ₁ (t), the phase time change X _N (t) due to the phase noise of the light source in the case of delay Nτ _Ref (N is a natural number) is calculated from the following equation:

Using the X _N (t), the phase fluctuation of the first beat signal is corrected, the first beat signal after the phase fluctuation correction is Fourier-transformed, and the spatial channel between the spatial multiplexed optical fibers to be measured It is characterized in that the delay time difference or the mode coupling of is evaluated.

According to a second aspect of the present invention, in the spatial multiplexing optical transmission line evaluation apparatus according to the first aspect, the delay τ _Ref of the second optical path is an average value of the delay of the measured spatial multiplexing optical fiber. As τ _FUT , τ _Ref <| τ _FUT −τ _Lo | / 2 is satisfied, and in the calculation of X _N (t), the arithmetic processing means sets N to 3 or more and | τ _FUT −τ _Lo | / τ _Ref It is characterized by using the nearest integer.

The invention described in claim 3 is the spatial multiplexing optical path evaluation apparatus according to claim 1 or 2, wherein | τ _FUT −τ _Lo −Nτ _Ref | is smaller than the coherence time of the continuous light. And

The invention according to claim 4 is a spatially multiplexed optical transmission line evaluation method, comprising the steps of emitting a frequency-swept continuous light, demultiplexing the continuous light, a measured spatially multiplexed optical fiber, and the measured optical fiber. The measured spatial multiplexed light is incident on a first optical path that gives a delay τ _Lo to the measurement spatial multiplexing optical fiber and a second optical path that gives a delay τ _Ref to the first optical path, respectively. Combining the continuous light after propagation through the fiber and the continuous light after propagation through the first and second optical paths and outputting a beat signal; receiving the beat signal and converting it into an electrical signal; By applying a filtering process to the electrical signal, a first beat signal generated by interference between continuous light after propagation through the measured spatially multiplexed optical fiber and continuous light after propagation through the first optical path, and Continuous light after propagation through the first optical path Separating and extracting a second beat signal generated by interference with continuous light after propagating through the second optical path; and temporal change in phase of the second beat signal due to phase noise of the light source of the continuous light X ₁ (t) is used to calculate the time variation X _N (t) of the phase due to the phase noise of the light source in the case of delay Nτ _Ref (N is a natural number) from the following equation:

The step of correcting the phase fluctuation of the first beat signal using the X _N (t), the Fourier transform of the first beat signal after the phase fluctuation correction, and the space in the measured spatial multiplexing optical fiber Evaluating a delay time difference between channels or mode coupling.

According to a fifth aspect of the present invention, in the spatially multiplexed optical transmission line evaluation method according to the fourth aspect, the delay τ _Ref of the second optical path is an average value of the delay of the measured spatially multiplexed optical fiber τ. _The step of satisfying τ _Ref <| τ _FUT −τ _Lo | / 2 as the _FUT and correcting the phase fluctuation of the first beat signal is performed in the calculation of X _N (t) with N being 3 or more and | τ _FUT An integer closest to −τ _Lo | / τ _Ref is used.

The invention described in claim 6 is the spatial multiplexing optical transmission line evaluation method according to claim 4 or 5, wherein | τ _FUT −τ _Lo −Nτ _Ref | is smaller than the coherence time of the continuous light. And

  According to a seventh aspect of the present invention, in the spatially multiplexed optical transmission line evaluation method according to any one of the fourth to sixth aspects, the first optical path is accommodated in the same cable as the measured spatially multiplexed optical fiber. It is characterized by being an optical fiber.

  The invention according to claim 8 is the spatially multiplexed optical path evaluation method according to any one of claims 4 to 6, wherein the first optical path is accommodated in the same cable as the measured spatially multiplexed optical fiber. It is characterized by being an optical fiber.

  By using the present invention, the FMCW method can be implemented with fewer light receivers and A / D converters than in the past, so that the apparatus can be configured at low cost. Further, since the measurement can be performed with one A / D converter, it is not necessary to synchronize the data recording timing among a plurality of A / D converters as in the conventional case.

It is a block diagram which shows an example of the apparatus structure used with the conventional FMCW method. (A) is a block diagram which shows an example of the apparatus structure used by embodiment of this invention, (b) is an enlarged view of the connection point of a spatial multiplexing optical fiber. It is an example of the spectrum intensity | strength of the beat signal obtained by this invention. It is a flowchart which shows the flow of the measurement in embodiment of this invention.

  In the conventional FMCW method, the light is branched immediately after the light source, and the reference interferometer and the main measurement interferometer are configured independently at the lower part of the branch, whereas in the present invention, the reference interferometer is used as the local optical path of the main measurement interferometer. The above two interferometer signals are collected by one light receiver and A / D converter, and the beat signal of the reference interferometer and main measurement interferometer is separated by digital filtering processing to solve the above problem. To do. Specifically, it can be configured as shown in FIG. 2 as in an embodiment of the present invention described later.

In this apparatus configuration, the delay fiber used for the reference interferometer needs to satisfy τ _Ref ≠ τ _FUT −τ _Lo so that the beat signals of the reference interferometer and the main measurement interferometer do not overlap on the frequency axis. However, in order to compensate for the phase noise of the light source, a beat signal corresponding to τ _Ref ≒ τ _FUT- τ _Lo is required, so that it can be obtained with a delay fiber of τ _Ref ≠ τ _FUT- τ _Lo by the following signal processing Calculate the signal of τ _Ref ≒ τ _FUT -τ _{Lo from} the signal.

The time change X ₁ (t) of the beat signal of the reference interferometer in the apparatus configuration of the present invention is described by the following equation.

Here, a term not dependent on t that is unnecessary for phase noise compensation is ignored. In addition, the delay fiber of the reference interferometer is installed in an environment that is not affected by the disturbance, and the phase noise derived from the disturbance in the delay fiber can be ignored. By calculating the following equation using X ₁ (t), the time variation X _N (t) of the phase when the delay of the reference interferometer is Nτ _Ref (N is a natural number) is calculated by the following equation.

If a term that does not depend on t is ignored, the calculation result of Expression (10) is as follows.

Using X _N (t), I _FUT (t) is resampled with a time sequence t _i corresponding to an arbitrary phase change interval, as in the conventional method. As a result of resampling, the remaining phase noise Θ _N (t _i ) is as follows:

From equation (12), the phase noise of the light source is canceled when Nτ _Ref = τ _FUT −τ _Lo . Therefore, by using a delay fiber satisfying τ _Ref ≈ (τ _FUT −τ _Lo ) / N for the reference interferometer, the FMCW method can be implemented with one light receiver and A / D converter. Note that τ _Ref may not strictly satisfy τ _Ref = (τ _FUT −τ _Lo ) / N if | τ _FUT −τ _Lo −Nτ _Ref | is sufficiently smaller than the coherence time of the light source.

  Furthermore, in the configuration of the present invention, phase noise derived from disturbance added to the local optical path can be monitored by the reference interferometer, so that the measured optical fiber and the local optical path are in an environment where the same disturbance is applied to each other (in the same cable, etc.) Also, phase noise derived from disturbance can be compensated. When the disturbance applied in the measured optical fiber and the local optical path is the same, the following relationship is established.

From equation (13), equation (12) is described as:

When the delay fiber of the reference interferometer satisfies τ _Ref = (τ _FUT −τ _Lo ) / N, Φ _N (t _i ) becomes zero as shown in the following equation.

Therefore, according to the present invention, not only the light source but also the phase noise derived from the disturbance applied to the transmission path can be compensated.

Here, the case where only the local optical path is in the same environment as the fiber to be measured has been described. However, even if the delay fiber of the reference interferometer is also in the same environment as the local optical path, the phase noise derived from the disturbance Can be compensated. In this case, X ₁ (t) can be described as:

Here, φ _Ref (t) is phase noise due to disturbance applied to the delay fiber of the reference interferometer. When the disturbance applied to the delay fiber can be regarded as the same as the local optical path, the following relationship is established.

As a result of substituting equation (18) into equation (17), X ₁ (t) is the same as in equation (9), and in this case as well, phase noise derived from disturbance can be compensated.

Next, the condition of the value of N that can be used in the present invention will be described below. The point to be noted in the present invention is that a signal obtained by receiving light includes the following three types of beat signals, and it is necessary to design each beat signal so as not to overlap on the frequency axis.
# 1: Beat signal of transmitted light of measured optical fiber and local light (transmitted light of optical path different from delay fiber of reference interferometer) # 2: Beat signal of transmitted light of measured optical fiber and delayed fiber transmitted light of reference interferometer # 3: Reference interferometer beat signal

FIG. 3 shows the spectral intensity of the beat signal obtained by the present invention. In the present invention, the beat signals # 1 to # 3 are separated by digital filtering, and phase noise compensation is performed on the signal # 1 using the phase of the beat signal # 3. If the delay fiber of the reference interferometer satisfies τ _Ref = (τ _FUT −τ _Lo ) / N, the beat frequencies of # 1 to # 3 are as follows.
Beat frequency of # 1: γ (τ _FUT -τ _Lo )
Beat frequency of # 2: γ (1-1 / N) (τ _FUT −τ _Lo )
Beat frequency of # 3: γ (τ _FUT −τ _Lo ) / N
Therefore, since these three frequencies do not overlap each other, in the present invention, it is necessary to use an integer of 3 or more for N.

  Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Here, as an example, a case where a multimode single core optical fiber is used as the optical fiber to be measured will be described.

  FIG. 2 shows the configuration of a spatially multiplexed optical transmission line evaluation apparatus according to an embodiment of the present invention. In FIG. 2, the optical fiber other than the optical fiber 221 to be measured is configured by a single mode single core optical fiber. The optical fiber 221 to be measured is not included in the spatial multiplexing optical transmission line evaluation apparatus, but is installed in the spatial multiplexing optical transmission path evaluation apparatus at the time of measurement. The optical multiplexer 211 and the optical demultiplexing are detachable. Connected to the device 214.

  A light source having a frequency sweep means is used as the light source 201, and continuous light that is frequency swept linearly with respect to time is emitted. The emitted continuous light is branched into two by the first optical demultiplexer 211, one being test light incident on the measured optical fiber 221 and the other being local light. The test light enters the measured optical fiber 221 through the off-axis connection point, and a plurality of propagation modes are excited in the measured optical fiber 221. Here, instead of the off-axis connection point, a plurality of propagation modes may be excited using other means such as a mode multiplexer.

The local light is further branched into two by the second optical demultiplexer 212 after propagating through the optical fiber 222 having the delay τ _Lo , one of which propagates through the optical fiber 223 having the delay τ _Ref . Beat signals generated by combining and combining all three propagation lights branched by the first and second optical demultiplexers 211 and 212 by the first and second optical multiplexers 213 and 214. Is converted into an electric signal by the light receiver 204 and converted into a digital signal by the A / D converter 205.

Note that the spatial multiplexing optical transmission line evaluation apparatus of the present invention does not need to include the optical fiber 222 having the delay τ _Lo and the optical fiber 223 having the delay τ _Ref , similarly to the optical fiber 221 to be measured. An external optical fiber may be connected to the wave multiplexers 211 and 212 and the optical multiplexers 213 and 214 as an optical fiber 222 having a delay τ _Lo and an optical fiber 223 having a delay τ _Ref .

  Next, the arithmetic processing unit 206 evaluates the inter-mode delay time difference and the mode coupling in the measured optical fiber 221 using the obtained beat signal. FIG. 4 shows a flowchart of calculation processing performed by the arithmetic processing unit 206.

  First, the beat signal is Fourier transformed to analyze the beat frequency (step S1). If the beat frequencies of # 1 to # 3 are known, step S1 is not necessarily performed.

  Next, digital filtering processing is performed on the beat signal, and the beat signals # 1 and # 3 are extracted (step S2).

Next, the time change X ₁ (t) of the phase of the beat signal when the delay output from the reference interferometer 203 is τ _Ref using the beat signal I _Ref (t) of # 3 extracted in step S2 is next It is obtained by an equation (step S3).

Here, H {I _Ref (t)} is a Hilbert transform of I _Ref (t).

Next, using X ₁ (t) obtained in step S3, equation (10) is calculated to obtain the time change X _N (t) of the phase of the beat signal when the delay is Nτ _Ref (step S4).

Next, determine the time sequence t _i corresponding to a phase change of the predetermined interval (step S5). The time sequence t _i is a time sequence satisfying X _N (t _i ) = iδ (δ is a phase interval).

Then, the # 1 beat signal extracted in step S2 is resampled based on the time sequence t _i obtained in step S5 (step S6).

  Finally, the # 1 beat signal resampled in step S6 is Fourier transformed to obtain a spectrum intensity waveform (step S7). The obtained spectral intensity waveform corresponds to a so-called impulse response waveform, and mode coupling and delay time difference between spatial channels can be evaluated.

  In this embodiment, a multimode single-core optical fiber is evaluated. However, the present invention is not limited to this, and the present invention is not limited to this, and may be implemented on a single-mode multicore optical fiber or a multimode multicore optical fiber. A fan-in / fan-out device or the like corresponding to a multi-core optical fiber may be used as means for exciting the light.

  By using the present invention, the FMCW method can be implemented with fewer light receivers and A / D converters than in the past, so that the apparatus can be configured at low cost. Further, since the measurement can be performed with one A / D converter, it is not necessary to synchronize the data recording timing among a plurality of A / D converters as in the conventional case. Especially in the situation of transmission line evaluation after laying, it is necessary to carry out measurement between two distant points in the conventional method and it is difficult to synchronize the measurement timing. However, according to the present invention, only one point on the light receiving side is required. It becomes possible to record data.

  Furthermore, when a fiber accommodated in the same cable as the optical fiber to be measured is used for the local optical path, not only the device configuration is simplified by sharing the cable between the two optical paths, but also phase noise derived from disturbance can be reduced. The present invention has a great advantage over the conventional method in terms of maintenance and operation of the optical transmission line.

101, 201 Frequency swept light source 102, 202 Main measurement interferometer 103, 203 Reference interferometer 104, 106, 204 Light receiver 105, 107, 205 A / D converter 108 Data recording timing synchronization means 109, 206 Arithmetic processing device 211, 212 Optical demultiplexers 213 and 214 Optical multiplexer 221 Optical fiber to be measured 222 Optical fiber with delay τ _Lo 223 Optical fiber with delay τ _Ref

Claims (8)

A light source that emits frequency-swept continuous light;
Including a plurality of optical paths that demultiplex the continuous light to give different delays, and combine the continuous light after propagation through the measured spatially multiplexed optical fiber and the continuous light after propagation through the plurality of optical paths to generate a beat signal The plurality of optical paths include a first optical path that gives a delay τ _Lo to the measured spatially multiplexed optical fiber, and a delay τ _Ref for the first optical path. The interferometer comprising: a second optical path providing:
Light detection means for detecting the beat signal;
Computation processing means for evaluating a delay time difference or mode coupling between spatial channels of the measured spatial multiplexing optical fiber using the beat signal,
The arithmetic processing means performs a filtering process on the beat signal obtained by the light detection means, so that continuous light after propagation through the measured spatially multiplexed optical fiber and continuous light after propagation through the first optical path Separating and extracting the first beat signal generated by interference with the second beat signal generated by interference between the continuous light after propagation through the first optical path and the continuous light after propagation through the second optical path; Phase variation due to phase noise of the light source in the case of delay Nτ _Ref (N is a natural number) using phase variation X ₁ (t) due to phase noise of the light source of the second beat signal X _N (t) is calculated from the following equation:

Using the X _N (t), the phase fluctuation of the first beat signal is corrected, the first beat signal after the phase fluctuation correction is Fourier-transformed, and the spatial channel between the spatial multiplexed optical fibers to be measured An apparatus for evaluating a spatially multiplexed optical transmission line, characterized by evaluating a delay time difference or mode coupling. The delay τ _Ref of the second optical path satisfies τ _Ref <| τ _FUT −τ _Lo | / 2, where τ _FUT is the average value of the delay of the measured spatial multiplexing optical fiber,
2. The spatial multiplexing according to claim 1, wherein the arithmetic processing unit uses an integer equal to or greater than 3 and closest to | τ _FUT −τ _Lo | / τ _Ref in the calculation of X _N (t). Optical transmission line evaluation device. 3. The spatially multiplexed optical transmission line evaluation apparatus according to claim 1, wherein | τ _FUT −τ _Lo −Nτ _Ref | is smaller than the coherence time of the continuous light. Emitting a frequency-swept continuous light;
The continuous light is demultiplexed, a measured spatially multiplexed optical fiber, a first optical path that gives a delay τ _Lo to the measured spatially multiplexed optical fiber, and a delay τ to the first optical path Each incident light enters a second optical path that gives a _Ref, and _combines the continuous light after propagation through the measured spatially multiplexed optical fiber and the continuous light after propagation through the first and second optical paths, and outputs a beat signal. And steps to
Receiving the beat signal and converting it into an electrical signal;
By applying a filtering process to the electrical signal, a first beat signal generated by interference between continuous light after propagation through the measured spatially multiplexed optical fiber and continuous light after propagation through the first optical path, and Separating and extracting a second beat signal generated by interference between continuous light after propagation through the first optical path and continuous light after propagation through the second optical path;
Phase due to the phase noise of the light source in the case of a delay Nτ _Ref (N is a natural number) using the time variation X ₁ (t) of the phase due to the phase noise of the light source of the continuous light of the second beat signal. time change X _N (t) is the calculated from the following equation,

Correcting the phase fluctuation of the first beat signal using the X _N (t);
Fourier transforming the first beat signal after the phase fluctuation correction, and evaluating a delay time difference or mode coupling between spatial channels in the measured spatial multiplexing optical fiber;
A spatially multiplexed optical transmission line evaluation method characterized by comprising: The delay τ _Ref of the second optical path satisfies τ _Ref <| τ _FUT −τ _Lo | / 2, where τ _FUT is the average value of the delay of the measured spatial multiplexing optical fiber,
The step of correcting the phase fluctuation of the first beat signal is characterized in that an integer closest to | τ _FUT −τ _Lo | / τ _Ref is used for N in the calculation of X _N (t). The method of evaluating a spatially multiplexed optical transmission line according to claim 4. 6. The spatially multiplexed optical transmission line evaluation method according to claim 4, wherein | τ _FUT −τ _Lo −Nτ _Ref | is smaller than the coherence time of the continuous light.   7. The spatially multiplexed optical transmission line evaluation method according to claim 4, wherein the first optical path is an optical fiber accommodated in the same cable as the measured spatially multiplexed optical fiber. .   7. The spatially multiplexed optical transmission line evaluation method according to claim 4, wherein the first optical path is an optical fiber accommodated in the same cable as the measured spatially multiplexed optical fiber. .

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