DEVICE FOR MEASURING VIBRATION DISTRIBUTION AND METHOD FOR THIS

Description

[Technical Field]

[0001] The present disclosure relates to a vibration distribution measuring apparatus and a method thereof.[Background Art]

[0002] For general prior art, reference is made to [NPL 3], [NPL 4], [NPL 5] and [PL 1].

[0003] [NPL 3] describes a numerical investigation of the statistical property of a local optical spectrum to confirm the beat frequency offset tolerance and to verify optical frequency domain reflectometry (OFDR) based distributed acoustic sensing (DAS) measurements with the tolerance.

[0004] [NPL 4] discloses an investigation of the effects of a vibration-induced frequency modulation in DAS based on OFDR. A shorter measurement time than the period of the frequency modulation approximates the frequency modulation as a frequency offset, and this offset behaves in the same way as a Doppler shift in a frequency modulated continuous wave (FMCW) radar. The frequency offset becomes a distance offset when a backscattered light waveform is measured. The distance offset shifts the distance of the targeted segment in each measurement and forces to analyze the spectra at unintentional distances. The spectra analyzed in each measurement are uncorrelated. Cross-correlation between the uncorrelated spectra shows a spurious vibration, namely a measured waveform that is different from the actual vibration waveform, which constitutes a measurement error in DAS. Further, a distance tracking technique for the targeted segment is proposed and demonstrated to compensate for the spurious vibration.

[0005] [NPL 5] describes a direct measure of the optical phase variation during a laser scan at a particular location. Applied sequentially along the length of the sensor fiber the total accumulated optical phase variations, which build up along the fiber, can be mapped out. A measurement example is given in which the frequency and location of three vibration sources along the length of a fiber sensor are detected with 0.17 m resolution over the 20 m sensor length and with 80 Hz frequency resolution over a 2560 Hz range.

[0006] [PL 1] discloses an optical SSB modulator that applies frequency modulation to coherence light from a light source to generate continuous light whose frequency changes stepwise per time interval T. An optical fiber vibration measuring system separates the continuous light into test light and reference light, and multiplexes reflection light generated by the test light being reflected by each point of a test object optical fiber and the reference light by a light multiplexer, and thereby obtains a test beat signal. Phase noise of the test beat signal is removed during sampling. A calculation processing unit applies N-point FFT to the obtained signal, and obtains the waveform of time-varying amplitude at any beat frequency.

[0007] In a Rayleigh scattered light, a spectrum shifts in response to a vibration. A distributed acoustic sensing (DAS), which analyzes vibration using the spectral shift, has been proposed (for example, refer to NPL 1).

[0008] In order to correctly measure the vibration, there are known a method using a sampling theorem in which a cycle of repeated measurement is made higher than a vibration frequency and a method using a spatial resolution in which a vibration analysis length is made shorter than a spatial expansion of vibration. However, the conditions for an amplitude of vibration are unclear.[Citation List][Non Patent Literature]

[0009] [NPL 1] Froggatt, Mark, and Jason Moore "High-spatial-resolution distributed strain measurement in optical fiber with Rayleigh scatter." Applied Optics 37.10 (1998): 1735-1740. [NPL 2] Koyamada, Yahei, et al. "Fiber-optic distributed strain and temperature sensing with very high measure and resolution over long range using coherent OTDR." Journal of Lightwave Technology 27.9 (2009): 1142-1146. [NPL 3] T. Okamoto, D. Iida and H. Oshida, "Investigation of Tolerance of OFDR-Based DAS to Vibration-Induced Beat Frequency Offset," 2020 Optical Fiber Communications Conference and Exhibition (OFC), San Diego, CA, USA, 2020, pp. 1-3. [NPL 4] T. Okamoto, D. Iida and H. Oshida, "Vibration-Induced Beat Frequency Offset Compensation in Distributed Acoustic Sensing Based on Optical Frequency Domain Reflectometry," in Journal of Lightwave Technology, vol. 37, no. 18, pp. 4896-4901, 15 September 2019. [NPL 5] Stephen T. Kreger, Justin W. Klein, Nur Aida Abdul Rahim, and Joseph J. Bos "Distributed Rayleigh scatter dynamic strain sensing above the scan rate with optical frequency domain reflectometry", Proc. SPIE 9480, Fiber Optic Sensors and Applications XII, 948006, 13 May 2015. [Patent Literature]

[0010] [PL 1] JP 2016 161512 A[Summary of Invention][Technical Problem]

[0011] In a vibration analysis using a spectral shift, in order to correctly measure vibration, it is necessary to appropriately set a condition for the amplitude of the vibration to be measured. Therefore, an object of the present disclosure is to enable the vibration analysis using the spectral shift under appropriate conditions according to a measurement object.[Solution to Problem]

[0012] According to a first aspect of the present invention, there is provided a vibration distribution measuring apparatus as defined in independent claim 1.

[0013] According to a second aspect of the present invention, there is provided a vibration distribution measuring method as defined in independent claim 4.[Advantageous Effects of Invention]

[0014] According to the present disclosure, it is possible to determine whether the DAS is applicable in consideration of an amplitude of vibration of a measurement object, and to optimize measurement conditions according to the measurement object.[Brief Description of Drawings]

[0015] [Fig. 1] Fig. 1 shows an example of a spectrum measured in DAS. [Fig. 2] Fig. 2 illustrates an example of the configuration of an OFDR. [Fig. 3] Fig. 3 shows an example of a relationship between a vibration analysis length w and a vibration amplitude of the measurement object. [Fig. 4] Fig. 4 shows an example of measurement conditions in the present disclosure; [Fig. 5] Fig. 5 is a measurement system used in an example of vibration distribution measurement. [Fig. 6A] Fig. 6A is an example of measuring the vibration distribution when a spatial resolution is 0.8 m. [Fig. 6B] Fig. 6B is an example of measuring the vibration distribution when a spatial resolution is 1.6 m. [Fig. 6C] Fig. 6C is an example of measuring the vibration distribution when a spatial resolution is 9.5 m. [Fig. 7] Fig. 7 shows an example of the spectrum measured using OFDR. [Fig. 8] Fig. 8 shows an example of measurement of dynamic distortion (vibration) using a temporal change of the spectral shift. [Fig. 9] Fig. 9 shows an example of the configuration of the vibration distribution measuring method according to the present disclosure. [Description of Embodiments]

[0016] Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the embodiment described below. These implementation examples are only illustrative, and this disclosure can be carried out in a form with various changes and improvements based on the knowledge of those skilled in the art. The scope of the present invention is defined by the appended claims.

[0017] Further, constituent elements with the same reference signs in the specification and the drawings are identical to each other.(DAS using OFDR)

[0018] Fig. 1 shows an example of the spectrum measured by a DAS. In the DAS, Rayleigh backscattered light from a plurality of different times is measured. Specifically, a reference measurement, a first measurement, and a second measurement are sequentially performed. Then, the spectrum (distortion) of the Rayleigh backscattered light waveform at distances z 1 ~z 2 at each point of time is analyzed to measure the time waveform of the vibration. Rayleigh backscattered light can be measured using, for example, optical frequency domain reflectometry (OFDR).

[0019] Fig. 2 shows an example of a system configuration according to the present disclosure. The vibration distribution measuring apparatus of the present disclosure is connected to an optical fiber 4 to be measured. The vibration distribution measuring apparatus of the present disclosure has a configuration similar to that of OFDR. Specifically, the vibration distribution measuring apparatus includes a frequency sweep light source 1, a coupler 2, a circulator 3, a coupler 5, a balance type photodetector 6, an A / D converter 7, and an analysis unit 8. The analysis unit 8 of the present disclosure can also be achieved by a computer and a program, and the program can be recorded in a recording medium or provided through a network.

[0020] The coupler 2 divides light from the frequency sweep light source 1 into a reference optical path for local light and a measurement optical path for probe light. The probe light divided into the measurement optical path enters an optical fiber 4 to be measured via the coupler 2 and the circulator 3. The coupler 5 multiplexes probe light which is backscattered light in the optical fiber 4 to be measured and local light divided by the coupler 2. The balance type photodetector 6 receives the interference light multiplexed by the coupler 5. The A / D converter 7 converts the output signal of the balance type photodetector 6 into a digital signal. The analysis unit 8 analyzes using the digital signal from the A / D converter 7.

[0021] The interference light incident on the balance type photodetector 6 has a beat frequency corresponding to the optical path length difference between the reference optical path and the measurement optical path. In the present disclosure, the backscattered light waveform in the optical fiber 4 to be measured is performed at least three times. The analysis unit 8 obtains an optical spectrum at a distance z 1 to z 2 in the optical fiber 4 to be measured using a time waveform of the interference light, and measures a vibration distribution in the optical fiber 4 to be measured on the basis of a temporal change of the optical spectrum. Thus, in the present disclosure, the vibration distribution in the optical fiber 4 to be measured is measured using the optical spectrum of a partial section defined by a window section in the optical fiber 4 to be measured.

[0022] The vibration analysis length w (light spectrum analysis length) obtained by the extraction of the window section is expressed by the following formula. [Math. 1] w = NΔz = N c 2 F

[0023] Here, parameters are as follows: N : Score of Rayleigh scattered light wave Δz : Spatial resolution of OFDR c : Light Speed of optical fiber F : Frequency sweep band of OFDR

[0024] The spectral shift amount Δν shift due to distortion is represented by the following formula (NPL 2): [Math. 2] Δν shift = 0.78 εν 0

[0025] Here, parameters are as follows: ε : Distortion of optical fiber ν 0 : Center frequency of probe light

[0026] The optical frequency resolution Δν of Rayleigh scattered light is expressed by the following formula. [Math. 3] Δν = c 2 w = F N (Method for Designing Frequency Sensitivity)

[0027] Fig. 3 shows an example of a relationship between the analysis length w and the vibration amplitude of measurement object. • represents the measured value. ε is a unit of strain representing an amount of expansion or contraction with respect to an original length. For example, when a length of 1 m expands or contracts 1 nm, it is expressed as distortion of 1nε.

[0028] The frequency resolution Δν of the optical spectrum of Rayleigh scattered light is given by the formula of (3). The higher the optical frequency resolution Δν is, the higher the sensitivity of the spectral shift amount Δν shift due to distortion is. According to the formula of (3), the longer the analysis length w is, the higher the optical frequency resolution Δν (sensitivity).

[0029] On the other hand, the spectrum shift with respect to the distortion of the Rayleigh scattered light spectrum is given by the formula (2): Hg2, and the spectral shift amount Δν shift is proportional to the distortion amount. Accordingly, as shown in Fig. 3, there is a trade-off relationship in which the longer the analysis length w, the higher the sensitivity for measuring a minute distortion (the smaller the noise of the measuring instrument), the lower the spatial resolution for analyzing the vibration.Therefore,

[0030] ▪ The sensitivity to vibration amplitude and the vibration analysis length w are in a trade-off relationship. ▪ The vibration analysis length w for correctly measuring the vibration needs to be smaller than the space expansion of the vibration which has been conventionally known, and needs to satisfy the condition of sensitivity to the vibration amplitude.

[0031] Therefore, according to the present disclosure, it is determined whether the OFDR-DAS is applicable from the vibration characteristics of the measurement object. More specifically, as shown in Fig. 4, the optimum measurement conditions are set depending on the analysis length w, that is, the spatial expansion and vibration amplitude of the vibration of the measurement object. Specifically, the window section is set so that the vibration amplitude of the measurement object extracted in the window section is larger than a threshold value determined in the window section.

[0032] In order to measure the vibration, the following three conditions need to be satisfied. (1) The window section w has a spatial frequency which is twice the wave number of the vibration to be measured, that is, the spatial frequency, or more. That is, the window section w is 1 / 2 times or smaller than the wavelength of the vibration. (2) Measurement is performed at a repetition frequency of the probe light, which has a time frequency that is at least twice the frequency of the vibration to be measured, that is, the time frequency. That is, the measurement cycle is 1 / 2 times or smaller than the vibration cycle. (3) The vibration sensitivity is higher than the amplitude of the vibration to be measured.

[0033] The window section w is related to the above-mentioned condition (1) and condition (3). Since the window section satisfying the condition (1) having the maximum width has high sensitivity to vibration, the optimum window section is 1 / 2 the wavelength of vibration. The spatial resolution of the OFDR determines the minimum value of the window section from the Formula (1). Therefore, the spatial resolution of the OFDR determines the minimum wavelength that can be measured.

[0034] Referring to Figs. 5 and 6, an example of measurement of the vibration distribution of the overhead cable is shown. Fig. 5 shows a measuring system. A pole #1 is disposed at a position of a distance of 15 m from the OFDR, and a pole #2 is disposed at a position of a distance of 45 m from the OFDR. In this measurement system, the vibration distribution of the overhead cable between two poles #1 and #2 was measured.

[0035] Fig. 6A shows a case where the spatial resolution Δz is 0.8 m, FIG. 6B shows a case where the spatial resolution Δz is 1.6 m, and FIG. 6C shows a case where the spatial resolution Δz is 9.5 m. When the spatial resolution Δz is 0.8 m, the measurement sensitivity is low and the SNR is low, as shown in FIG. 6A. When the spatial resolution Δz is 1.6 m, both the spatial resolution and the sensitivity satisfy the measurement conditions as shown in FIG. 6B. When the spatial resolution Δz is 9.5 m, as shown in FIG. 6C, the spatial resolution is large with respect to the spatial spread of the vibration to be measured, and the vibration distribution cannot be clearly measured.

[0036] As described above, the analysis unit 8 of the present disclosure calculates the optical frequency response of the window section, using the window section determined depending on the vibration amplitude of the measurement object in the optical fiber 4 to be measured. Thus, the present disclosure allows for correct measurement of vibration in the DAS.

[0037] Fig. 7 shows an example of the spectrum measured using OFDR. In OFDR, the optical frequency response of the entire fiber is measured. Therefore, the optical spectrum of a section having the loss distribution waveform can be analyzed. For example, a Fourier transform is performed on the optical frequency response r(v) of the entire optical fiber, and a loss distribution waveform r(τ) is obtained. Further, a window section is determined using the loss distribution waveform r(τ) and a spectrum S(v) of the window section is obtained by performing a Fourier transform of the window section.

[0038] On the other hand, the optical fiber can be modeled as an FBG having a random refractive index distribution. Therefore, as shown in Fig. 8, the spectrum analysis section can be designated, and the dynamic distortion (vibration) can be measured as the time change of the spectrum shift.

[0039] Fig. 9 shows an example of the vibration distribution measuring method according to the present disclosure. The vibration distribution measuring method according to the present disclosure includes executing steps S11 to S15 in order.

[0040] S11: An optical frequency response to the probe light of the optical fiber to be measured is repeatedly measured, and an optical frequency response r(v) of the entire optical fiber at each time is obtained.

[0041] S12: A window section for analyzing the vibration (static distortion) is designated.

[0042] S13: A backscattered light waveform at a measurement time n is measured.

[0043] S14: An optical spectrum of the designated section is analyzed.

[0044] S15: A time waveform of the frequency shift (distortion) is analyzed from the spectrogram.

[0045] In the present disclosure, in step S12, an optical frequency response r(v) is Fourier-transformed to a loss distribution waveform r(τ). Then, a window section is set using the amplitude of the loss distribution waveform r(τ). Then, the optical spectrum of the set window section is extracted from the loss distribution waveform r(τ) obtained through the reference measurement, the first measurement and the second measurement, and the vibration distribution of the measurement object in the optical fiber 4 to be measured is measured using the plurality of extracted optical spectra.

[0046] The window section is set to determine a spatial frequency and vibration sensitivity. If the vibration characteristics such as the wavelength and amplitude of the measurement object are known, the window section can be designated to satisfy the condition (1) and the condition (3) in the setting of the window section. When the vibration characteristics such as the wavelength and amplitude of the vibration to be measured are unknown, the window section is optimized according to the vibration characteristics by examining the window section, while changing the window section as shown in Fig. 6 in setting of the window section. Thus, according to the present disclosure, it can be determined whether the DAS can be applied in consideration of amplitude of vibration of the measurement object, and measurement conditions can be optimized depending on the measurement object.[Industrial Applicability]

[0047] The present disclosure is applicable to the information and communication industry, equipment monitoring, crime prevention, and disaster monitoring.[Reference Signs List]

[0048] 1Frequency sweep light source 2Coupler 3Circulator 4Optical fiber to be measured 5Coupler 6Balance type photodetector 7A / D converter 8Analysis unit

Claims

1. A vibration distribution measuring apparatus which is configured: to measure backscattered light in an optical fiber (4) to be measured a plurality of times at different times, to extract an optical spectrum of a window section determined from a plurality of backscattered light waveforms obtained by the measurement, and to measure a vibration distribution in the optical fiber (4) to be measured using optical spectra of the plurality of extracted backscattered light waveforms; wherein measurement is performed at a repetition frequency of probe light, which has a time frequency that is at least twice the frequency of the vibration to be measured; and wherein the vibration distribution measuring apparatus is configured to calculate the optical spectrum of the window section using a window section in which the vibration amplitude in the window section in the optical fiber (4) to be measured is larger than a threshold value defined in the window section; characterized in that: the window section has a spatial frequency which is twice the wave number of the vibration to be measured or more; and a vibration sensitivity is higher than the amplitude of the vibration to be measured.

2. The vibration distribution measuring apparatus according to claim 1, wherein: the backscattered light is an optical frequency response of a section extracted in a window section with respect to the probe light, the vibration distribution measuring apparatus is configured to generate a loss distribution waveform of each backscattered light measured a plurality of times using an optical frequency response of a section extracted in a window section with respect to the probe light, and to generate an optical spectrum in the same window section from each loss distribution waveform to extract an optical spectrum of a window section.

3. The vibration distribution measuring apparatus according to claim 2, wherein: the vibration distribution measuring apparatus is configured to measure vibration in the optical fiber (4) to be measured using a spectral shift in the same window section.

4. A vibration distribution measuring method of a vibration distribution measuring apparatus comprising: measuring backscattered light in an optical fiber (4) to be measured a plurality of times at different times, extracting an optical spectrum of a window section determined from a plurality of backscattered light waveforms obtained by the measurement, and measuring a vibration distribution in the optical fiber (4) to be measured using optical spectra of the plurality of extracted backscattered light waveforms; wherein measurement is performed at a repetition frequency of probe light, which has a time frequency that is at least twice the frequency of the vibration to be measured; and wherein the optical spectrum of the window section is calculated using a window section in which the vibration amplitude in the window section in the optical fiber (4) to be measured is larger than a threshold value defined in the window section; characterized in that: the window section has a spatial frequency which is twice the wave number of the vibration to be measured or more; and a vibration sensitivity is higher than the amplitude of the vibration to be measured.