Spatial mode dispersion measurement device

Abstract

The present disclosure provides a spatial mode dispersion measurement device and a spatial mode dispersion measurement method, wherein test light having a plurality of wavelengths is input to one end of an optical fiber to be tested, an intensity of reflected light returned to the one end of the optical fiber to be tested is measured, and spatial mode dispersion in the optical fiber to be tested is calculated on the basis of the wavelength dependence of the intensity of the reflected light.

Description

Spatial mode dispersion measurement device

[0001] The present disclosure relates to an apparatus for measuring spatial mode dispersion of a coupled multicore fiber.

[0002] Multicore fibers have attracted attention as a medium for realizing future high-capacity optical communications. Among these, coupled multicore fibers, which actively utilize optical coupling between cores, are one of the most promising optical fibers due to their excellent optical properties. In communication systems using coupled multicore fibers, spatial mode dispersion occurs due to signal propagation differences between channels and optical coupling between channels. This spatial mode dispersion is an important parameter in signal processing to restore transmitted signals after propagation through coupled multicore fibers.

[0003] Known methods for measuring spatial mode dispersion include a method for measuring transmitted light and a method for measuring reflected light. Examples of methods for measuring transmitted light include the wavelength sweep method and the impulse response method described in Non-Patent Documents 1 and 2. Known examples of methods for measuring reflected light include the optical frequency domain reflectometry (OFDR) described in Non-Patent Document 3.

[0004] T. Sakamoto et al., “Fiber twisting- and bending-induced adiabatic / nonadiabatic super-mode transition in coupled multicore fiber,” Journal of Lightwave Technology, vol. 34, no. 4, pp. 1228-1237, 2016.M. Uyama et al., “Bandwidth-decomposed analysis of spatial-mode dispersion of coupled 2-core fiber employing linear optical sampling,” Journal of Lightwave Technology, vol. 41, no. 10, pp. 3153-3163, 2023.S. Ohno et al., “Distributed spatial mode dispersion measurement along strongly coupled multicore fibers based on the correlation analysis of Rayleigh backscattering amplitudes,” Optics Express, vol. 25, no. 24, pp. 29650-29658, 2017.Recommendation ITU-T G.650.2(08 / 2015), International Telecommunication Union

[0005] In coupled multicore fibers, spatial mode dispersion is sensitive to changes in response to bending or twisting of the optical fiber. Therefore, when using coupled multicore fibers, it is necessary to measure spatial mode dispersion in a real environment after laying the optical cable. After laying, both ends of the optical cable are located at locations distant from each other. Therefore, the methods described in Non-Patent Documents 1 and 2, which require personnel and equipment to be simultaneously located at both ends of the optical cable, are not easy to implement in terms of operation and cost. Furthermore, the frequency domain reflectometry method described in Non-Patent Document 3 can measure spatial mode dispersion from one end of the fiber under test, but the measurement equipment is very expensive. In other words, it is difficult to measure the spatial mode dispersion of a coupled multicore fiber using inexpensive equipment without access to both ends.

[0006] The present disclosure has been made in light of the above circumstances, and aims to make it possible to measure spatial mode dispersion from one end of a coupled multi-core fiber using an inexpensive device.

[0007] The spatial mode dispersion measuring apparatus and spatial mode dispersion measuring method disclosed herein input test light of a plurality of wavelengths to one end of an optical fiber under test, measure the intensity of the reflected light returning to the one end of the optical fiber under test, and calculate the spatial mode dispersion in the optical fiber under test based on the wavelength dependency of the reflected light intensity.

[0008] The optical fiber under test is a coupled multicore fiber having a plurality of cores, and the spatial mode dispersion measurement device inputs test light into one of the plurality of cores, measures the intensity of the light reflected from the plurality of cores, and calculates the spatial mode dispersion using the intensity of the light reflected from the plurality of cores.

[0009] The spatial mode dispersion measuring apparatus may derive a time domain waveform of the reflected light intensity using the wavelength dependency of the reflected light intensity, and may calculate the spatial mode dispersion in the optical fiber under test using the derived time domain waveform.

[0010] The spatial mode dispersion measuring device may include: a test light input unit that generates and outputs test light of a plurality of wavelengths at different timings; an input / output unit that inputs the test light from the test light input unit to one end of one core provided in the coupled multicore fiber and outputs light that has returned to the one end of each core provided in the coupled multicore fiber; a receiving unit that converts the light output from the input / output unit into an electric signal; and a calculation unit that calculates the wavelength dependency of the reflected light intensity using the electric signal from the receiving unit and calculates the spatial mode dispersion in the coupled multicore fiber.

[0011] The above disclosures can be combined as much as possible.

[0012] According to the present disclosure, it is possible to measure spatial mode dispersion from one end of a coupled multi-core fiber using an inexpensive device.

[0013] 1 is an explanatory diagram of spatial mode dispersion; FIG. 2 is an example of an embodiment of a spatial mode dispersion measurement device according to the present disclosure; FIG. 3 is an example of a cross-sectional view of a coupled multicore fiber used in the present embodiment; FIG. 4 is an explanatory diagram of a measurement method according to the present disclosure for a case where a four-core fiber is targeted; FIG. 5 is a flowchart showing an example of a spatial mode dispersion measurement method according to the present disclosure; FIG. 6 is an example of a time-domain waveform according to the present disclosure; FIG. 7 is an example of a comparative example of accuracy with the prior art; FIG. 8 is an example of an embodiment of a spatial mode dispersion measurement device according to the present disclosure.

[0014] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0015] (Spatial Mode Dispersion) In a coupled multicore fiber, the output pulse broadens compared to the input pulse due to the difference in signal propagation velocity between channels and optical coupling. Figure 1 shows an example of the difference in signal propagation velocity and optical coupling that occurs in a coupled multicore fiber 90 with two cores. For example, if there is a difference in signal propagation velocity between channels in the coupled multicore fiber 90, a difference occurs in the time at which pulses are output from the two cores, as shown in Figure 1(a). Furthermore, if optical coupling exists in the coupled multicore fiber 90, a temporal broadening occurs in the output pulse, as shown in Figure 1(b). Spatial mode dispersion, which represents the magnitude of this broadening, is an important parameter in signal processing for restoring the transmitted signal received by a photodetector.

[0016] In the present disclosure, test light is incident on one core of a coupled multi-core fiber, and the intensity of the reflected light generated at each core is measured. This measurement is performed for multiple wavelengths, thereby obtaining the wavelength dependence of the reflected light intensity in the coupled multi-core fiber.

[0017] Spatial mode dispersion can be defined as the square root of the second moment of the time-domain waveform (see, for example, Non-Patent Document 4). Therefore, in this disclosure, the time-domain waveform is derived from the wavelength dependence of the reflected light intensity, and the spatial mode dispersion in a coupled multi-core fiber is calculated using the time-domain waveform.

[0018] Since the reflected light intensity can be measured from one end, the present disclosure makes it possible to measure spatial mode dispersion from one end of a coupled multicore fiber using an inexpensive device.

[0019] The present disclosure makes it possible to measure spatial mode dispersion after laying an optical cable, thereby ensuring that the coupled multi-core fiber after laying is in a state in which a transmitted signal can be restored by signal processing, and further makes it possible to restore the transmitted signal using the measured value. This will be described in detail below.

[0020] (First Embodiment) Fig. 2 shows an example embodiment of the spatial mode dispersion measurement device of the present disclosure. A spatial mode dispersion measurement device 91 of this embodiment includes a test light input unit 11, a receiving unit 12, and a calculating unit 13, and executes the spatial mode dispersion measurement method of the present disclosure. The test light input unit 11 generates test light of multiple wavelengths at different timings and inputs the test light to a coupled multicore fiber 92. As a result, the test light is reflected and scattered in the coupled multicore fiber 92. The receiving unit 12 receives the light reflected by the coupled multicore fiber 92 and outputs an electrical signal corresponding to the intensity of the reflected light. The calculating unit 13 calculates the wavelength dependency of the reflected light intensity using the electrical signal from the receiving unit 12, and calculates the spatial mode dispersion in the coupled multicore fiber 92.

[0021] The test light input unit 11 has a wavelength swept light generator 21, an optical circulator 22, and an input / output device 23. The wavelength swept light generator 21 outputs wavelength swept light of an arbitrary wavelength width. The wavelength swept light generator 21 may be a combination of a wavelength swept light source or a continuous light source of a fixed wavelength and a modulator. In this embodiment, the optical circulator 22 is used to separate the test light input to the coupled multi-core fiber 92 from the reflected light from the coupled multi-core fiber 92.

[0022] The receiving unit 12 converts the light output from the input / output device 23 into an electrical signal. The receiving unit 12 can employ a means capable of measuring the light intensity of the reflected light from the coupled multicore fiber 92, and in this embodiment, a power meter 24 equipped with a photoelectric converter and an AD converter is shown. The photoelectric converter has a function of converting the light intensity of the received reflected light into an electrical signal, and is, for example, a photodiode. The AD converter has a function of converting the analog electrical signal into a digital signal.

[0023] In this embodiment, a four-core fiber having four cores C1, C2, C3, and C4 as shown in Fig. 3 is used as the coupled multi-core fiber 92. The four-core fiber of this embodiment is a coupled four-core fiber in which the cores are coupled to each other.

[0024] A terminal of the input / output device 23 on the coupled multicore fiber 92 side is connected to each of the multiple cores C1, C2, C3, and C4 that appear at one end of the coupled multicore fiber 92. The input / output device 23 connects each of the cores C1, C2, C3, and C4 provided in the coupled multicore fiber 92 to different optical fibers 231, 232, 233, and 234. The input / output device 23 can use any means that can connect to each core of the coupled multicore fiber 92, and in this embodiment, a fan-in / fan-out device for multicore fibers is used.

[0025] In the present disclosure, as shown in Fig. 4 , test light is input to one of the cores included in the coupled multicore fiber 92, and reflected light from each core is received. In this embodiment, an example is shown in which test light is input to core C1. In this case, the optical circulator 22 is connected to an optical fiber 231 connected to core C1. The input / output device 23 functions as an "input / output section," inputs test light from the test light input section 11 to one end of one core C1 included in the coupled multicore fiber 92, and outputs light that has returned to the one end of each of the cores C1, C2, C3, and C4 included in the coupled multicore fiber 92.

[0026] When wavelength swept light is input to core C1 appearing at one end of the optical transmission line, the wavelength swept light propagates through the optical transmission line and is reflected at the open far end 92E to generate reflected light in core C1. Furthermore, since the coupled multicore fiber 92 is a four-core fiber, the wavelength swept light is also coupled to the other cores C2, C3, and C4, and reflected light is also generated in these cores.

[0027] The light reflected at the far end 92E of the core C1 is received by the receiving unit 12 via the input / output device 23 and the optical circulator 22. The light reflected at the far ends 92E of the other cores C2, C3, and C4 is received by the receiving unit 12 via the input / output device 23. As a result, the intensity of the reflected light generated in the coupled multicore fiber 92 at the far end 92E of each core is measured by the receiving unit 12.

[0028] The calculation unit 13 includes a spatial mode dispersion calculation unit 25. The spatial mode dispersion calculation unit 25 calculates the wavelength dependence of the insertion loss of each of the cores C1, C2, C3, and C4 using the reflected light intensity of each of the cores C1, C2, C3, and C4 measured by the receiving unit 12. Then, the spatial mode dispersion calculation unit 25 calculates the spatial mode dispersion of the coupled multicore fiber 92 based on the wavelength dependence of the insertion loss of each of the cores C1, C2, C3, and C4.

[0029] 5 is a flowchart illustrating the operation of the calculation unit 13. The calculation unit 13 executes steps S01, S02, S03, S04, and S05.

[0030] (Step S01) In step S01, the wavelength dependency of the insertion loss of one core to be measured is calculated from the wavelength dependency of the reflected light intensity of each core. For example, in this embodiment, the reflected light intensity P of the core C1 to be measured at the wavelength λ is calculated. １ (λ) and the reflected light intensity of the other cores P ２ (λ), P ３ (λ), P ４ Using (λ), the insertion loss I Ｌ (λ) is calculated using Equation 1. Ｌ (λ) = P １ (λ) / {P １ (λ) + P ２ (λ) + P ３ (λ) + P ４ (λ)} (1)

[0031] (Step S02) In step S02, the insertion loss I Ｌ The wavelength dependency of (λ) is converted into a function of frequency. If the high speed is c, the wavelength λ and the frequency f have the following relationship: (Equation 2) f = λ / c (2) Then, the insertion loss I Ｌ Dividing (λ) by the speed of light c gives the insertion loss I as a function of frequency. Ｌ (f) is obtained.

[0032] (Step S03) In step S03, the insertion loss I obtained in step S02 is Ｌ(f) is converted into a time domain waveform. For example, the insertion loss I Ｌ By performing an inverse Fourier transform on (f), the frequency domain waveform can be converted into a time domain waveform. An example of the time domain waveform P(δτ) obtained in this embodiment is shown in “Measured” in FIG.

[0033] (Step S04) In step S04, the time domain waveform P(δτ) is used to calculate the spatial mode dispersion Δτround trip of the coupled multi-core fiber 92. As a method for calculating the spatial mode dispersion, for example, the square root σ of the second moment of the time domain waveform P(δτ) as shown in FIG. Ｒ is defined as spatial mode dispersion (see, for example, Non-Patent Document 4, pages 29-30, "6.1.4.4.2.5 PMD calculation for fibers with random mode coupling"). In this method, the spatial mode dispersion is calculated as the square root σ of the second moment of the time domain waveform P(δτ) by applying the time domain waveform P(δτ) to the following equation: Ｒ It can be calculated as:

[0034] (Step S05) When the length of the coupled multi-core fiber 92 is L, the spatial mode dispersion Δτ round trip calculated in step S04 is a value for a round-trip length 2L of the coupled multi-core fiber 92. Therefore, in step S05, the spatial mode dispersion Δτ round trip calculated in step S04 is used to calculate the spatial mode dispersion Δτ for the one-way length L of the coupled multi-core fiber.

[0035] Here, the spatial mode dispersion of the coupled multi-core fiber 92 is proportional to the square root of the length of the coupled multi-core fiber 92. Therefore, the spatial mode dispersion Δτ in the one-way length L of the coupled multi-core fiber can be calculated as follows (Equation 4): Δτ=Δτround trip / √2 (4)

[0036] As described above, the present disclosure makes it possible to input test light of multiple wavelengths to one end of a coupled multicore fiber 92, which is an optical fiber under test, measure the intensity of the reflected light returning to the one end of the coupled multicore fiber 92, and calculate the spatial mode dispersion Δτ of the optical fiber under test based on the wavelength dependency of the reflected light intensity. Since the present disclosure makes it possible to apply an inexpensive configuration capable of measuring the reflected light intensity to the spatial mode dispersion measurement device 91, the device is inexpensive and economical.

[0037] (Comparison with conventional technology) A comparison of accuracy with conventional technology is shown in Figure 7. In Figure 7, the horizontal axis represents conventional technology, and the vertical axis represents the technology of the present invention. In this disclosure, five types of coupled multimode fibers were prepared, and spatial mode dispersion was measured using the conventional technology and the technology of the present invention. The measurement method used in the conventional technology was the method described in Non-Patent Documents 1 and 2, in which a light source and a power meter are connected to an optical fiber. The slope of the line is 1, which shows that measurements can be made with the same accuracy as conventional technology.

[0038] 8 shows an example embodiment of a spatial mode dispersion measurement device according to the present disclosure. The functional units of a spatial mode dispersion measurement device 91 are housed in a housing 16, and a terminal 17 for connecting to a coupled multi-core fiber 92 is mounted on the housing 16. The functional units provided in the housing 16 include a test light input unit 11, a receiving unit 12, and a calculation unit 13. The housing 16 may also be equipped with a measurement result display unit 15.

[0039] The terminal 17 is a terminal connectable to each core of the coupled multicore fiber 92, and may include an optical connector for a multicore fiber. The number of cores in the terminal 17 is arbitrary. For example, one or more terminals 17 with a predetermined number of cores may be provided. When there are multiple terminals 17, the number of cores of the terminals 17 may be the same or different. When there are multiple terminals 17, the optical circulator 22 and the input / output device 23 are provided for each terminal 17.

[0040] Furthermore, the terminal 17 may be replaceable depending on the number of cores in the coupled multicore fiber 92. In this case, the optical circulator 22, the input / output device 23, and the terminal 17 constitute a single optical device, and the entire optical device is replaced. The optical device can be connected to the wavelength swept light generating unit 21 using a single-core optical connector, and the optical device can be connected to the power meter 24 using an array-type optical connector.

[0041] When spatial mode dispersion increases, the optical signal output from the optical fiber spreads. The greater the spread, the greater the burden on the signal processing of the receiver. Therefore, in order to keep the load on the signal processing of the receiver within an acceptable range, it is necessary to set a certain reference value for the magnitude of spatial mode dispersion and prevent it from increasing beyond that. Furthermore, if spatial mode dispersion exceeds the reference value, it is necessary to make a decision, for example, to redo the laying of the optical fiber. Therefore, the calculation unit 13 includes a spatial mode dispersion determination unit 26.

[0042] The spatial mode dispersion determiner 26 stores a reference value of spatial mode dispersion as a reference in advance and makes a determination by comparing it with the measurement result. The measurement result display unit 15 displays the measurement result of the spatial mode dispersion of the coupled multicore fiber 92. For example, the following aspects can be exemplified. (i) When the spatial mode dispersion obtained by the measurement exceeds the reference value and it is necessary to redo the laying of the optical fiber, the calculator 13 displays on the measurement result display unit 15 a message to the effect that the laying of the optical fiber will be redo. (ii) When the spatial mode dispersion obtained by the measurement is within the reference value, the calculator 13 displays on the measurement result display unit 15 a message to the effect that it is within the reference value. (iii) The calculator 13 displays the value of spatial mode dispersion on the measurement result display unit 15.

[0043] Although the coupled multicore fiber in the above-described embodiment has four cores, the number of cores in the coupled multicore fiber of the present disclosure may be any number equal to or greater than 2. Furthermore, although the reflected light intensity at the far end 92E is measured in this embodiment, the length L may be any reflection point whose length from the incident end is predetermined.

[0044] The calculation unit 13 of the present invention can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network. The program of the present disclosure is a program for causing a computer to realize each function of the calculation unit 13 according to the present disclosure, and is a program for causing a computer to execute each procedure of the method executed by the calculation unit 13 according to the present disclosure.

[0045] 11: Test light input section 12: Receiving section 13: Calculating section 15: Measurement result display section 16: Housing 17: Terminal 21: Wavelength swept light generating section 22: Optical circulator 23: Input / output device 24: Power meter 25: Spatial mode dispersion calculating section 26: Spatial mode dispersion determining section 91: Spatial mode dispersion measuring device 90, 92: Coupled multi-core fiber

Claims

1. A spatial mode dispersion measuring device comprising: inputting test light of multiple wavelengths into one end of an optical fiber under test; measuring the intensity of the reflected light returning to said end of the optical fiber under test; and calculating the spatial mode dispersion in said optical fiber under test based on the wavelength dependency of the intensity of the reflected light.

2. The spatial mode dispersion measuring device according to claim 1, further comprising: a time domain waveform of the reflected light intensity is derived using the wavelength dependency of the reflected light intensity; and a spatial mode dispersion in the optical fiber under test is calculated using the derived time domain waveform.

3. The spatial mode dispersion measuring device according to claim 1, wherein the optical fiber under test is a coupled multicore fiber having a plurality of cores, and comprises: a test light input unit that generates test light of a plurality of wavelengths at different timings and inputs the test light to the optical fiber under test; an input / output unit that inputs the test light from the test light input unit to one end of one core provided in the coupled multicore fiber and outputs light that has returned to the one end of each core provided in the coupled multicore fiber; a receiving unit that converts the light output from the input / output unit into an electrical signal; and a calculation unit that calculates the wavelength dependency of the reflected light intensity using the electrical signal from the receiving unit and calculates the spatial mode dispersion in the coupled multicore fiber.

4. A spatial mode dispersion measuring method comprising: inputting test light of a plurality of wavelengths into one end of an optical fiber under test; measuring the intensity of the reflected light returning to said end of the optical fiber under test; and calculating the spatial mode dispersion in said optical fiber under test based on the wavelength dependency of the intensity of the reflected light.

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