Vibration Distribution Measuring Device and Method

Abstract

The object of the present disclosure is to be able to perform vibration analysis using spectral shift under appropriate conditions suitable for the measurement object. The vibration distribution measurement device of the present disclosure is a vibration distribution measurement device that measures the backward scattered light in the fiber to be measured multiple times at different times, extracts the spectra of a determined window interval from a plurality of measured backward scattered light waveforms, and measures the vibration distribution in the fiber to be measured using the spectra of the plurality of extracted backward scattered light waveforms; the vibration distribution measurement device calculates the spectrum of the window interval using the window interval in which the vibration amplitude of the fiber to be measured in the window interval is larger than a threshold value determined in the window interval.

Description

Technical Field

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

[0002] The spectrum of Rayleigh scattered light is shifted in frequency with vibration. Distributed Acoustic Sensing (DAS) that analyzes vibration using this frequency shift has been proposed (for example, refer to Non-Patent Document 1).

[0003] In order to accurately measure vibration, although there are known methods using the sampling theorem with a period of repeated measurement higher than the vibration frequency and a method using a spatial resolution with a vibration analysis length shorter than the spatial extension of vibration, the conditions corresponding to the amplitude of vibration are not clear.

[0004] Prior Art Documents

[0005] Non-Patent Documents

[0006] Non-Patent Document 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.

[0007] Non-Patent Document 2: Koyamada, Yahei, et al. “Fiber-optic distributed strain and temperature sensing with very high measurand resolution over long range using coherent OTDR.” Journal of Lightwave Technology 27.9 (2009): 1142 - 1146. Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] In vibration analysis using frequency shift, in order to accurately measure vibration, it is necessary to appropriately set the conditions corresponding to the amplitude of the vibration to be measured. Therefore, an object of the present disclosure is to be able to perform vibration analysis using frequency shift under appropriate conditions suitable for the measurement object.

[0010] Means for Solving the Problems

[0011] The vibration distribution measurement device of the present disclosure is

[0012] performing multiple measurements on the backward scattered light in the fiber under measurement at different times,

[0013] extracting the spectra of a determined window interval from multiple backward scattered light waveforms obtained by the measurement,

[0014] and using the spectra of the multiple backward scattered light waveforms extracted to measure the vibration distribution in the fiber under measurement;

[0015] The vibration distribution measurement device calculates the spectrum of a window interval where the vibration amplitude of the fiber under measurement in the window interval is larger than a threshold determined in the window interval.

[0016] The vibration distribution measurement method of the present disclosure is

[0017] performing multiple measurements on the backward scattered light in the fiber under measurement at different times,

[0018] extracting the spectra of a determined window interval from multiple backward scattered light waveforms obtained by the measurement,

[0019] and using the spectra of the multiple backward scattered light waveforms extracted to measure the vibration distribution in the fiber under measurement, which is a vibration distribution measurement method executed by the vibration distribution measurement device;

[0020] The vibration distribution measurement method calculates the spectrum of a window interval where the vibration amplitude of the fiber under measurement in the window interval is larger than a threshold determined in the window interval.

[0021] Advantages of the Invention

[0022] According to the present disclosure, it is possible to determine whether DAS is applicable by taking into account the amplitude of the vibration of the measurement object, and it is possible to optimize the measurement conditions corresponding to the measurement object. Brief Description of the Drawings

[0023] Figure 1 Showing an example of the spectrum measured in DAS.

[0024] Figure 2 Showing an example of the structure of OFDR.

[0025] Figure 3 Showing an example of the relationship between the vibration analysis length w and the vibration amplitude of the measurement object.

[0026] Figure 4 Showing an example of the measurement conditions in the present disclosure.

[0027] Figure 5 It is a measurement system used in the vibration distribution measurement example.

[0028] Figure 6A This is an example of measuring the vibration distribution when the spatial resolution is 0.8 m.

[0029] Figure 6B This is an example of measuring the vibration distribution when the spatial resolution is 1.6 m.

[0030] Figure 6C This is an example of measuring the vibration distribution when the spatial resolution is 9.5 m.

[0031] Figure 7 This represents an example of the spectrum measured using OFDR.

[0032] Figure 8 This represents an example of measuring dynamic strain (vibration) in which the spectral shift changes with time.

[0033] Figure 9 This represents an example of the vibration distribution measurement method of the present disclosure. Detailed implementation mode

[0034] Hereinafter, the implementation mode of the present disclosure will be described in detail with reference to the drawings. In addition, the present disclosure is not limited to the implementation modes shown below. These examples of implementation are merely examples, and the present disclosure can be implemented in various modified ways based on the knowledge of those skilled in the art. In addition, in this specification and the drawings, technical features with the same reference numerals represent the same technical features.

[0035] (DAS using OFDR)

[0036] Figure 1 This represents an example of the spectrum measured in DAS. In DAS, Rayleigh backscattered light at different times is measured. Specifically, a reference measurement, a first measurement, and a second measurement are performed in sequence. Then, the spectrum (strain) of the Rayleigh backscattered light waveform at distances z1 to z2 at each moment is analyzed, and the time waveform of the vibration is measured. Rayleigh backscattered light can be measured using, for example, OFDR (Optical Reflection Domain Reflection).

[0037] Figure 2 This shows an example of the system structure of the present disclosure. The vibration distribution measurement device of the present disclosure is connected to the fiber optic cable 4 to be measured. The vibration distribution measurement device of the present disclosure has the same structure as OFDR. Specifically, the vibration distribution measurement device includes a frequency-swept light source 1, a coupler 2, a circulator 3, a coupler 5, a balanced photoreceiver 6, an A / D converter 7, and an analysis unit 8. The analysis unit 8 of the present disclosure can be implemented by a computer or a program, and the program can be stored in a storage medium or provided through a network.

[0038] The coupler 2 branches the light from the frequency-swept light source 1 into a reference optical path for local light and a measurement optical path for probe light. The probe light branched into the measurement optical path is incident on the fiber under measurement 4 via the coupler 2 and the circulator 3. The coupler 5 multiplexes the backward scattered light in the fiber under measurement 4, i.e., the probe light, and the local light branched by the coupler 2. The balanced optical receiver 6 receives the interference light multiplexed by the coupler 5. The A / D converter 7 converts the output signal of the balanced optical receiver 6 into a digital signal. The analysis unit 8 analyzes using the digital signal from the A / D converter 7.

[0039] The interference light incident on the balanced optical receiver 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 backward scattered light waveform in the fiber under measurement 4 is performed at least 3 times. The analysis unit 8 uses the time waveform of the interference light to obtain the spectrum at distances z1 to z2 in the fiber under measurement 4, and measures the vibration distribution in the fiber under measurement 4 based on the time change of the spectrum. Thus, in the present disclosure, the vibration distribution in the fiber under measurement 4 is measured using a part of the spectrum determined in the window interval in the fiber under measurement 4.

[0040] The vibration analysis length w (spectrum analysis length) obtained by extracting the window interval is expressed by the following formula.

[0041] [Mathematical formula 1]

[0042]

[0043] Among them, the parameters are as follows.

[0044] N: The number of points of Rayleigh backward scattered light.

[0045] Δz: The spatial resolution of OFDR.

[0046] c: The light speed in the optical fiber.

[0047] F: The frequency sweep band of OFDR.

[0048] The spectral shift amount Δν caused by strain shift Is expressed by the following formula (Non-Patent Document 2).

[0049] [Mathematical formula 2]

[0050] Δv shift = 0.78εv0 (2)

[0051] Among them, the parameters are as follows.

[0052] ε: The strain of the optical fiber.

[0053] v0: The center frequency of the probe light.

[0054] The optical frequency resolution Δν of Rayleigh scattered light is expressed by the following formula.

[0055] [Mathematical formula 3]

[0056]

[0057] (Design method of vibration sensitivity)

[0058] Figure 3 An example showing the relationship between the analysis length w and the vibration amplitude of the measurement object is shown. ● represents the measured value. ε is a unit representing the strain indicating how much the original length has expanded or contracted. For example, when an object with a length of 1 m expands or contracts by 1 nm, it is expressed as a strain of 1 nε.

[0059] The frequency resolution Δν of the spectrum of Rayleigh scattered light is given by Equation (3). The higher the optical frequency resolution Δν, the higher the sensitivity of the spectral shift amount Δν shift caused by strain. According to Equation (3), the longer the analysis length w, the higher the optical frequency resolution Δν (sensitivity).

[0060] On the other hand, the spectral shift corresponding to the strain of the Rayleigh scattered light spectrum is given by Equation (2), and the spectral shift amount Δν shift is proportional to the amount of strain. Therefore, although as Figure 3 shown, the longer the analysis length w, the more capable of measuring minute strain (higher sensitivity, lower measurement noise), there is a trade-off relationship in which the spatial resolution of the analyzed vibration deteriorates.

[0061] Therefore,

[0062] · The sensitivity corresponding to the vibration amplitude and the vibration analysis length w are in a trade-off relationship.

[0063] · Regarding the vibration analysis length w capable of correctly measuring vibration, in addition to being smaller than the spatial expansion of the vibration known in the past, it is also necessary to satisfy the condition of the sensitivity corresponding to the vibration amplitude.

[0064] Therefore, the present disclosure determines whether OFDR-DAS is applicable based on the vibration characteristics of the measurement object. Specifically, as Figure 4 shown, the optimal measurement conditions are set according to the analysis length w, that is, the spatial expansion of the vibration of the measurement object and the vibration amplitude. Specifically, a window interval is set so that the vibration amplitude of the measurement object extracted in the window interval is larger than the threshold value determined in the window interval.

[0065] In order to measure vibration, the following three conditions need to be satisfied:

[0066] (1) A window section w having a spatial frequency that is more than twice as high as the spatial frequency of the vibration being measured, i.e., the wavenumber. That is, a window section w that is less than half the vibration wavelength.

[0067] (2) Measurement is performed at a repetition frequency of the probe light having a temporal frequency that is more than twice as high as the temporal frequency of the vibration being measured, i.e., the vibration number. That is, a measurement period that is less than half the vibration period.

[0068] (3) The vibration sensitivity is higher than the amplitude of the vibration being measured.

[0069] The window section w is related to the above conditions (1) and (3). Since the window section with the maximum width in the window sections that satisfy condition (1) has high sensitivity to vibration, the optimal window section is half the vibration wavelength. In addition, according to Equation (1), the spatial resolution of OFDR determines the minimum value of the window section. Therefore, the spatial resolution of OFDR determines the minimum wavelength that can be measured.

[0070] Refer to Figure 5 and FIG. 6, which show an example of measuring the vibration distribution of an aerial cable. Figure 5 It shows the measurement system. A utility pole #1 is set at a position 15 m away from the OFDR, and a utility pole #2 is set at a position 45 m away from the OFDR. In this measurement system, the vibration distribution of the aerial cable between the two utility poles #1 and #2 is measured.

[0071] Figure 6A shows the case where the spatial resolution Δz is 0.8 m, Figure 6B shows the case where the spatial resolution Δz is 1.6 m, Figure 6C shows the case where the spatial resolution Δz is 9.5 m. When the spatial resolution Δz is 0.8 m, as Figure 6A shown, the measurement sensitivity is low and the SNR is low. When the spatial resolution Δz is 1.6 m, as Figure 6B shown, both the spatial resolution and the sensitivity satisfy the measurement conditions. When the spatial resolution Δz is 9.5 m, as Figure 6C shown, the spatial resolution is large relative to the spatial expansion of the vibration of the measurement object, and the vibration distribution cannot be clearly measured.

[0072] As described above, the analysis unit 8 of the present disclosure uses a window section determined according to the vibration amplitude of the measurement object in the measured optical fiber 4, and calculates the optical frequency response of the window section. Thus, the present disclosure can correctly measure vibration in DAS.

[0073] Figure 7An example of the spectrum measured using OFDR is shown. OFDR measures the optical frequency response of the entire optical fiber. Therefore, it is possible to analyze the spectrum in the section where the loss distribution waveform exists. For example, the Fourier transform of the optical frequency response r(ν) of the entire optical fiber is performed to obtain the loss distribution waveform r(τ). Then, the window section is determined using the loss distribution waveform r(τ), and the spectrum S(ν) of the window section is obtained by performing the Fourier transform of the window section.

[0074] On the other hand, the optical fiber can be modeled as an FBG with a random refractive index distribution. Therefore, as Figure 8 shown, it is possible to specify the spectrum analysis section and measure the dynamic strain (vibration) as the time change of the spectrum shift.

[0075] Figure 9 An example of the vibration distribution measurement method of the present disclosure is shown. The vibration distribution measurement method of the present disclosure sequentially performs steps S11 to S15.

[0076] S11: Repeatedly measure the optical frequency response of the measured optical fiber to the probe light to obtain the optical frequency response r(ν) of the entire optical fiber at each moment.

[0077] S12: Specify the window section for vibration (static strain) analysis.

[0078] S13: Measure the backscattered light waveform at the measurement time n.

[0079] S14: Analyze the spectrum of the specified section.

[0080] S15: Analyze the time waveform of the frequency shift (strain) based on the spectrogram.

[0081] In the present disclosure, in step S12, the Fourier transform of the optical frequency response r(ν) is performed to convert it into the loss distribution waveform r(τ). Then, the window section is set using the amplitude of the loss distribution waveform r(τ). Then, based on the loss distribution waveforms r(τ) obtained from the reference measurement, the first measurement, and the second measurement, the spectrum of the set window section is extracted, and the vibration distribution of the measurement object in the measured optical fiber 4 is measured using the extracted spectra.

[0082] Setting the window section determines the spatial frequency and vibration sensitivity. If the vibration characteristics such as the wavelength and amplitude of the measurement object are known, in the setting of the window section, the window section can be specified to satisfy conditions (1) and (3). When the vibration characteristics such as the wavelength and amplitude of the vibration of the measurement object are unknown, in the setting of the window section, as shown in FIG. 6, by investigating while changing the window section, the window section is optimized according to the vibration characteristics. Thus, the present disclosure can determine whether DAS is applicable in consideration of the amplitude of the vibration of the measurement object, and can optimize the measurement conditions corresponding to the measurement object.

[0083] Industrial Applicability

[0084] The present disclosure can be applied to the information and communication industry, equipment monitoring, anti-theft, and disaster monitoring.

[0085] Description of Reference Numerals

[0086] 1: Frequency Scanning Light Source

[0087] 2: Coupler

[0088] 3: Circulator

[0089] 4: Optical Fiber Under Test

[0090] 5: Coupler

[0091] 6: Balanced Optical Receiver

[0092] 7: A / D Converter

[0093] 8: Analysis Unit.

Claims

1. A vibration distribution measurement device, which measures the backward scattered light in the optical fiber to be measured multiple times at different times, extracts the spectra of the window intervals from the multiple backward scattered light waveforms obtained by the measurement, and uses the spectra of the multiple backward scattered light waveforms extracted to measure the vibration distribution in the optical fiber to be measured; the vibration distribution measurement device uses probe light having a repetition frequency more than twice the temporal frequency of the vibration to be measured, in the extraction of the spectrum, a window interval is used in which the vibration amplitude in the window interval is larger than a threshold value determined in the window interval and is less than or equal to 1 / 2 times the vibration wavelength of the object to be measured.

2. The vibration distribution measurement device according to claim 1, wherein the backward scattered light is the optical frequency response of the interval extracted in the window interval corresponding to the probe light; using the optical frequency response of the interval extracted in the window interval corresponding to the probe light, generating a loss distribution waveform of each backward scattered light after multiple measurements; generating a spectrum of the same window interval through each loss distribution waveform; extracting the spectrum of the window interval.

3. The vibration distribution measurement device according to claim 2, measuring the vibration in the optical fiber to be measured using the frequency shift of the same window interval.

4. A vibration distribution measurement method, which measures the backward scattered light in the optical fiber to be measured multiple times at different times, extracts the spectra of the window intervals from the multiple backward scattered light waveforms obtained by the measurement, and uses the spectra of the multiple backward scattered light waveforms extracted to measure the vibration distribution in the optical fiber to be measured; the vibration distribution measurement method uses probe light having a repetition frequency more than twice the temporal frequency of the vibration to be measured, in the extraction of the spectrum, a window interval is used in which the vibration amplitude in the window interval is larger than a threshold value determined in the window interval and is less than or equal to 1 / 2 times the vibration wavelength of the object to be measured.

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