Optical Fiber Sensing Device and Optical Fiber Sensing Method

Abstract

The purpose of the present invention is to provide an optical fiber sensing device and an optical fiber sensing method that, even when temperature changes and deformation being measured are large, enable accurate measurement thereof.　The present invention pertains to an optical fiber sensing device using OFDR, said optical fiber sensing device comprising an analysis circuit that executes: acquiring of a change rate of a measured spectral shift by performing time differentiation of the spectral shift; detecting of a discontinuous point in the spectral shift from the change rate; calculating of a predicted value of the change rate with respect to the discontinuous point; estimating of an estimated value of the change rate at the discontinuous point by applying a Kalman filter to the predicted value and the change rate at the discontinuous point; and acquiring of a corrected spectral shift by performing time integration of the estimated value estimated at the discontinuous point and the change rate other than that at the discontinuous point.

Description

Technical Field

[0001] The present disclosure relates to an optical fiber sensing device using OFDR (Optical Frequency Domain Reflectometry) and a sensing method thereof.

Background Art

[0002] FIG. 1 is a diagram for explaining a sensing principle using OFDR. OFDR employs frequency-swept light as probe light. Then, the spectrum S(ν) (FIG. 1(B)) can be analyzed by performing a Fourier transform on the waveform r(τ) (FIG. 1(A)) of the Rayleigh backscattered light with respect to the probe light in the optical fiber (see, for example, Non-Patent Document 1).

[0003] The spectrum S(ν) of the backscattered light varies (spectrum shift) with respect to the strain and temperature of the optical fiber. Therefore, by detecting how much the reference spectrum S ref has shifted at each measurement (spectrum shift Δν), the change amounts of the strain and temperature of the optical fiber can be calculated as follows.

Equation

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0005] When performing OFDR, if the vibration applied to the optical fiber is large, it causes changes in the polarization state of the probe light (see, for example, Non-Patent Document 2) and optical frequency modulation (see, for example, Non-Patent Document 3), and the state of the probe light varies with time. Due to this variation of the probe light, the optical spectrum does not have a consistent spectral structure as shown in Fig. 2(A), and the spectral shift obtained from the cross-correlation (Fig. 2(B)) has discontinuous measurement errors as shown in Fig. 2(C).

[0006] That is, in optical fiber sensing by OFDR, when the strain or temperature change of the measurement target is large, there is a problem that accurate measurement is difficult. Therefore, an object of the present invention is to provide an optical fiber sensing device and an optical fiber sensing method that enable accurate measurement even when the strain or temperature change of the measurement target is large in order to solve the above problems.

Means for Solving the Problems

[0007] In order to achieve the above object, the optical fiber sensing device and its method according to the present invention apply a Kalman filter to the discontinuous points of the spectral shift to estimate the time change of the correct spectral shift.

[0008] Specifically, the optical fiber sensing device according to the present invention is an optical fiber sensing device using OFDR, obtaining the change rate of the spectral shift by differentiating the measured spectral shift with respect to time, detecting the discontinuous points of the spectral shift from the change rate, calculating a predicted value of the change rate for the discontinuous points, applying a Kalman filter to the change rate and the predicted value at the discontinuous points to estimate an estimated value of the change rate at the discontinuous points, and obtaining a corrected spectral shift by integrating the change rate other than the discontinuous points and the estimated value estimated at the discontinuous points with respect to time characterized by comprising an analysis circuit that executes the above.

[0009] Further, the optical fiber sensing method according to the present invention is an optical fiber sensing method using OFDR, and obtaining a rate of change of the spectral shift by time-differentiating the measured spectral shift; detecting a discontinuous point of the spectral shift from the rate of change; calculating a predicted value of the rate of change with respect to the discontinuous point; applying a Kalman filter to the rate of change at the discontinuous point and the predicted value to estimate an estimated value of the rate of change at the discontinuous point, and obtaining a corrected spectral shift by time-integrating the rate of change other than the discontinuous point and the estimated value estimated at the discontinuous point. It is characterized by the above.

[0010] The optical fiber sensing device can eliminate the discontinuous points of the spectral shift generated when the strain or temperature change of the measurement target is large by using a Kalman filter, and can smooth the time waveform of the spectral shift. Therefore, the present invention can provide an optical fiber sensing device and an optical fiber sensing method capable of accurate measurement even when the strain or temperature change of the measurement target is large.

[0011] Before applying the Kalman filter, first, a predicted value of the rate of change of the spectral shift is obtained for the detected discontinuous point under the condition that the rate of change of the spectral shift does not change rapidly. For example, if the predicted value is the rate of change at the time immediately before the time indicating the discontinuous point.

[0012] Next, under the condition that the uncertainty of the predicted value of the rate of change of the spectral shift is smaller than its own noise, a Kalman filter is applied to the detected discontinuous point to estimate the correct rate of change (estimated value) of the spectral shift. Specifically, the Kalman filter estimates the estimated value by Equation (C1).

Equation

[0013] Here, the noise σ m and the uncertainty σ p are set such that σ m > σ p and the standard deviation of the corrected spectral shift with respect to time is smaller than the standard deviation of the spectral shift with respect to time.

[0014] Note that the analysis circuit can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided through a network.

[0015] In addition, the above inventions can be combined as much as possible.

Advantages of the Invention

[0016] The present invention can provide an optical fiber sensing device and an optical fiber sensing method that enable accurate measurement even when the strain or temperature change of the measurement target is large.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

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Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0018] 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. In the present specification and drawings, components having the same reference numerals are assumed to be the same as each other.

[0019] FIG. 3 is a diagram for explaining the optical fiber sensing device 301 of the present embodiment. The optical fiber sensing device 301 is a sensing system including a measuring device 11 and an analysis circuit 12. The measuring device 11 is an OFDR that inputs probe light whose frequency has been swept once into the optical fiber 13 and acquires the spectrum of Rayleigh backscattered light in the optical fiber 13.

[0020] The analysis circuit 12 acquires a spectral shift S from the time variation of the spectrum obtained by the measuring device 11. Then, the analysis circuit 12 differentiates the spectral shift S with respect to time to detect a discontinuous point of the spectral shift. Further, the analysis circuit 12 applies a Kalman filter (see, for example, Non-Patent Document 4) to the discontinuous point to estimate the time change (estimated value) of the correct spectral shift. Finally, the analysis circuit 12 integrates the time change of the spectral shift to obtain a corrected spectral shift. The details of the analysis method of the analysis circuit 12 will be described below.

[0021] FIG. 4 is a diagram for explaining the analysis method performed by the analysis circuit 12. The analysis circuit 12 acquires the rate of change dS / dt of the spectral shift S by differentiating the measured spectral shift S with respect to time (step S01), detects the discontinuous point of the spectral shift S from the rate of change dS / dt (step S02), the time t indicating the discontinuous point nCalculate a predicted value p of the rate of change dS / dt with respect to it (step S03). The time t indicating the discontinuity point n Apply a Kalman filter to the measured value m and the predicted value p of the rate of change at the time t indicating the discontinuity point to estimate the estimated value m^ of the rate of change at the time t n indicating the discontinuity point (step S04), and Time-integrate the rate of change dS / dt other than the discontinuity point and the estimated value m^ estimated at the discontinuity point to obtain a corrected spectral shift S’ (step S05). Perform it.

[0022] [Step S01] Calculate the rate of change dS / dt by differentiating the obtained spectral shift S with respect to time.

[0023] [Step S02] Detect the discontinuity point of the spectral shift S from the value of the rate of change dS / dt. For example, the time when the value of the rate of change dS / dt exceeds a predetermined threshold can be set as the discontinuity point. FIG. 5 is a diagram for explaining the rate of change of the spectral shift S. The vertical axis is the rate of change dS / dt, and the horizontal axis is time. The thin line is the rate of change dS / dt obtained by differentiating the obtained spectral shift S with respect to time, and peaks occur at random times. The time when this peak occurs (for example, time t n Let's say.) is the discontinuity point of the spectral shift.

[0024] [Step S03] Normally, since the sampling rate of the A / D converter of the measuring device 11 is sufficiently higher than the frequency of the measurement target (vibration or temperature change received by the optical fiber 13), the rate of change dS / dt does not change rapidly. Utilize this characteristic, for the time t n indicating the discontinuity point of the spectral shift, predict with the rate of change at time t n-1 . Specifically, replace the value of the peak of the thin line in FIG. 5 (the value of the rate of change dS(t n ) / dt at time t n ) with the rate of change (predicted value p) at the immediately preceding time t n-1 . Predicted value p = dS(tn-1 ) / dt

[0025] [Step S04] Assuming that the noise of the optical fiber sensing device 301 is larger than the uncertainty of prediction, in the Kalman filter, the estimated value m^ is estimated by Equation (C1). [Number] However, m^ is the estimated value, m is the change rate (observed value) at the discontinuous point, p is the predicted value, and σ m is the noise of the optical fiber sensing device 301, and σ p is the uncertainty of prediction.

[0026] The Kalman filter cannot function correctly without grasping the measurement device noise σ m and the uncertainty σ p of the predicted value. σ m represents the standard deviation of the discontinuity of the spectral shift. Since the statistical properties regarding the discontinuity of the spectral shift cannot be obtained, σ m is not exactly known. Therefore, in this embodiment, σ m is set to a large value and is assumed to be able to take any value.

[0027] On the other hand, σ p represents the uncertainty of the predicted value p. If the sampling rate of the A / D converter included in the measuring device 11 is sufficiently higher than the frequency of the phenomenon to be measured (vibration or temperature change), it is considered that the change rate of the spectral shift does not change significantly between time t n and time t n-1 For this reason, σ p is set to a value smaller than σ m

[0028] [Step S05] ​The thick line in FIG. 5 is the rate of change of the spectral shift after replacing the rate of change of the discontinuous points of the spectral shift S (observed value m) estimated in step S04 with the estimated value m^ in this step. Integrating this rate of change of the spectral shift with respect to time gives the corrected spectral shift S'. FIG. 6 is a diagram comparing the spectral shift S (thin line) and the corrected spectral shift S' (thick line). The corrected spectral shift S' has a smoother waveform compared to the spectral shift S.

[0029] At this time, for the analysis circuit 12, σ is set so that the standard deviation of the corrected spectral shift S' with respect to time is smaller than the standard deviation of the spectral shift S with respect to time. m and σ p It is preferable to set the values of.

[0030] [Effect] The results of measuring vibrations with large amplitudes using the optical fiber 13 are shown in FIG. 7. FIG. 7(A) is a diagram showing the vibration distribution measured by the measuring device 11 without analysis by the analysis circuit 12. FIG. 7(B) is a diagram showing the corrected vibration distribution after analysis by the analysis circuit 12. In FIG. 7(A), the vibration distribution in the section with large amplitudes (2500 - 3250 m) is unclear due to measuring device noise. On the other hand, in FIG. 7(B), the measuring device noise is reduced and the vibration distribution is clear. In this way, the optical fiber sensing device 301 can measure the vibration distribution of vibration phenomena with large vibration amplitudes while suppressing the measuring device noise.

Explanation of Signs

[0031] 11: Measuring device 12: Analysis circuit 13: Optical fiber 301: Optical fiber sensing device

Claims

1. An optical fiber sensing device using OFDR, comprising: differentiating the measured spectral shift with respect to time to obtain a change rate of the spectral shift; detecting a discontinuity point of the spectral shift from the change rate; calculating a predicted value of the change rate for the discontinuity point; applying a Kalman filter to the change rate at the discontinuity point and the predicted value to estimate an estimated value of the change rate at the discontinuity point; and obtaining a corrected spectral shift by time-integrating the change rate other than the discontinuity point and the estimated value estimated at the discontinuity point An optical fiber sensing device characterized by comprising an analysis circuit that executes the above.

2. The optical fiber sensing device according to claim 1, wherein the predicted value is the change rate at a time immediately before the time indicating the discontinuity point.

3. The optical fiber sensing device according to claim 1, wherein the Kalman filter estimates the estimated value by formula (C1). 【Number C1】 However, m^ is the estimated value, m is the change rate at the discontinuous point, p is the predicted value, and σ ｍ is the noise of the optical fiber sensing device, and σ ｐ is the uncertainty of the prediction.

4. the noise σ ｍ and the uncertainty σ ｐ are σ ｍ > σ ｐ wherein a standard deviation of the correction spectral shift with respect to time is set to a value smaller than a standard deviation of the spectral shift with respect to time, the optical fiber sensing device according to claim 3.

5. An optical fiber sensing method using OFDR, comprising: differentiating the measured spectral shift with respect to time to obtain a change rate of the spectral shift; detecting a discontinuity point of the spectral shift from the change rate; calculating a predicted value of the change rate for the discontinuity point; applying a Kalman filter to the change rate at the discontinuity point and the predicted value to estimate an estimated value of the change rate at the discontinuity point; and obtaining a corrected spectral shift by time-integrating the change rate other than the discontinuity point and the estimated value estimated at the discontinuity point An optical fiber sensing method characterized by the above.

6. The optical fiber sensing method according to claim 5, wherein the predicted value is the change rate at a time immediately before the time indicating the discontinuity point.

7. The optical fiber sensing method according to claim 5, wherein the Kalman filter estimates the estimated value by formula (C1). 【Number C1】 However, m^ is the estimated value, m is the change rate at the discontinuous point, p is the predicted value, and σ ｍ is the noise of the optical fiber sensing device, and σ ｐ is the uncertainty of the prediction.

8. the noise σ ｍ and the uncertainty σ ｐ are σ ｍ >σ ｐ and setting the standard deviation of the correction spectral shift with respect to time to a value smaller than the standard deviation of the spectral shift with respect to time, the optical fiber sensing method according to claim 7, characterized in that.

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