JP7720014B2 - Analysis device and optical fiber sensing method - Google Patents

Description

This disclosure relates to an analysis device for an optical fiber sensing device using OFDR (Optical Frequency Domain Reflectometry) and its sensing method.

Figure 1 illustrates the sensing principle using OFDR. OFDR uses frequency-swept light as the probe light. Then, by Fourier transforming the waveform r(τ) (Figure 1(A)) of the Rayleigh backscattered light corresponding to the probe light from the optical fiber, the spectrum S(ν) (Figure 1(B)) can be analyzed (see, for example, Non-Patent Document 1).

The spectrum S(ν) of the backscattered light varies (spectral shift) depending on the strain and temperature of the optical fiber. Therefore, by detecting how much the reference spectrum S _ref obtained in the reference measurement has shifted between measurements (spectral shift Δν), the amount of change in the strain and temperature of the optical fiber can be calculated using the following equation:
where Δν is the spectral shift, ν ₀ is the central frequency of the probe light, ε is the strain, and T is the temperature.

M. Frogatt and J. Moore, “High-spatial-resolution distributed strain measurement in optical fiber with Rayleigh scatter,” Appl. Opt. , vol. 37, no. 10, pp. 1735-1740, 1998. Okamoto, Tatsuya, Daisuke Iida, and Hiroyuki Oshida. “Vibration-induced beat frequency offset compensation in distributed acoustic sensing based on optical frequency domain reflectometry”. Journal of Lightwave Technology 37.18 (2019): 4896-4901. Okamoto, Tatsuya, Daisuke Iida, and Hiroyuki Oshida. “Investigation of tolerance of OFDR-based DAS to vibration-induced beat frequency offset”. , 2020 Optical Fiber Communications Conference and Exhibition (OFC). IEEE, 2020.

OFDR-based optical fiber sensing devices have a fundamental issue in that optical frequency modulation caused by vibrations in front of the analysis position to be analyzed changes the distance to the analysis position, making it difficult to obtain a consistent optical spectrum and perform accurate vibration analysis.

Figure 2 illustrates the basic problem. Figure 2(A) shows the waveform r(τ) of Rayleigh backscattered light in response to a probe light beam from an optical fiber. Optical fibers typically vibrate. Therefore, the position of the waveform r(τ) to be analyzed fluctuates over time due to vibrations (frequency offset 21) of the optical fiber before the analysis position (the section from time _τ1 to _τ2 ) (toward the probe light input end). This causes waveforms r(τ) that are not the subject of analysis to be included in the section from time _τ1 to _τ2 . In other words, because the waveform r(τ) to be analyzed changes over time, the optical spectral structure of the spectrum S(ν) obtained by Fourier transforming the waveform r(τ) also changes, making it difficult to calculate the amount of spectral shift (amount of distortion) (Figure 2(B)).

To address this fundamental issue, Non-Patent Document 2 discloses a method for mitigating the effects of misalignment due to an increase in the analysis length. This method utilizes the fact that misalignment can be suppressed (has resistance to vibration) if the sensor length (the section from time _τ1 to _τ2 ) is sufficiently long relative to the frequency offset 21. However, this method presents a new problem: the spatial resolution of strain analysis deteriorates when the analysis length is increased.

In response to this fundamental issue, Non-Patent Document 3 discloses a method for correcting positional deviations using signal processing. This method involves signal processing that estimates the distance offset using the cross-correlation of the loss distribution waveforms of the vibration sensors and tracks the position of the vibration sensor to be analyzed. However, this method presents a new issue: the signal load due to the cross-correlation is large.

In order to solve the above-mentioned problems, the present invention aims to provide an analysis device for an optical fiber sensing device and an optical fiber sensing method that can suppress the effects of positional deviation due to vibration with a low processing load without degrading spatial resolution.

To achieve the above objective, the optical fiber sensing device and method of the present invention calculate the amount of optical frequency modulation for each section of the optical fiber from the spectral shift for that section, and correct the positional deviation of the analysis position using the cumulative value of the optical frequency modulation for each section located before the analysis position.

Specifically, the analysis device for an optical fiber sensing device according to the present invention is an analysis device provided in an optical fiber sensing device using OFDR,
calculating an amount of optical frequency modulation for each section of the optical fiber from the spectral shift measured for each section;
The amounts of optical frequency modulation of the section located closer to the analysis position of the optical fiber are accumulated to calculate an accumulated value, and the analysis position is corrected based on the accumulated value.

Further, an optical fiber sensing method according to the present invention is an optical fiber sensing method using OFDR, comprising:
measuring the spectral shift for each section of optical fiber;
calculating an amount of optical frequency modulation for each of the sections from the spectral shift;
The method is characterized in that the optical frequency modulation amounts of the sections located closer to the analysis position of the optical fiber are accumulated to calculate an accumulated value, and the analysis position is corrected based on the accumulated value.

In an optical fiber sensing device, the cumulative strain in each section is considered to be the amount of expansion and contraction of the optical fiber due to vibration. The optical fiber is divided into multiple sections, the amount of optical frequency modulation is measured, and the positional deviation of the analysis position is corrected using the cumulative value of the optical frequency modulation in each section located before the analysis position. Therefore, the present invention can provide an analysis device for an optical fiber sensing device and an optical fiber sensing method that can suppress the effects of positional deviation due to vibration with a low processing load and without degrading spatial resolution.

Calculating the cumulative value may involve accumulating the amount of optical frequency modulation from the section at the near end to the section immediately preceding the analysis position, or accumulating the amount of optical frequency modulation from any of the sections to the section immediately preceding the analysis position.

The present invention is a program for causing a computer to function as the analysis device.

In addition, the analysis device can also be realized using a computer and a program, and the program can be recorded on a recording medium or provided via a network.

The above inventions can be combined as much as possible.

The present invention provides an analysis device and an optical fiber sensing method for an optical fiber sensing device that can suppress the effects of positional deviation due to vibration with a low processing load without degrading spatial resolution.

FIG. 1 is a diagram illustrating the sensing principle using OFDR. FIG. 1 is a diagram illustrating a problem to be solved by the present invention. 1 is a diagram illustrating an optical fiber sensing device according to the present invention. 1A to 1C are diagrams illustrating an optical fiber sensing method according to the present invention. 1A to 1C are diagrams illustrating the principle of an optical fiber sensing method according to the present invention. 1A to 1C are diagrams illustrating the effects of the optical fiber sensing method according to the present invention. 1 is a diagram illustrating an optical fiber sensing device according to the present invention. 1A to 1C are diagrams illustrating the effects of the optical fiber sensing method according to the present invention. 1 is a diagram illustrating an optical fiber sensing device according to the present invention.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components with the same reference numerals in this specification and drawings are considered to be identical to each other.

(Embodiment 1)
3 is a diagram illustrating an optical fiber sensing device 301 according to this embodiment. The optical fiber sensing device 301 is a sensing system including a measurement device 11 and an analysis device 12. The measurement device 11 is an OFDR that inputs probe light whose frequency has been swept once into an optical fiber 13 and acquires the spectrum of the Rayleigh backscattered light in the optical fiber 13.
In this specification, of the two ends of the optical fiber 13, the end connected to the measuring device 11 may be referred to as the "near end" and the other end as the "far end."

The analysis device 12 obtains the spectral shift Δν from the time variation of the spectrum obtained by the measurement device 11. Then, the analysis device 12 divides the optical fiber 13 into a plurality of sections #i ("i" is the section number and is an integer from 0 to N) in the longitudinal direction, calculates the optical frequency modulation amount f(x _i , t) of each section of the optical fiber 13 from the spectral shift Δν(x _i , t) of that section, and corrects the positional deviation of the analysis position using the cumulative value f _offset (t) of the optical frequency modulation amount of each section located before the analysis position.
The correction of the positional deviation of the analysis position performed by the analysis device 12 will be described in detail below.

FIG. 4 is a diagram illustrating the optical fiber sensing method performed by the analysis device 12. In FIG.
The analysis device 12
Measuring the spectral shift Δν(x _i , t) for each section #i of the optical fiber 13 using the measurement device 11 (steps S01, S02, Si1, Si2);
calculating an optical frequency modulation amount f(x _i , t) for each of the sections from the spectral shift;
accumulating the optical frequency modulation amounts of the sections located closer to the analysis position of the optical fiber 13 to calculate an accumulated value f _offset (t) (step S03, step Si3); and correcting the analysis position x _i based on the accumulated value (step Sa1).
Do the following.

5 is an image diagram illustrating the correction of positional deviation of the analysis position in this optical fiber sensing method. The optical fiber 13 is divided into multiple sections #i (i is an integer from 0 to N) for analysis. _xi is the length of section #i, t is time, and ε( _xi , t) is the amount of strain of the optical fiber 13 in section #i. The length _xi is set to be longer than the spatial resolution L of strain detectable by the measurement device 11.

This optical fiber sensing method analyzes the optical frequency modulation amount f(x _i , t) from the strain amount ε(x _i , t) of each section #i, and utilizes the fact that the optical frequency modulation amount of the probe light propagated from the near end to section #i is given by the accumulation of the optical frequency modulation amounts from sections #0 to #i. Note that this optical fiber sensing method expresses the strain amount ε as a spectral shift Δν(x _i , t).

[Step S01]
The analysis device 12 acquires a spectrogram S(x ₀ , t, v) from the light returning from section #0 (position at a distance x ₀ from the near end) among the backscattered light acquired by the measurement device 11, where v is the optical frequency.
[Step S02]
The analysis device 12 then analyzes the spectral shift Δν(x _{0 , t) from the spectrogram S(x 0 ,} t, ν). The method for analyzing the spectral shift Δν from the spectrogram S may be the method disclosed in Non-Patent Document 1.
[Step S03]
The analyzer 12 calculates the amount of optical frequency modulation f(x ₀ , t) from Δν(x ₀ , t) using the following equation: The relationship between the amount of distortion ε and the amount of phase change θ will be explained in Appendix A.
Furthermore, the analyzing device 12 calculates the beat frequency offset f _offset (t) of the probe light that has propagated round trip through section #0 (distance x ₀ ).
Since equation (2) is the frequency modulation amount of the probe light propagated from the distance x _i to x _j , the frequency offset in section #0 is given by
f _offset (t)=2f(x ₀ , t)
is.
[Step Sc1]
Section numbers range from 0 to 1.
[Step Sa1]
6 is a diagram illustrating step Sa1. The analysis device 12 corrects the positional deviation of the vibration analysis position (distance x _i-1 ; distance x ₀ after step S03, distance x _i-1 after step Si3) using the following equation:
That is, the analyzer 12 corrects the positional deviation of the vibration analysis position using the accumulated optical frequency modulation amounts f _offset (t) of sections #0 to #i-1.
[Step Si1]
The analysis device 12 obtains a spectrogram S(x _i , t, v) from the light returning from section #i (position at a distance x _i from the near end) among the backscattered light obtained by the measurement device 11, where v is the optical frequency.
[Step Si2]
Then, the analysis device 12 analyzes the spectral shift Δν(x _i , t) from the spectrogram S(x _i , t, ν).
Here, the analysis device 12 may calculate the distortion amount ε( _xi ,t) for each section #i from the spectral shift Δν( _xi ,t) for each section #i. The distortion amount ε( _xi ,t) needs to be calculated only for the desired analysis position. Specifically, the distortion amount ε( _xi ,t) is calculated using equation (A2) described in Appendix A at the position _xi after correcting the positional deviation.
[Step Si3]
Using equation (2), the beat frequency offset f _offset (t) of the light that has propagated round trip from the distance x ₀ to x _i is calculated.
If i=N, the process ends without performing this step.

After step Si3, the section number is counted up by one in step Sc2, and the process returns to step Sa1, where the positional deviation of the vibration analysis position (distance x _i-1 ) is corrected by equation (3).

[Example]
7 is a diagram illustrating the experimental system. The optical fiber 13 was divided into two sections: the optical fiber 13a at the near end and the optical fiber 13b at the far end. The length of the optical fiber 13a was 60 m, and the length of the optical fiber 13b was 160 m. A 30 Hz sinusoidal vibration was applied only to the optical fiber 13a.
The spatial resolution L of the measuring device 11 is 66 cm.
8A and 8B are diagrams illustrating the effects of the present invention. Fig. 8A shows the results of distortion measurement in the experimental system of Fig. 7 without performing misalignment correction by the analyzer 12 (Comparative Example). Fig. 8B shows the results of distortion measurement in the experimental system of Fig. 7 with misalignment correction by the analyzer 12 (Example).
In the comparative example, the strain ε can be measured correctly up to a distance of approximately 0 to 30 m, but measurement errors occur after that. This means the following: The spatial resolution of the measurement device 11 is strong enough to withstand the amount of accumulated frequency modulation when the probe light propagates up to a distance of approximately 0 to 30 m, but is insufficient to withstand the amount of accumulated frequency modulation at distances beyond that. Measurement errors occur frequently, particularly around the time when the positional deviation is at its maximum or the time when the sinusoidal distortion becomes zero.
On the other hand, in the embodiment, the positional deviation is corrected in both the section of the optical fiber 13a and the section of the optical fiber 13b to which vibration is not applied, and the strain ε is measured correctly.
(supplement)
As shown in equation (A3), the strain ε imparts a phase shift θ to the probe light. The time derivative of the phase shift θ is the optical frequency modulation amount f. In this experiment, a sinusoidal distortion was applied, so the optical frequency modulation amount (positional deviation amount) reaches its maximum when the distortion becomes zero (the time derivative of the sine wave reaches its maximum when it passes through the zero point). The experimental results certainly demonstrate this, with measurement errors occurring near the time when the distortion becomes zero.
The amount of optical frequency modulation f due to the strain ε accumulates as the probe light propagates. Therefore, the strain ε can be measured correctly up to a distance of approximately 0 to 30 m, but beyond that, the accumulated value of the amount of optical frequency modulation exceeds the spatial resolution of the measuring device 11, causing measurement errors.
(End of supplement)

[effect]
As explained in FIG. 8, the optical fiber sensing method according to the present invention requires less signal processing load (differentiation and addition) than the conventional method (cross-correlation), and can measure vibration distribution without degrading the spatial resolution of strain analysis.

In the above embodiment, the optical frequency modulation amount f(x _i , t) from the near-end section #0 to the section #N-1 immediately before the analysis position is accumulated to perform position correction. However, the optical fiber sensing method according to the present invention can calculate the frequency offset f _offset (t) for any section (x i to x _j ) as explained in equation (2). Therefore, it is also possible to analyze the amount of positional deviation by accumulating the optical frequency modulation amount f(x _i , t) from a section at any point, not just the near _- end, to the section #N-1 immediately before the analysis position. Therefore, the optical fiber sensing method according to the present invention can improve convenience as well as the accuracy of strain measurement.

(Embodiment 2)
The analysis device 12 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.
9 shows a block diagram of a system 100. The system 100 includes a computer 105 connected to a network 135.

Network 135 is a data communications network. Network 135 may be a private network or a public network, and may include any or all of the following: (a) a personal area network, e.g., covering a room; (b) a local area network, e.g., covering a building; (c) a campus area network, e.g., covering a campus; (d) a metropolitan area network, e.g., covering a city; (e) a wide area network, e.g., covering an area spanning city, region, or country boundaries; or (f) the Internet. Communications are conducted over network 135 by electronic and optical signals.

Computer 105 includes processor 110 and memory 115 connected to processor 110. Although computer 105 is depicted herein as a standalone device, it is not limited to such and may rather be connected to other devices not shown in a distributed processing system.

Processor 110 is an electronic device consisting of logic circuits that respond to and execute instructions.

Memory 115 is a tangible, computer-readable storage medium on which a computer program is encoded. In this regard, memory 115 stores data and instructions, i.e., program code, that are readable and executable by processor 110 to control the operation of processor 110. Memory 115 may be implemented as random access memory (RAM), a hard drive, read-only memory (ROM), or a combination thereof. One component of memory 115 is program module 120.

Program modules 120 include instructions for controlling processor 110 to perform the processes described herein. Although operations are described herein as being performed by computer 105 or a method or process or sub-process thereof, those operations are actually performed by processor 110.

The term "module" is used herein to refer to a functional operation that may be embodied as either a stand-alone component or an integrated configuration of multiple subcomponents. Thus, program module 120 may be implemented as a single module or as multiple modules operating in concert with one another. Furthermore, while program module 120 is described herein as being installed in memory 115 and thus implemented in software, it may be implemented in any of hardware (e.g., electronic circuitry), firmware, software, or a combination thereof.

While program module 120 is shown as already loaded into memory 115, it may be configured to reside on storage device 140 for later loading into memory 115. Storage device 140 is a tangible, computer-readable storage medium that stores program module 120. Examples of storage device 140 include compact discs, magnetic tape, read-only memory, optical storage media, a memory unit consisting of a hard drive or multiple parallel hard drives, and a universal serial bus (USB) flash drive. Alternatively, storage device 140 may be random access memory or another type of electronic storage device located in a remote storage system (not shown) and connected to computer 105 via network 135.

System 100 further includes data source 150A and data source 150B, collectively referred to herein as data sources 150, and communicatively connected to network 135. In practice, data source 150 may include any number of data sources, i.e., one or more data sources. Data source 150 may include unstructured data and may include social media.

The system 100 further includes a user device 130 operated by the user 101 and connected to the computer 105 via a network 135. The user device 130 includes an input device, such as a keyboard or a voice recognition subsystem, that allows the user 101 to communicate information and command selections to the processor 110. The user device 130 also includes an output device, such as a display device, a printer, or a voice synthesizer. A cursor control, such as a mouse, trackball, or touch-sensitive screen, allows the user 101 to manipulate a cursor on the display device to communicate further information and command selections to the processor 110.

The processor 110 outputs the results 122 of the execution of the program module 120 to the user device 130. Alternatively, the processor 110 may provide the output to a storage device 125, such as a database or memory, or via a network 135 to a remote device (not shown).

For example, a program that performs the flowchart of Figure 4 may be the program module 120. The system 100 may be operated as the analysis device 12.

The terms "comprises" or "comprising" should be interpreted as specifying the presence of the stated feature, integer, step, or component, but not excluding the presence of one or more other features, integers, steps, or components, or groups thereof. The terms "a" and "an" are indefinite articles and therefore do not exclude embodiments having a plurality thereof.

(Other embodiments)
The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. In short, the present invention is not limited to the above-described embodiment, and the components can be modified and embodied in the implementation stage without departing from the spirit of the present invention.

In addition, various inventions can be formed by appropriately combining multiple components disclosed in the above embodiments. For example, some components may be deleted from all of the components shown in the embodiments. Furthermore, components from different embodiments may be appropriately combined.

[Appendix A]
The relationship between the distortion amount ε, the spectrum shift Δ, and the phase change amount θ will be described.
The distortion and temperature dependence of the spectral shift of Rayleigh backscattered light is given by:
Considering a situation where there is no temperature change T in the optical fiber and only strain ε is applied, the spectral shift Δν of the Rayleigh backscattered light is given by the following equation:
When a section of length L changes by an amount of expansion/contraction ΔL due to strain ε, the amount of phase change θ is given by the following equation.

[Definition]
The parameters used herein are as follows:
x: longitudinal position of the optical fiber (distance from the near end)
t: time n: refractive index of optical fiber Δν: spectral shift ν ₀ : central optical frequency of probe light c: speed of light in vacuum L: spatial resolution of strain that can be detected by the measurement device k: wave number T: temperature change of optical fiber f ₀ : vibration analysis position at t=0 γ: frequency sweep speed of OFDR (measurement device)

11: Measuring device 12: Analysis circuit 13: Optical fiber 100: System 101: User 105: Computer 110: Processor 115: Memory 120: Program module 122: Results 125: Storage device 130: User device 135: Network 140: Storage device 150: Data source 301: Optical fiber sensing device

Claims (7)

An analysis device provided in an optical fiber sensing device using OFDR,
calculating an amount of optical frequency modulation for each section of the optical fiber from the spectral shift measured for each section;
an analysis device that calculates a cumulative value by accumulating the optical frequency modulation amounts of the section located closer to the analysis position of the optical fiber; and correcting the analysis position based on the cumulative value. The analysis device described in claim 1, wherein calculating the cumulative value is accumulating the amount of optical frequency modulation from the section at the near end to the section immediately before the analysis position. The analysis device described in claim 1, wherein calculating the cumulative value is accumulating the amount of optical frequency modulation from any one of the sections to the section immediately preceding the analysis position. An optical fiber sensing method using OFDR, comprising:
measuring the spectral shift for each section of optical fiber;
calculating an amount of optical frequency modulation for each of the sections from the spectral shift;
an optical fiber sensing method comprising: calculating a cumulative value by accumulating the optical frequency modulation amounts of the section located closer to an analysis position of the optical fiber; and correcting the analysis position based on the cumulative value. The optical fiber sensing method described in claim 4, wherein calculating the cumulative value is accumulating the amount of optical frequency modulation from the section at the near end to the section immediately before the analysis position. The optical fiber sensing method described in claim 4, wherein calculating the cumulative value is accumulating the amount of optical frequency modulation from any one of the sections to the section immediately preceding the analysis position. A program for causing a computer to function as an analysis device according to any one of claims 1 to 3.