Optical fiber sensing method

Abstract

The objective of the present invention is to provide an optical fiber sensing device and method that are capable of measuring a plurality of optical fibers simultaneously in a short time, and that enable planar simultaneous measurement of an entire area employing an optical fiber network.　An optical fiber sensing device 301 according to the present invention is characterized by comprising one optical tester 10, and an optical switch 25 that connects the optical pulse tester 10 to each of a plurality of optical fibers (50-1, 50-2, 50-3, …, 50-n) included in an existing optical fiber network, wherein the optical switch 25 sequentially switches the connected optical fiber for each light pulse output by the optical tester 10.

Description

Optical fiber sensing method

[0001] The present disclosure relates to fiber optic sensing technology.

[0002] 1 is a diagram illustrating optical fiber sensing. Optical fiber sensing is an optical measurement technique in which an optical tester 10 receives backscattered light 35 generated by test light 30 incident on an optical fiber 50 and measures the distribution of the state of the optical fiber 50. Optical fiber sensing has the following characteristics (see, for example, Non-Patent Document 1):

[0003] Measurements can be made at only one end of the optical fiber (no device needs to be installed at the other end). The optical tester 10 inputs optical pulses or CW (continuous wave) test light and observes the backscattered light generated in the optical fiber. Backscattered light includes Rayleigh scattering, Brillouin scattering, Raman scattering, etc. When an external disturbance (bending, temperature change, strain, vibration, etc.) is applied to the optical fiber, its state changes, and the state of the backscattered light also changes accordingly. Observing the change in the state of the backscattered light is equivalent to observing the change in the state of the optical fiber, and it is possible to observe the state of the applied disturbance. In other words, by treating the optical fiber as a sensor and observing the backscattered light, it is possible to measure or estimate the disturbance applied to the optical fiber (bending, temperature change, strain change, vibration, etc.). The disturbances that can be measured depend on the measurement method and the type of scattered light being observed.

[0004] For this reason, consideration is being given to treating communication optical fiber networks as sensors and making measurements (see, for example, Non-Patent Document 2). Communication optical fibers are already laid throughout the city, and simply measuring them can be expected to estimate the state of the surrounding environment, and this information can be used in a variety of applications.

[0005] Distributed Acoustic Sensing (DAS), a vibration sensing method using a coherent OTDR, is known as one type of optical fiber sensing technology. Fig. 2 illustrates a DAS system. The DAS system is comprised of an optical tester 10 and an optical fiber 50 to be measured. The optical tester 10 includes a light source 11, an optical coupler 12, a modulator 13, an optical amplifier 14, an optical circulator 15, a control unit 17, a coupler and balance PD 18, a digitizing means 19, and a computer 20.

[0006] The DAS system of Figure 2 performs coherent detection of backscattered light, thereby obtaining backscattered light waveforms 41, as shown in Figure 3(A), for the number of incident light pulses. Note that there is an interval T between adjacent backscattered light waveforms 41, which is equal to or greater than the time it takes for light to travel back and forth through the optical fiber 50. In other words, the interval T between light pulses incident on the optical fiber is set to be equal to or greater than the time it takes for light to travel back and forth through the optical fiber 50 (see Figure 3(B)). Adding and averaging these multiple waveforms 41 results in a waveform obtained by so-called coherent OTDR, enabling high S / N ratio measurements. Meanwhile, arranging these waveforms 41 in pulse order enables the time change in scattered light at each scattering position, i.e., vibration measurement.

[0007] In the DAS system of Fig. 2, optical pulses with different optical frequencies (optical frequency multiplexing) may be input to the optical fiber 50 (FDM DAS). In the case of a single optical frequency, an interval T is required as shown in Fig. 3. However, in the case of optical frequency multiplexing, optical pulses with different optical frequencies can be input to the optical fiber 50 without waiting for the time it takes for the light to travel back and forth through the optical fiber 50 as shown in Fig. 4, so that a larger number of backscattered light waveforms 42 can be generated in the same time period than in the case of a single optical frequency. １ ~42 ５ In other words, it is possible to shorten the measurement time and improve accuracy by averaging (see, for example, Patent Documents 1 and 2).

[0008] JP 2017-026503 A JP 2020-169904 A

[0009] "Advances in distributed vibration sensing for optical communication fiber state visualization," Optical Fiber Technology, Vol. 57, 102263. 2020 "Approach to smart engineering / maintenance aiming for operational innovation," Business Communication, 2022, Vol. 59, No. 4 (https: / / www.bcm.co.jp / site / 2022 / 04 / ntt-as / 2204-ntt-as-01-06.pdf, retrieved June 29, 2023).

[0010] The optical fiber sensing technology described above involves injecting test light into a single optical fiber and measuring backscattered light that propagates in the reverse direction through the same optical fiber from various points within the optical fiber and returns to the optical tester. Therefore, the backscattered light must be measured for each optical fiber. When attempting to measure many optical fibers deployed as communication fibers using a single optical tester, as in Non-Patent Document 2, the following problems arise: (1) Because measurements are performed for each optical fiber, there is a measurement time difference between each optical fiber. (2) Because an optical fiber network includes many optical fibers, it takes a long time to measure the entire area. In other words, optical fiber sensing technology has the problem of difficulty in simultaneously measuring the entire area using an optical fiber network.

[0011] Therefore, in order to solve the above problems, the present invention aims to provide an optical fiber sensing device and method that can measure multiple optical fibers simultaneously and in a short time, and that enables simultaneous surface measurement of an entire area using an optical fiber network.

[0012] In order to achieve the above object, the optical fiber sensing device according to the present invention performs switching for each test light pulse and simultaneously measures a plurality of optical fibers within the limits imposed by the switching speed.

[0013] Specifically, the optical fiber sensing device according to the present invention comprises an optical pulse tester and an optical switch that connects the optical pulse tester to each of a plurality of optical fibers included in an existing optical fiber network, and the optical switch sequentially switches the optical fiber to which it is connected for each optical pulse output by the optical pulse tester.

[0014] Furthermore, an optical fiber sensing method according to the present invention is characterized in that an optical pulse tester is connected to each of a plurality of optical fibers included in an existing optical fiber network by an optical switch, and the optical switch sequentially switches the optical fiber to which the optical pulse tester is connected for each optical pulse output from the optical pulse tester.

[0015] Because test optical pulses are simply injected into all optical fibers connected to the optical switch, a single optical pulse tester can simultaneously measure the backscattered light from all optical fibers, enabling area-wide measurements to be made across the entire area of ​​an optical fiber network.

[0016] Therefore, the present invention can provide an optical fiber sensing device and method that can measure multiple optical fibers simultaneously and in a short period of time, and enable simultaneous surface measurement of an entire area using an optical fiber network.

[0017] In the optical fiber sensing device and method according to the present invention, the optical pulse may include a plurality of optical frequencies, which can improve the signal-to-noise ratio and measurement accuracy by averaging.

[0018] In the optical fiber sensing device and method according to the present invention, the optical pulse tester may have a light source that outputs light of one optical frequency and an optical modulator that DSB-modulates the light, and output optical pulses containing two optical frequencies. Two types of optical pulses can be generated from one optical pulse, which can easily improve the signal-to-noise ratio and measurement accuracy through averaging.

[0019] In the optical fiber sensing device and method according to the present invention, the optical time domain tester may receive backscattered light from the optical fiber using a polarization diversity method. By receiving one backscattered light beam as X and Y polarization components, the number of waveforms can be reduced to two, which can easily improve the signal-to-noise ratio and measurement accuracy through averaging.

[0020] The above inventions can be combined as much as possible.

[0021] The present invention can provide an optical fiber sensing device and method that can measure multiple optical fibers simultaneously and in a short time, and that enables planar simultaneous measurement of an entire area using an optical fiber network.

[0022] FIG. 1 is a diagram illustrating optical fiber sensing. FIG. 2 is a diagram illustrating a DAS system. FIG. 3 is a diagram illustrating backscattered light obtained with optical pulses of a single optical frequency. FIG. 4 is a diagram illustrating backscattered light obtained with optical pulses of different optical frequencies. FIG. 5 is a diagram illustrating an optical fiber sensing device according to the present invention. FIG. 6 is a diagram illustrating backscattered light obtained with optical pulses of a single optical frequency. FIG. 7 is a diagram illustrating backscattered light obtained with optical pulses of different optical frequencies.

[0023] The following description of the preferred embodiments of the present invention will be given with reference to the accompanying drawings. The preferred embodiments described below are examples of the present invention, and the present invention is not limited to the preferred embodiments. In this specification and the drawings, components having the same reference numerals are intended to represent the same components.

[0024] (Embodiment 1) Figure 5 is a diagram illustrating an optical fiber sensing device 301 of this embodiment. The optical fiber sensing device 301 includes: an optical tester 10; and an optical switch 25 that connects the optical pulse tester 10 to each of multiple optical fibers (50-1, 50-2, 50-3, ..., 50-n) included in an existing optical fiber network. The optical switch 25 sequentially switches the optical fiber to which the optical tester 10 is connected for each optical pulse output by the optical tester 10. n is a natural number. The optical fibers (50-1, 50-2, 50-3, ..., 50-n) are, for example, optical fibers in an optical fiber network already laid throughout a city.

[0025] Figure 6(B) shows the optical pulse 30 output by the optical tester 10. Optical pulse 30-1 is input to optical fiber 50-1, optical pulse 30-2 to optical fiber 50-2, optical pulse 30-3 to optical fiber 50-3, ..., optical pulse 30-n to optical fiber 50-n. Figure 6(A) is a diagram explaining the waveform 41 of the backscattered light received by the optical tester 10. Waveform 41-1 is that of the backscattered light from optical fiber 50-1, waveform 41-2 from optical fiber 50-2, waveform 41-3 from optical fiber 50-3, ..., waveform 41-n from optical fiber 50-n.

[0026] The optical fiber sensing device 301 switches the optical switch 25 for each pulse, acquires the waveform of the backscattered light from different optical fibers 50, and simultaneously measures multiple optical fibers (50-1, 50-2, 50-3, ..., 50-n). To achieve simultaneous measurement, a certain relationship is required between the parameters of the switch speed, measurement vibration frequency, fiber length, and number of optical fibers.

[0027] Here, let L be the maximum fiber length among the optical fibers 50, n be the number of optical fibers to be measured, Δt be the switching speed of the optical switch 25, and T be the pulse interval of the optical pulse 30. In this embodiment, the optical tester 10 outputs the optical pulse 30 at a single optical frequency.

[0028] (Example 1) Let L = 10 km, n = 10, and Δt = 10 ms. In this example, the measurement time τ for one pulse in one optical fiber 50 isａ must be less than the pulse interval and longer than the time it takes for light to travel back and forth through the longest optical fiber, 2L / v, where v is the propagation speed of light in the optical fiber. In other words, T>τ ａ >2L / v, and τ ａ The interval τ of the waveform 41 for each optical fiber 50 is about 100 μs. ｂ (the interval between the light pulses 30) is n×T>τ ｂ >n×(2L / v+Δt), where n×Δt is approximately 100 ms. In this case, the maximum measurement frequency F for one optical fiber 50 is F=1 / (2nT) to 1 / (2nΔt). Specifically, F is approximately 5 Hz. In other words, it is possible to measure all optical fibers 50 within the waveform interval range of 100 ms×n.

[0029] (Example 2) Let L = 10 km, n = 10, and Δt = 1 μs. Because the switching speed is high, the time it takes for light to travel back and forth through the optical fiber becomes relatively long, and the pulse interval T becomes approximately 2L / v. As explained in Example 1, the interval τ of the waveform 41 for each optical fiber 50 ｂ (the interval between the light pulses 30) is n×T>τ ｂ >n×(2L / v+Δt), so in this example, the interval τ ｂ Specifically, the interval τ ｂ is approximately 1 ms. The maximum measurement frequency F for one optical fiber 50 is F = 1 / (2nT) ~ v / (2n 2L). Specifically, F is approximately 500 Hz. In other words, it is possible to measure all optical fibers 50 within the waveform interval range of 1 ms x n.

[0030] If the optical pulse 30 is injected into each optical fiber in this way, all optical fibers can be measured simultaneously with a time resolution of n×T. Here, the following points should be noted: There is a trade-off between the number n of optical fibers 50 and the maximum measurement frequency F. If the switching speed Δt is slower than the time it takes for light to make a round trip through the optical fiber, 2L / v, then (1) there is a trade-off between Δt and n, and (2) there is a trade-off between Δt and F. On the other hand, if the switching speed Δt is faster than the time it takes for light to make a round trip through the optical fiber, 2L / v, then (1) there is a trade-off between L and n, and (2) there is a trade-off between L and F.

[0031] (Embodiment 2) The configuration of the optical fiber sensing device of this embodiment is the same as that of the optical fiber sensing device 301 in Fig. 5. The optical fiber sensing device of this embodiment is characterized in that, unlike the optical fiber sensing device 301, the optical pulse 30 includes multiple optical frequencies. In other words, this embodiment is an FDM DAS system. In the case of FDM, optical pulses of multiple optical frequencies are input to each optical fiber, so that multiple backscattered light waveforms can be measured continuously for each optical fiber. The waveforms of the backscattered light for each of these optical frequencies can be averaged to reduce speckle noise and improve S / N.

[0032] 7B shows the optical pulses 30 output by the optical tester 10. Optical pulse 30-1 is input to optical fiber 50-1, optical pulse 30-2 to optical fiber 50-2, optical pulse 30-3 to optical fiber 50-3, ..., optical pulse 30-n to optical fiber 50-n. Each optical pulse 30 is composed of multiple (m) optical frequency chips (multiplexing number m). The width of each optical frequency chip is w, and the width dt of each optical pulse 30 is w × m.

[0033] 7A is a diagram illustrating the waveform 42 of backscattered light received by the optical tester 10. Waveform 42-1 is that of backscattered light from optical fiber 50-1, waveform 42-2 is that of backscattered light from optical fiber 50-2, waveform 41-3 is that of backscattered light from optical fiber 50-3, ..., waveform 41-n is that of backscattered light from optical fiber 50-n. Each waveform 41 has a waveform (42) corresponding to each of the multiple (m number of) optical frequency chips included in the optical pulse 30. １ , 42 ２ , ..., 42 ｍ ) is included.

[0034] The parameters not explained in this example are the same as those explained in Example 1.

[0035] The interval τ of the waveform 41 for each optical fiber 50 ｂ (the interval between the light pulses 30) is n×T>τ ｂ >n×(m×w+2L / v+Δt), and the maximum measured frequency is F=1 / (2n×(mw+2L / v+Δt)). Here, w is much smaller than 1 μs when the fiber length L is on the order of km, so it can be ignored. Therefore, the calculation is the same as that described in the first embodiment.

[0036] When the switching speed is Δt=10 ms, the interval τ ｂ is n×Δt, which is 10×10 ms=100 ms, and the maximum measurement frequency F=1 / 200 ms=5 Hz, which is the same as in the first embodiment. ｂ is n×2L / v, which is about 1 ms, and the maximum measurement frequency F=½ ms=500 Hz, which is the same as in the first embodiment.

[0037] In other words, the FDMDAS system can also improve the S / N ratio by averaging, and the measurement waveform interval and maximum measurement vibration frequency can be performed under the same conditions as the DAS system. Therefore, the trade-off relationship described in the first embodiment is also exactly the same.

[0038] (Embodiment 3) The configuration of an optical fiber sensing device of this embodiment is the same as that of the optical fiber sensing device 301 of Fig. 5. The optical fiber sensing device of this embodiment differs from the optical fiber sensing device 301 in that an optical tester 10 includes a light source 11 that outputs light of one optical frequency and an optical modulator 13 that DSB-modulates the light, and outputs an optical pulse 30 that includes two optical frequencies.

[0039] In this embodiment, the frequency modulation in the modulator 13 of the optical tester 10 is DSB (Double Side Band) modulation. The modulator 13 receives a carrier frequency f ０ For light of one modulation frequency Δf, ０ +Δf and f ０ Therefore, the optical fiber sensing device of this embodiment can simultaneously generate optical pulses 30 of two different optical frequencies and input them to each optical fiber 50, without generating the optical pulses 30 by frequency multiplexing as in the second embodiment. In other words, the optical fiber sensing device of this embodiment can simply average the waveform of the backscattered light.

[0040] (Embodiment 4) The configuration of an optical fiber sensing device of this embodiment is the same as that of the optical fiber sensing device 301 in Fig. 5. The optical fiber sensing device of this embodiment is characterized in that, compared to the optical fiber sensing device 301, the optical tester 10 receives backscattered light from the optical fiber 50 using a polarization diversity method.

[0041] The optical tester 10 performs polarization diversity reception when receiving backscattered light. By performing polarization diversity reception, the backscattered light can be separated into an x-polarized component and a y-polarized component and received. If the x-polarized component and the y-polarized component are considered to be separate signals, two waveforms can be obtained from one backscattered light. In other words, the optical fiber sensing device of this embodiment can easily average the waveform of the backscattered light.

[0042] (Other Embodiments) The techniques described in the third and fourth embodiments (DSB modulation and polarization diversity reception) can be used in combination. Optical pulses of two optical frequencies are generated without frequency multiplexing modulation taking into account the frequency band, and are input into each optical fiber, and the backscattered light is received using polarization diversity, thereby obtaining a total of four waveforms per optical fiber. In other words, the optical fiber sensing device of this embodiment can simply average the waveforms of the backscattered light.

[0043] (Effects of the Invention) In order to measure an optical network across a surface, multiple measurement devices are required unless an optical switch is used. However, by using an optical switch according to the present invention, only one measurement device is required, which can be realized at low cost.

[0044] 10: Optical tester 11: Light source 12: Optical coupler 13: Modulator 14: Optical amplifier 15: Optical circulator 17: Control unit 18: Coupler + balanced PD 19: Digitization means 20: Calculator 25: Optical switch 30, 30-1, 30-2, ..., 30-n: Light pulse 35: Backscattered light 41, 41-1, 41-2, ..., 41-n: Waveform of backscattered light 42, 42-1, 42-2, ..., 42-n: Waveform of backscattered light 42 １ , 42 ２ , 42 ３ , ..., 42 ｍ : Waveform of backscattered light 50, 50-1, 50-2, ..., 50-n: Optical fiber

Claims

1. An optical fiber sensing device comprising: an optical pulse tester; and an optical switch that connects said optical pulse tester to each of a number of optical fibers included in an existing optical fiber network, said optical switch sequentially switching the optical fiber to which said optical pulse tester is connected for each optical pulse output by said optical pulse tester.

2. The optical fiber sensing device according to claim 1, wherein the optical pulse includes a plurality of optical frequencies.

3. The optical fiber sensing device according to claim 1, characterized in that the optical pulse tester has a light source that outputs light of one optical frequency, and an optical modulator that DSB-modulates the light, and outputs the optical pulse containing two optical frequencies.

4. The optical fiber sensing device according to claim 1, wherein said optical pulse tester receives the backscattered light from said optical fiber using a polarization diversity method.

5. An optical fiber sensing method comprising the steps of: connecting one optical pulse tester to each of a number of optical fibers included in an existing optical fiber network using an optical switch; and sequentially switching the optical fiber connected to each of the optical pulses output by the optical pulse tester using the optical switch.

6. The optical fiber sensing method according to claim 5, wherein said optical pulse includes a plurality of optical frequencies.

7. The optical fiber sensing method according to claim 5, wherein in the optical pulse tester, light of one optical frequency output from a light source is DSB-modulated to generate the optical pulse containing two optical frequencies.

8. The optical fiber sensing method according to claim 5, wherein in said optical pulse tester, the backscattered light from said optical fiber is received by a polarization diversity method.

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