JP2018205014A - Method and device for measuring dispersion of spatial mode - Google Patents

Abstract

To provide a method and a device for measuring a spatial mode that can measure the SMD of a strong-coupling SDM transmission path non-destructively and distributedly.SOLUTION: The spatial mode distribution measuring device according to the present invention acquires an autocorrelation peak distribution for an arbitrary section component extracted from an amplitude distribution waveform related to a propagation delay time of rear Rayleigh scattered light in a strong-coupling SDM transmission path, and uses the fact that the secondary moment of the peak distribution (other than the center peak) is the SMD of the arbitrary section.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a technique for measuring a distribution of propagation delay time spread in a spatially multiplexed optical transmission line using a multimode optical fiber, a multicore optical fiber, or the like.

  As a technique for expanding the optical transmission capacity per optical fiber, there is a spatial multiplexing transmission technique (hereinafter referred to as 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 signal processing at the receiving end becomes complicated if characteristics such as loss and propagation delay time differ between spatial channels. Therefore, a transmission path in which the dependence of the above characteristics on the spatial channel is reduced by actively utilizing coupling between propagation modes using a strong coupling multicore optical fiber or the like has been recently proposed (for example, see Non-Patent Document 1). ).

  In such a spatially multiplexed transmission line with strong mode coupling (hereinafter, strongly coupled SDM transmission line), the propagation delay time has a Gaussian distribution due to random mode coupling during light propagation. This phenomenon is similar to the coupling between polarization modes in a conventional single mode optical fiber. Like polarization mode dispersion (hereinafter referred to as PMD), spatial phenomenon (hereinafter referred to as spatial mode dispersion) is used as an index for evaluating a strongly coupled SDM transmission line. SMD) is defined.

  It is known that the difference in delay time between spatial channels increases in proportion to the propagation distance in a transmission line with weak mode coupling, whereas SMD in a strong coupling SDM transmission line is proportional to the square root of the propagation distance. Due to such characteristics, the strong coupling SDM transmission line is expected as a large-capacity transmission line for long distances.

  SMD can be measured by the same method as conventional PMD measurement. For example, in Non-Patent Document 1, a fixed analyzer method (for example, see Non-Patent Document 2) standardized as an alternative test method for PMD measurement is used. SMD is measured. In addition, SMD measurement is also possible using low-coherence interferometry (see, for example, Non-Patent Document 2), which is also an alternative test method for PMD.

T.A. Sakamoto et al. , “Fiber Twisting- and Bending-Induced Adiabatic / Nondiabatic Super-Mode Transition in Coupled Multicore Fiber”, J. et al. Lightwave Technol. , Vol. 34, no. 4, p. 1228 (2016). ITU-T Recommendation G. 650.2, “Definitions and test methods for statistical and non-linear related attributes of single-mode fibers and cables” (2015).

  As reported in Non-Patent Document 1, it is known that SMD changes sensitively by bending or twisting of an optical fiber. Therefore, it is assumed that the local change occurs in the transmission path depending on the cable structure, the laying condition, and the like. However, although the conventional fixed analyzer method and low coherence interference method can measure the SMD of the entire transmission line, there is a problem that the SMD in the middle of the transmission line cannot be measured non-destructively.

  Accordingly, an object of the present invention is to provide a spatial mode dispersion measuring method and a spatial mode dispersion measuring apparatus capable of non-destructively measuring SMD of a strongly coupled SDM transmission line in order to solve the above-described problems.

  The spatial mode dispersion measuring method and the spatial mode dispersion measuring apparatus according to the present invention calculate the SMD of the strongly coupled SDM transmission line by calculating the autocorrelation of an arbitrary interval component of the backward Rayleigh scattered light amplitude distribution observed in the light reflection measurement. Non-destructive and distributed acquisition is possible.

Specifically, the spatial mode dispersion measurement method according to the present invention is:
A measurement procedure for measuring an amplitude distribution waveform with respect to a propagation delay time of backward Rayleigh scattered light in a spatially multiplexed optical transmission line;
An arbitrary section component of the amplitude distribution waveform is extracted, an autocorrelation of the extracted amplitude distribution waveform is calculated, and a second moment of the correlation peak distribution excluding the central peak of the autocorrelation is calculated as the arbitrary section of the spatial multiplexing optical transmission line A calculation procedure for spatial mode dispersion in
I do.

The spatial mode dispersion measuring apparatus according to the present invention is
Measuring means for measuring an amplitude distribution waveform with respect to a propagation delay time of backward Rayleigh scattered light in a spatially multiplexed optical transmission line;
An arbitrary section component of the amplitude distribution waveform is extracted, an autocorrelation of the extracted amplitude distribution waveform is calculated, and a second moment of the correlation peak distribution excluding the central peak of the autocorrelation is calculated as the arbitrary section of the spatial multiplexing optical transmission line Arithmetic means for spatial mode dispersion in
Is provided.

  The present invention measures an amplitude distribution waveform with respect to the propagation delay time of backward Rayleigh scattered light in a spatially multiplexed optical transmission line, calculates an autocorrelation for an arbitrary interval component extracted from the amplitude distribution waveform, and analyzes an autocorrelation peak distribution To do. The present invention utilizes the fact that the second moment of the peak distribution is the SMD of the arbitrary section.

  Therefore, the present invention can provide a spatial mode dispersion measuring method and a spatial mode dispersion measuring apparatus capable of non-destructively measuring SMD of a strongly coupled SDM transmission line in a non-destructive manner.

  In order to measure SMD in an arbitrary section, a sufficiently small delay resolution (less than ps order) is required for SMD. For this reason, when acquiring an amplitude distribution waveform by this invention, it is preferable to utilize an optical frequency domain reflectometer (OFDR: Optical Frequency Domain Reflectometry).

That is, in the measurement procedure of the spatial mode dispersion measurement method according to the present invention,
Branch the continuous light swept in frequency,
One of the branched continuous lights is incident on the spatially multiplexed optical transmission line;
Detecting the beat signal by combining the back Rayleigh scattered light generated in the spatially multiplexed optical transmission line and the other of the branched continuous light;
The beat signal is Fourier transformed to obtain the amplitude distribution waveform.

Further, the measuring means of the spatial mode dispersion measuring apparatus according to the present invention comprises:
A light source that emits frequency-swept continuous light;
An optical branching device for branching continuous light emitted from the light source;
One of the continuous lights branched by the optical splitter is incident on the spatially multiplexed optical transmission line, and backward Rayleigh scattered light generated in the spatially multiplexed optical transmission line and the continuous light split by the optical splitter. Detecting a beat signal by combining with the other, an optical detector for Fourier-transforming the beat signal to obtain the amplitude distribution waveform;
It is characterized by having.

  The present invention can provide a spatial mode dispersion measuring method and a spatial mode dispersion measuring apparatus capable of non-destructively measuring SMD of a strongly coupled SDM transmission line.

It is a conceptual diagram which shows the measurement principle of the spatial mode dispersion | distribution measuring method which concerns on this invention. 1 is a block diagram illustrating a spatial mode dispersion measuring apparatus according to the present invention. It is a flowchart explaining the spatial mode dispersion | distribution measuring method which concerns on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

[Measurement principle]
FIG. 1 is a conceptual diagram showing the measurement principle of the spatial mode dispersion measurement method of this embodiment. In the light reflection measurement, the amplitude distribution of the backward Rayleigh scattered light in the optical fiber can be observed as a function of the propagation delay time (FIG. 1B). At this time, when light having a sufficiently long coherence time with respect to the delay resolution is used as the test light, the observed amplitude distribution is observed in a random zigzag waveform (hereinafter, a scattering signature) due to the interference of the scattered light. When the light reflection measurement is performed on the strongly coupled SDM transmission line, the delay time corresponding to the scattering point has a plurality of delay times in a Gaussian distribution due to random mode coupling during propagation of the backscattered light. That is, duplicated scatter signatures exist in a Gaussian distribution on the delay axis, and a waveform of their superposition is observed.

τは遅延時間、δτ_ｍとθ_ｍはそれぞれｍ番目のモード結合成分の遅延時間シフトと位相変化、ε（τ）は結合を考慮しない場合の後方散乱光振幅である。 When the delay spread of the backscattered light amplitude assuming Gaussian standard deviation sigma _a, backscattered light amplitude observed it can be described as follows.

τ is the delay time, δτ _m and θ _m are the delay time shift and phase change of the m-th mode coupling component, respectively, and ε (τ) is the backscattered light amplitude when coupling is not considered.

つまり、ＳＭＤはΔτを求めることで得られることになる。 Since SMD is defined as twice the standard deviation of the delay spread of the impulse response (light intensity), there is a relationship of the following equation with respect to the delay spread σ _a of the optical amplitude.

That is, SMD can be obtained by obtaining Δτ.

モード結合によって生じる散乱シグネチャの複製は互いに相関を持つことから、遅延広がりは観測される後方散乱光振幅分布の任意区間成分の自己相関により解析できる（図１（Ｃ））。自己相関Ｒ（τ’）（τ’は相対遅延）は次式で計算される。

ここで＊は複素共役を表す。δτ_ｎとθ_ｎはそれぞれｎ番目のモード結合成分の遅延時間シフトと位相変化である。なお、式（３）では散乱シグネチャが光ファイバ中のランダムな屈折率揺らぎに起因する不規則なジグザグ波形であることから、次式の関係が成り立つと仮定した。

Equation (1) can be described as the following equation using Δτ of equation (1a).

Since the copy of the scattering signature caused by mode coupling has a correlation with each other, the delay spread can be analyzed by the autocorrelation of an arbitrary interval component of the observed backscattered light amplitude distribution (FIG. 1C). Autocorrelation R (τ ′) (τ ′ is a relative delay) is calculated by the following equation.

Here, * represents a complex conjugate. δτ _n and θ _n are the delay time shift and phase change of the nth mode coupling component, respectively. In Equation (3), since the scattering signature is an irregular zigzag waveform caused by random refractive index fluctuations in the optical fiber, it is assumed that the relationship of the following equation holds.

  An example of the autocorrelation waveform calculated by Expression (3) is shown in FIG. As described in Equation (3), the first term, a strong correlation peak appears at τ ′ = 0 due to the correlation with the backscattered light amplitude waveform itself. On the other hand, as shown in the second term of Equation (3), small correlation peaks are distributed even in the region of τ ′ ≠ 0 due to the correlation between the duplicates of the scattered signatures existing at different delay times.

  The correlation peak intensity at τ ′ ≠ 0 has irregular variations due to the addition of the random phase terms in the second term of Equation (3), but takes a larger value as the | τ ′ | The square root of the second moment of the correlation peak distribution at '≠ 0 corresponds to Δτ. Therefore, by analyzing the autocorrelation peak distribution in an arbitrary section of the backscattered light amplitude distribution waveform, the SMD in the middle of the transmission path can be obtained nondestructively.

[Embodiment]
Embodiments of the present invention will be described with reference to the accompanying drawings. Here, as an example, a case will be described in which an optical frequency domain reflection measurement method (OFDR) is used for measuring the backscattered light amplitude distribution, and an arbitrary core is measured using a strongly-coupled multicore optical fiber as the fiber to be measured.

FIG. 2 is a diagram illustrating the spatial mode dispersion measuring apparatus 301 of the present embodiment. The spatial mode dispersion measuring apparatus 301 is
Measuring means for measuring an amplitude distribution waveform with respect to a propagation delay time of backward Rayleigh scattered light in a spatially multiplexed optical transmission line;
An arbitrary section component of the amplitude distribution waveform is extracted, an autocorrelation of the extracted amplitude distribution waveform is calculated, and a second moment of the correlation peak distribution excluding the central peak of the autocorrelation is calculated as the arbitrary section of the spatial multiplexing optical transmission line Arithmetic means for spatial mode dispersion in
Is provided.

In addition, the spatial mode dispersion measuring method using the spatial mode dispersion measuring apparatus 301 is:
A measurement procedure for measuring an amplitude distribution waveform with respect to a propagation delay time of backward Rayleigh scattered light in a spatially multiplexed optical transmission line;
An arbitrary section component of the amplitude distribution waveform is extracted, an autocorrelation of the extracted amplitude distribution waveform is calculated, and a second moment of the correlation peak distribution excluding the central peak of the autocorrelation is calculated as the arbitrary section of the spatial multiplexing optical transmission line A calculation procedure for spatial mode dispersion in
I do.

In this embodiment, the spatially multiplexed optical transmission line is the measured fiber 50.
And since OFDR is used, the measuring means is
A light source 11 that emits continuous light having a frequency sweep;
An optical branching device 12 for branching continuous light emitted from the light source 11;
One of the continuous lights branched by the optical branching device 12 is incident on the spatially multiplexed optical transmission line, and backward Rayleigh scattered light generated in the spatially multiplexed optical transmission line and the continuous light branched by the optical splitter 12 Detecting a beat signal by combining with the other, an optical detector for Fourier-transforming the beat signal to obtain the amplitude distribution waveform;
Have
The optical detector includes a light receiver 15, an A / D converter 16, and an arithmetic processing device 17.

  First, the spatial mode dispersion measuring apparatus 301 obtains a beat signal by coherently detecting backscattered light for an arbitrary core of the measured fiber 50. In FIG. 2, a fiber other than the fiber to be measured 50 is configured by a single mode single core optical fiber.

  A light source having frequency sweeping means is used as the light source 10, and continuous light that is frequency swept linearly with respect to time is emitted. The emitted continuous light is bifurcated by the optical splitter 12, one is used as test light that enters the fiber to be measured 50, and the other is used as local light when coherently detecting backscattered light. The test light is incident on an arbitrary core of the measured fiber 50, and a part of the test light is Rayleigh scattered in the measured fiber 50. At this time, Rayleigh scattered light propagates behind the incident direction while being randomly coupled to a plurality of cores or modes. The backscattered light emitted from the core on which the test light is incident is separated from the test light by the optical circulator 13 and combined with the local light by the optical multiplexer 14. A beat signal resulting from the combination of the backscattered light and the local light is converted into an electric signal by the light receiver 15 and converted into a digital signal by the A / D converter 16.

  Next, in the arithmetic processing unit 17, the SMD at an arbitrary point of the measured fiber 50 is obtained using the beat signal. FIG. 3 is a flowchart for explaining a calculation procedure of SMD analysis performed by the calculation processing device 17.

First, as step 1, the arithmetic processing unit 17 performs Fourier transform on the beat signal to obtain a backscattered light amplitude distribution with respect to the propagation delay time, and then stores it in the data storage unit 18.
Next, as step 2, the arithmetic processing unit 17 cuts out an arbitrary section component centered on the point where SMD is measured from the amplitude distribution obtained in step 1.
Next, in step 3, the arithmetic processing unit 17 calculates the autocorrelation of the amplitude distribution waveform cut out in step 2 using equation (3).
Next, as step 4, the arithmetic processing unit 17 removes a correlation peak centered at τ ′ = 0 in the autocorrelation waveform obtained at step 3. At this time, the width of the correlation peak to be removed is larger than the delay resolution of OFDR and smaller than SMD.
Next, in step 5, the arithmetic processing unit 17 uses the absolute value | R ′ (τ ′) | of the autocorrelation obtained by removing the central peak in step 4 to obtain Δτ as SMD by the following equation.

  When obtaining the SMD of another point of the measured fiber 50, the arithmetic processing unit 17 changes the center position of the section in which the backscattered light amplitude distribution waveform is cut out in step 2, and performs steps 3 to 5. At this time, the backscattered light amplitude distribution stored in Step 1 may be used again to perform Step 2 and the subsequent steps, and it is not necessary to perform beat signal acquisition and Fourier transform again.

  In this embodiment, OFDR is performed on an arbitrary core of a strongly coupled multicore optical fiber. However, the present invention is not limited to this, and measurement is performed across multiple cores of a multicore optical fiber, or single core multimode light is used. You may measure on fiber. Further, as a measurement means, a light reflection measurement method other than OFDR may be used as long as it has a sufficiently small delay resolution (ps order or less) with respect to SMD.

[effect]
By using the present invention, SMD in a strongly coupled SDM transmission line can be measured nondestructively, so that SMD evaluation can be performed not only after fiber production but also after cable installation and after laying. Furthermore, since the present invention completes the measurement at one end of the transmission line, when evaluating the transmission line after laying, the SMD can be monitored remotely from the station building. In particular, in a strong coupling SDM transmission line, it is assumed that the SMD changes locally depending on the cable structure and the installation environment as described above. Therefore, the present invention is a conventional measurement method from the viewpoint of maintenance and operation of the transmission line. Has a great advantage over

11: Light source 12: Optical splitter 13: Optical circulator 14: Optical multiplexer 15: Light receiver 16: A / D converter 17: Arithmetic processing device 18: Data storage means 50: Fiber to be measured 301: Spatial mode dispersion measuring device

Claims (4)

A measurement procedure for measuring an amplitude distribution waveform with respect to a propagation delay time of backward Rayleigh scattered light in a spatially multiplexed optical transmission line;
An arbitrary section component of the amplitude distribution waveform is extracted, an autocorrelation of the extracted amplitude distribution waveform is calculated, and a second moment of the correlation peak distribution excluding the central peak of the autocorrelation is calculated as the arbitrary section of the spatial multiplexing optical transmission line A calculation procedure for spatial mode dispersion in
Spatial mode dispersion measurement method. In the measurement procedure,
Branch the continuous light swept in frequency,
One of the branched continuous lights is incident on the spatially multiplexed optical transmission line;
Detecting the beat signal by combining the back Rayleigh scattered light generated in the spatially multiplexed optical transmission line and the other of the branched continuous light;
The spatial mode dispersion measuring method according to claim 1, wherein the beat signal is Fourier transformed to obtain the amplitude distribution waveform. Measuring means for measuring an amplitude distribution waveform with respect to a propagation delay time of backward Rayleigh scattered light in a spatially multiplexed optical transmission line;
An arbitrary section component of the amplitude distribution waveform is extracted, an autocorrelation of the extracted amplitude distribution waveform is calculated, and a second moment of the correlation peak distribution excluding the central peak of the autocorrelation is calculated as the arbitrary section of the spatial multiplexing optical transmission line Arithmetic means for spatial mode dispersion in
A spatial mode dispersion measuring apparatus comprising: The measuring means includes
A light source that emits frequency-swept continuous light;
An optical branching device for branching continuous light emitted from the light source;
One of the continuous lights branched by the optical splitter is incident on the spatially multiplexed optical transmission line, and backward Rayleigh scattered light generated in the spatially multiplexed optical transmission line and the continuous light split by the optical splitter. Detecting a beat signal by combining with the other, an optical detector for Fourier-transforming the beat signal to obtain the amplitude distribution waveform;
The spatial mode dispersion measuring apparatus according to claim 3, wherein

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