Vibration detection device and vibration detection method

Abstract

The present invention provides a vibration detection device and a vibration detection method, which can avoid locations where the detection sensitivity deteriorates in vibration detection of a scattered light intensity monitor based on optical frequency multiplexing. The present invention considers a multi-dimensional space having a dimension corresponding to the number of used frequencies, makes the change in the scattered light intensity at each optical frequency correspond to each axis, depicts the scattered light signal as a vector on the space, performs a straight line approximation passing through the origin on the space for the plurality of depicted vector points as original data, and uses the direction vector of the approximate straight line obtained as the approximation result for vibration detection.

Description

Technical Field

[0001] The present invention relates to a vibration detection device based on frequency multiplexing using time domain reflectometry and a vibration detection method thereof. Background Art

[0002] Describe the problem of vibration detection of a scattered light intensity monitor using probe light of a single optical frequency. The probe light pulse of pulse number m is incident on the fiber to be measured, and at time t m Arrive at a location at a distance z from the incident end, and let the intensity of the OTDR scattered light waveform in which the probe light pulse is Rayleigh backscattered and returned be I(z, t m ). Regarding the amount of deformation applied to the location z, taking time t0 as the reference state and time t m When it is set to ε(t m ), then when ε(t m ) is small below the 10 1 nanostrain level, according to experiments and other experiences, it can be regarded that the intensity change linearly changes with respect to the amount of deformation at multiple locations.

[0003] [Mathematical formula 1]

[0004]

[0005] However, strictly speaking, the coefficient C(z) depends on the shape or intensity of the probe light pulse and the distribution of Rayleigh scatterers at each location, and as a result, it becomes a variable such as I(z, T0).

[0006] The coefficient C(z) is affected by the attenuation noise caused by the interference of the scattered light of multiple Rayleigh scatterers randomly distributed at each location, and both the sign and magnitude are different according to each location, that is, the distance z from the incident end. If the noise applied at the time of intensity measurement at each time is set to n(t m ), then the magnitude of the measured signal becomes the following formula.

[0007] [Mathematical formula 2]

[0008]

[0009] The tilde is used to distinguish the measured value from the ideal value. It can be seen that the sensitivity of vibration detection deteriorates at locations where the coefficient C is small.

[0010] In the method of measuring optical loss using OTDR, as a method for eliminating attenuation noise, a method of optical frequency multiplexing is known (for example, refer to Non-Patent Document 1). This method uses frequency multiplexing f k(k = 1, 2, …, K) frequency - multiplexed pulses are used as the probe light for each pulse number m. And, in this method, the scattered light intensity waveform represented by the following formula is calculated by signal processing based on the OTDR waveform obtained at each moment, and this scattered light intensity waveform is averaged as it is and used as a signal. k The scattered light intensity waveform under the following, and this scattered light intensity waveform is averaged as it is and used as a signal.

[0011] [Mathematical formula 2a]

[0012]

[0013] Prior art documents

[0014] Non - Patent Document 1: Hiroyuki Iida, Yusuke Koshikiya, Fumihiko Ito, and Kuniaki Tanaka, “High - Sensitivity Coherent Optical Time Domain Reflectometry Employing Frequency - Division Multiplexing,” J. Lightwave Technol. 30, 1121 - 1126 (2012)

[0015] Non - Patent Document 2: Yahei Koyamada, Mutsumi Imahama, Kenya Kubota, and Kazuo Hogari, “Fiber - Optic Distributed Strain and Temperature Sensing With Very High Measurand Resolution Over Long Range Using Coherent OTDR,” J. Lightwave Technol. 27, 1142 - 1146 (2009)

[0016] Regarding using the scattered light intensity as a signal, the vibration detection of the scattered light intensity monitor is the same. However, in the case of vibration detection, when averaging the scattered light intensity waveform of Mathematical formula 2a as it is, compared with the measurement at a single frequency, there is a problem that it is difficult to avoid locations where sensitivity degradation occurs.

[0017] The following details this problem. The change in the scattered light intensity of the k - th frequency becomes the following formula.

[0018] [Mathematical formula 3]

[0019]

[0020] Coefficient Ck Both the symbol and the magnitude are independent. In addition, the noise n k (z, t m ) is also independent of the frequency number k and the time t m .

[0021] In the method of simply calculating the intensity average, as described below, the signal intensity at time t with respect to the reference time t0 is calculated. K represents the frequency reuse number. m .

[0022] [Equation 4]

[0023]

[0024] From the above equation, it can be seen that the effect of noise reduction is obtained through averaging. C k can take positive and negative values. Therefore, it can be seen that compared with the noise reduction effect, the proportionality coefficient of the following equation multiplied by ε(z, t m )

[0025] [Equation 4a]

[0026]

[0027] is smaller than the original C k , and the effect becomes larger, and there may be locations where the sensitivity deteriorates.

[0028] That is, for the vibration detection of the signal of any frequency before frequency averaging, there may be locations where the sensitivity of the vibration detection using the averaged signal deteriorates. Thus, in the vibration detection of the scattered light intensity monitor, it is not necessarily considered that the method of simply averaging the scattered light intensities of each frequency is most suitable for improving the sensitivity of vibration detection.

[0029] In addition, in the optical frequency reuse method, generally, coherent detection is performed on the backscattered light, and according to the difference in the bandwidth in the frequency domain after Fourier transform of the detected signal, the signals of each optical frequency are separated. Using the values of the I (Inphase; in-phase) component and the Q (Quadrature; quadrature) component of each separated optical frequency, a signal equivalent to the scattered light intensity is obtained according to the sum of the square value of the I component and the square value of the Q component. However, it is also possible to use the square root of the sum of the square value of the I component and the square value of the Q component, that is, the length of the vector in the IQ plane with the I component as the horizontal axis and the Q component as the vertical axis as the signal. It is also possible to perform vibration detection based on the change in the length of this vector.

[0030] In this case, in the deformation amount ε(t m) When the change of m ) is small enough, the same conclusion can be obtained in the same way as the case related to the scattered light intensity. Specifically, the scattered light intensity I(z, t m ) corresponds to the square of the length L(z, t m ) of the vector. Therefore, if the respective changes are set to ΔI(z, t m ) and ΔL(z, t

[0031] ), then within the range of small changes, it becomes m ) = 2L(z, t m ) · ΔL(z, t m ).

[0032] If mathematical formula 1 is used, it is the following formula.

[0033] [Mathematical formula 4b]

[0034]

[0035] Therefore, if C’(z) = c(z) / 2L(z, t m ), the vector length and the deformation amount are proportional with the proportionality coefficient C’(z). In addition, regarding this proportional relationship, according to the empirical rule of the inventors' experiments, if the deformation amount is below the 10 1 nanometer strain level, it is considered to hold at multiple locations. From this, it can be seen that even when the change in the vector length on the IQ plane is used as a signal, when the vector lengths of each optical frequency are set to L k (z, t m ), it cannot be considered that the method of simply averaging them is most suitable for improving the sensitivity. Summary of the Invention

[0036] Therefore, in order to solve the above problems, an object of the present invention is to provide a vibration detection device and a vibration detection method that can avoid locations where the detection sensitivity deteriorates in the vibration detection of a scattered light intensity monitor based on optical frequency multiplexing.

[0037] In order to achieve the above object, the vibration detection device of the present invention performs vibration detection through two processes: preliminary measurement and formal measurement.

[0038] In a preliminary measurement, first, while varying the multiplexed optical frequency for each pulse, Rayleigh scattered light intensity is obtained by time domain reflectometry. Also, a direction vector having, as an element, the time variation of the Rayleigh scattered light intensity obtained in the preliminary measurement is detected for the number obtained by subtracting 1 from the number of pulses used in the preliminary measurement, and statistical processing is performed on the detected plurality of direction vectors, thereby creating an approximate direction vector for each location of the optical fiber.

[0039] In an official measurement, vibration is detected by processing a signal obtained in the official measurement using the approximate direction vector.

[0040] By using the approximate direction vector, selection of an optimal optical frequency at which the sensitivity to deformation changes at each location of the optical fiber becomes optimal, etc. can be performed, and sensitivity improvement can be achieved compared to the aforementioned method of simply averaging the vector lengths on the IQ plane.

[0041] Specifically, the vibration detection device of the present invention is a vibration detection device based on time domain reflectometry, including:

[0042] An optical tester that causes an optical frequency multiplexed pulse including a plurality of basic optical frequencies to be incident on an optical fiber and obtains the intensity of Rayleigh backscattered light; and

[0043] A control arithmetic unit that performs:

[0044] Causing the optical tester to perform a preliminary measurement, the preliminary measurement causing the basic optical frequency to vary only by a moving optical frequency for each optical frequency multiplexed pulse and be incident on the optical fiber, and obtaining the intensity of Rayleigh backscattered light;

[0045] Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, for each location of the optical fiber, detecting a direction vector having, as an element, the time variation of the Rayleigh backscattered light intensity with respect to the basic optical frequency, and creating an approximate direction vector after performing statistical processing on the direction vectors for each moving optical frequency;

[0046] Using the created approximate direction vector, selecting an optimal basic optical frequency at which the sensitivity to deformation becomes optimal for each location of the optical fiber; and

[0047] Causing the optical tester to perform an official measurement, the official measurement causing the optical frequency multiplexed pulse of the plurality of basic optical frequencies to be incident on the optical fiber, and detecting vibration for each location of the optical fiber based on the time variation of the intensity of the Rayleigh backscattered light with respect to the optimal basic optical frequency.

[0048] The vibration detection method is a vibration detection method based on time domain reflectometry, including:

[0049] Perform a preliminary measurement, in which the multiple basic optical frequencies included in the optical frequency multiplexed pulse are incident on the optical fiber by changing only the moving optical frequency for each of the optical frequency multiplexed pulses, and obtain the intensity of the Rayleigh backscattered light;

[0050] Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, at each location of the optical fiber, detect a direction vector having as elements the time variation of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency, and create an approximate direction vector obtained by statistically processing the direction vectors for each of the moving optical frequencies;

[0051] Use the created approximate direction vector to select, at each location of the optical fiber, an optimal basic optical frequency with the best sensitivity to deformation; and

[0052] Perform a formal measurement, in which the optical frequency multiplexed pulse of the multiple basic optical frequencies is incident on the optical fiber, and at each location of the optical fiber, detect vibration based on the time variation of the intensity of the Rayleigh backscattered light with respect to the optimal basic optical frequency.

[0053] In addition, another vibration detection device of the present invention is a vibration detection device based on time domain reflectometry, including:

[0054] An optical tester that makes an optical frequency multiplexed pulse including multiple basic optical frequencies incident on an optical fiber and obtains the intensity of Rayleigh backscattered light; and

[0055] A control arithmetic unit that performs:

[0056] Make the optical tester perform a preliminary measurement, in which the basic optical frequency is incident on the optical fiber by changing only the moving optical frequency for each of the optical frequency multiplexed pulses, and obtain the intensity of the Rayleigh backscattered light;

[0057] Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, at each location of the optical fiber, detect a direction vector having as elements the time variation of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency, and create an approximate direction vector obtained by statistically processing the direction vectors for each of the moving optical frequencies; and

[0058] Make the optical tester perform a formal measurement, in which the optical frequency multiplexed pulse of the multiple basic optical frequencies is incident on the optical fiber, at each location of the optical fiber, detect a direction vector having as elements the time variation of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency, and detect vibration based on the inner product of the direction vector and the approximate direction vector.

[0059] The vibration detection method is a vibration detection method based on time domain reflectometry,

[0060] A preliminary measurement is performed. In the preliminary measurement, multiple basic optical frequencies included in an optical frequency multiplexed pulse are made to change only the moving optical frequency for each optical frequency multiplexed pulse and are incident on an optical fiber, and the intensity of Rayleigh backscattered light is obtained;

[0061] Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, at each location of the optical fiber, a direction vector having as elements the time change of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency is detected, and an approximate direction vector obtained by statistically processing the direction vectors for each moving optical frequency is produced; and

[0062] A formal measurement is performed. In the formal measurement, the optical frequency multiplexed pulse of the multiple basic optical frequencies is incident on the optical fiber, and at each location of the optical fiber, a direction vector having as elements the time change of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency is detected, and vibration is detected based on the inner product of the direction vector and the approximate direction vector.

[0063] The present vibration detection device and method perform a preliminary measurement in which the multiplexed optical frequencies are intentionally changed for each pulse in such a way that the best results can be obtained for each location of the optical fiber, produce approximate direction vectors for each location of the optical fiber, select the best optical frequencies with the best sensitivity to deformation changes, etc., and thus can prevent local deterioration of sensitivity. Therefore, the present invention can provide a vibration detection device and a vibration detection method that can avoid locations where detection sensitivity deteriorates in vibration detection based on a scattered light intensity monitor using optical frequency multiplexing.

[0064] In addition, the above-described inventions can be combined as much as possible.

[0065] The present invention can provide a vibration detection device and a vibration detection method that can avoid locations where detection sensitivity deteriorates in vibration detection based on a scattered light intensity monitor using optical frequency multiplexing. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 is a diagram illustrating the vibration detection device of the present invention.

[0067] Figure 2 is a diagram illustrating the vibration detection method of the present invention.

[0068] Figure 3 is a diagram illustrating the principle of the vibration detection method of the present invention.

[0069] Figure 4 is a diagram illustrating the vibration detection method of the present invention. Detailed implementation mode

[0070] The implementation modes of the present invention will be described with reference to the accompanying drawings. The following described implementation modes are embodiments of the present invention, and the present invention is not limited to the following implementation modes. In addition, in this specification and the accompanying drawings, components with the same reference numerals represent the same components.

[0071] (Embodiment 1)

[0072] Figure 1 It is a diagram for explaining the vibration detection device of this embodiment. This vibration detection device is a vibration detection device based on time domain reflectometry, including:

[0073] An optical tester 21 that makes an optical frequency multiplexed pulse containing a plurality of basic optical frequencies f k incident on the optical fiber 5 and obtains the intensity of Rayleigh backscattered light; and

[0074] A control arithmetic unit 22 that performs:

[0075] Making the optical tester 21 perform a preliminary measurement, the preliminary measurement makes the basic optical frequency f k change the moving optical frequency Δf(m) only for each of the optical frequency multiplexed pulses and be incident on the optical fiber 5, and obtain the intensity of Rayleigh backscattered light;

[0076] Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, at each location of the optical fiber 5, detect the direction vector having, as an element, the time change of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency f k and make an approximate direction vector obtained by statistically processing the direction vectors for each of the moving optical frequencies Δf(m);

[0077] Using the made approximate direction vector, select the optimal basic optical frequency with the best sensitivity to deformation at each location of the optical fiber 5; and

[0078] Making the optical tester 21 perform a formal measurement, the formal measurement makes the optical frequency multiplexed pulses of a plurality of basic optical frequencies f k incident on the optical fiber 5, and at each location of the optical fiber 5, detect vibration according to the time change of the intensity of the Rayleigh backscattered light with respect to the optimal basic optical frequency.

[0079] First, the optical tester 21 will be described in detail. CW laser is output from the laser 1 and branched into two parts by the coupler 2 to produce local light and probe light. The probe light is passed through the modulator 3 to be shaped into a probe pulse multiplexed with an optical frequency, and is incident on the optical fiber 5 via the circulator 4. The scattered light generated by Rayleigh backscattering of the incident probe pulse and the local light are incident on the optical 90-degree mixer 6 together. The in-phase component is received by the balanced photodiode 7, the quadrature component is received by the balanced photodiode 8, and photoelectric conversion is performed. The in-phase component after photoelectric conversion is converted into a digital signal by the AD converter 9, and the quadrature component after photoelectric conversion is converted into a digital signal by the AD converter 10.

[0080] Next, the control arithmetic unit 22 will be described in detail. The storage unit 12 stores the digital signals from the AD converters (9, 10) as data. The calculation unit 13 performs the calculations described later using the stored data. In addition, the instruction unit 11 instructs the modulator 3 of the optical tester 21 to change the optical frequency.

[0081] Figure 2 It is a flowchart showing the vibration detection method performed by this vibration detection device. This vibration detection method is a vibration detection method based on time domain reflectometry, and includes:

[0082] A preliminary measurement is performed. In the preliminary measurement, a plurality of basic optical frequencies included in the optical frequency multiplexed pulse are changed to move the optical frequency only for each optical frequency multiplexed pulse and are incident on the optical fiber, and the intensity of the Rayleigh backscattered light is obtained (step S01).

[0083] Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, at each location of the optical fiber 5, a direction vector having as an element the time change of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency is detected (step S02), and an approximate direction vector obtained by statistically processing the direction vectors for each of the moving optical frequencies is produced (step S03).

[0084] Using the produced approximate direction vector, the optimal basic optical frequency with the best sensitivity to deformation is selected at each location of the optical fiber 5 (step S04), and

[0085] A formal measurement is performed. In the formal measurement, the optical frequency multiplexed pulse of the plurality of basic optical frequencies is incident on the optical fiber (step S05), and at each location of the optical fiber, vibration is detected based on the time change of the intensity of the Rayleigh backscattered light with respect to the optimal basic optical frequency (step S06).

[0086] Use Figure 2 It will be described in more detail. This vibration detection method consists of the following steps:

[0087] [Step A] Prepare frequency multiplexed pulses with a frequency change applied at a moving frequency and obtain preliminary data.

[0088] [Step B] Select the optimal fundamental frequency at each location based on the analysis of the preliminary data, and

[0089] [Step C] Perform a formal measurement using frequency multiplexed pulses with the frequency fixed at the fundamental frequency and detect vibrations based on the change in the scattered light intensity of the optimal fundamental optical frequency.

[0090] [Step A]

[0091] Pulses numbered from pulse number 1 to pulse number M (pulse number m; m is an integer from 1 to M) are used to obtain preliminary data. M also depends on the magnitude of the noise of the detector, but usually a value around a dozen is sufficient. The probe light in each pulse number is a frequency multiplexed pulse with K (optical frequency number k; k is an integer from 1 to K) frequencies multiplexed. In particular, if the optical frequency used for the formal measurement in Step C is set to f k (k = 1, 2,..., K) and is called the fundamental frequency, then in this step, the frequency of the frequency multiplexed pulse constituting each pulse number m is set to f k +Δf(m) using the moving frequency Δf(m). That is, when the oscillation frequency of the laser 1 is set to f0, the external modulator 3 applies a frequency modulation of f k +Δf(m) - f0 to the laser light, generating a frequency multiplexed pulse containing K optical frequency components f k +Δf(m). Among them, Δf(m) does not depend on the frequency number k. That is, the frequency multiplexed pulse of pulse number 1 contains K optical frequency components with each of the K optical frequencies f k shifted by Δf(1). The frequency multiplexed pulse of pulse number 2 contains K optical frequency components with each of the K optical frequencies f k shifted by Δf(2). And the frequency multiplexed pulse of pulse number M contains K optical frequency components with each of the K optical frequencies f k shifted by Δf(M). In addition, even when the frequency change of Δf(m) is applied, the probe light intensity at each optical frequency is adjusted to be fixed.

[0092] As a method for generating such pulses, for example, by applying a modulation of +f k -f0 through an optical SSB modulator and applying a modulation of +Δf(m) through an acousto-optic modulator, it is possible to apply a total of f kModulation of +Δf(m) - f0. However, as long as the modulation as the objective can be achieved and the condition that the degradation of the line width inside the pulse of the frequency multiplexed pulse with respect to the line width of the laser 1 used for measurement is below the level where vibration can be detected is satisfied, any modulator 3 can be used.

[0093] Measurement is performed using frequency multiplexed pulses having the frequencies set in the above-described manner, and the obtained data is saved in the storage unit 12 (step S01). For each frequency number k (k = 1, 2,..., K), that is, for each frequency f k +Δf(m), the calculation unit 13 calculates the change in the scattered light intensity of pulse number m with respect to the scattered light intensity of pulse number 1 obtained.

[0094] [Mathematical formula 4c]

[0095]

[0096] , and the following vector is depicted in the K-dimensional space (step S02).

[0097] [Mathematical formula 5]

[0098]

[0099] The change in the optical frequency of the probe light from f k to f k +Δf(m) can be regarded as equivalent to the generation of the following deformation in the optical fiber 5 while the optical frequency of the probe light remains unchanged.

[0100] [Mathematical formula 5a]

[0101]

[0102] Here, the coefficient A is a constant having a value of about 0.78 in the case of a normal single-mode optical fiber (for example, refer to Non-Patent Document 2).

[0103] The change caused by the frequency modulation of the probe light in Mathematical formula 5a is such that since the magnitude of f k -f0 is small enough with respect to f0, using f0 instead of f k , it can be approximated as

[0104] [Mathematical formula 5b]

[0105]

[0106] . Therefore, considering the actually applied deformation change and noise, the vector in Mathematical formula 5 can be expanded as the following formula.

[0107] [Mathematical formula 6]

[0108]

[0109] Here, if Δf(1) in m = 1 is set to zero, the apparent deformation amount obtained by summing the actual deformation applied to the optical fiber and the change caused by the frequency modulation of the probe light becomes

[0110] [Mathematical formula 6a]

[0111]

[0112] If Δf(m) is made small enough to satisfy Mathematical formula 1 and randomly varies within a range larger than the noise, the correlation between the apparent deformation amount and the actual deformation disappears, and it can vary with an amplitude larger than the noise level. That is, it is possible to create a state equivalent to the state where the deformation with an amplitude larger than the noise changes over time at each location. Regarding the actual setting of Δf(m), it also depends on the magnitude of the noise of the detector. However, if it is considered that Mathematical formula 1 holds for deformation amounts below the 10 1 nanostrain level, it is possible to set it in the range of about -10 MHz to +10 MHz, with random values for magnitude and sign for each pulse number, etc., to cope with it.

[0113] As described above, in step A, for the M pulses up to m = 1, 2,..., M, the vectors of Mathematical formula 6 are depicted in the K-dimensional space.

[0114] [Step B]

[0115] Figure 2 This is a schematic diagram explaining this step. In this step, in the calculation unit 13, these M vectors are used as raw data and fitted to a straight line passing through the origin. For example, the usual least squares method or the like can be used. The unit vector (approximate direction vector) of the following formula representing the direction of the approximate straight line is obtained through this fitting

[0116] [Mathematical formula 6b]

[0117] d(z) = (d1(z), d2(z),..., d K (z) (6b)

[0118] (Step S03).

[0119] The calculation unit 13 compares the magnitudes |d k (z)| of the respective components of the approximate direction vector of Mathematical formula 6(b), and sets the index number of the component providing the maximum value to k opt (z). And the calculation unit 13 sets the optimal frequency at position z to f opt (z) = f kopt (z).

[0120] The meaning of this step will be described. If the influence of noise is excluded, the vector in Mathematical Expression 6 is a constant multiple of the vector (C1(z), C2(z), …, C k (z)). Therefore, if an approximate straight line passing through the origin is calculated for the measurement data, the direction of this straight line can be used as an estimated value of the direction of the vector (C1(z), C2(z), …, C k (z)). Since the estimated vector direction becomes the approximate direction vector of Mathematical Expression 6b, if the estimation accuracy is high enough, comparing the magnitudes of the components of the approximate direction vector of Mathematical Expression 6b is the same as comparing the magnitudes of C k (z). As a result, f opt (z) becomes the optical frequency with the best sensitivity to deformation among the multiplexed fundamental frequencies within the range where Mathematical Expression 1 holds. Therefore, f opt (z) can also be called the optimal fundamental frequency.

[0121] [Step C]

[0122] This step is the formal measurement. After pulse number M + 1, a pulse multiplexing the optical frequency f k (k = 1, 2, …, K) (a pulse without offset Δf(m)) is generated by modulator 3 and incident on optical fiber 5 as the formal measurement to measure the scattered light intensity of Mathematical Expression 2a (Step S05).

[0123] Specifically, for the (M + 1)-th pulse number, the scattered light intensity at each position z in the optimal frequency f opt (z) is set as

[0124] [Mathematical Expression 6c]

[0125]

[0126] , and for pulse numbers after M + 2, the value obtained by subtracting the intensity

[0127] [Mathematical Expression 6d]

[0128]

[0129] of the (M + 1)-th pulse number from the measured scattered light intensity

[0130] [Mathematical Expression 6e]

[0131]

[0132] is set as the signal s(z, t m ), and vibration detection is performed based on the change of this signal. That is,

[0133] [Mathematical Expression 7]

[0134]

[0135] The meaning of this step is explained. Since the intensity change in the light frequency f opt is used as the signal s(z, t m ), if the estimation of f opt in step B is correct, it is possible to exclude the cases with poor vibration detection sensitivity among all the frequencies used, and prevent the deterioration of sensitivity in vibration detection.

[0136] Even when the estimation of f opt in step B is incorrect due to large noise influence, the deformation amount generated in the optical fiber during the preliminary measurement being greater than the range where Equation 1 holds, Equation 1 not holding for the light frequencies used in the measurement, or other reasons, since any one of the multiplexed light frequencies is selected as the signal, it is also possible to prevent the occurrence of locations with deteriorated vibration detection sensitivity in principle compared to the case of using only one of the signals of each light frequency used.

[0137] In addition, except for the process of calculating the scattered light intensity of Equation 2a at each frequency, the method of vibration detection and signal processing of the scattered light intensity monitor at a single light frequency remains unchanged, so the calculation results can be output at high speed. Also, the signal s(z, t m ) is calculated based on the reference that it becomes zero at the time t (M+1) when it can be initially measured in the formal measurement of step C.

[0138] In this embodiment, during the acquisition of preliminary data (step A), the optical frequency of the probe light pulse is deliberately changed from the frequency used in the formal measurement. However, when the optical fiber section to be measured is limited and it is known that vibrations naturally occur in this section, or when vibrations can be artificially applied to this section, it is also possible to use the basic optical frequency used in the formal measurement as it is without the need for the change of Δf(m), and preliminary data can be acquired without deliberately applying a change. In addition, when vibration detection is performed based on the acquisition of all measurement data, it is also possible not to distinguish between preliminary data and formal measurement data. After all measurement data are measured, all measurement data are used to proceed to step B. As step C, vibration detection is performed based on the change in the scattered light intensity at the optical frequency f opt . In this case, if the calculation process up to step B is saved, no new additional calculation needs to be performed in step C.

[0139] In addition, in the present embodiment, the change in the intensity of the scattered light is processed as a signal, but the change in the vector length on the IQ plane may also be processed as a signal.

[0140] (Embodiment 2)

[0141] The structure of the vibration detection device of the present embodiment is the same as that of Figure 1 the vibration detection device. Figure 4 is a flowchart for explaining the vibration detection method performed by the present vibration detection device. The present vibration detection method is a vibration detection method based on time domain reflectometry, and includes:

[0142] Performing a preliminary measurement, in which the plurality of basic optical frequencies included in the optical frequency multiplexed pulse are changed only by the moving optical frequency for each of the optical frequency multiplexed pulses and incident on the optical fiber, and the intensity of the Rayleigh backscattered light is obtained (step S01),

[0143] Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, at each location of the optical fiber, a direction vector having as an element the time change of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency is detected, and an approximate direction vector obtained by statistically processing the direction vectors for each of the moving optical frequencies is produced (steps S02 to S03); and

[0144] Performing a formal measurement, in which the optical frequency multiplexed pulses of the plurality of basic optical frequencies are incident on the optical fiber, and at each location of the optical fiber, a direction vector having as an element the time change of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency is detected, and vibration is detected based on the inner product of the direction vector and the approximate direction vector.

[0145] Using Figure 4 is described in more detail. The present vibration detection method consists of the following steps:

[0146] [Step D] Prepare an optical frequency multiplexed pulse with a deliberately applied frequency change and obtain preliminary data;

[0147] [Step E] Calculate the approximate direction vector at each location based on the analysis of the preliminary data; and

[0148] [Step F] Detect vibration based on the inner product of the direction vector of the scattered light intensity change obtained by performing a formal measurement using an optical frequency multiplexed pulse with a fixed frequency and the approximate direction vector.

[0149] [Step D]

[0150] Is the same as step A described in Embodiment 1.

[0151] [Step E]

[0152] Same as step S03 of step B described in Embodiment 1. However, in this step, the selection of the optimal optical frequency in step S04 is not performed.

[0153] [Step F]

[0154] This step is an official measurement. After pulse number M + 1, a pulse multiplexing optical frequencies f k (k = 1, 2,..., K) (a pulse without offset Δf(m)) is generated by modulator 3 and incident on optical fiber 5 as an official measurement to measure the scattered light intensity of Mathematical Formula 2a (step S15).

[0155] Specifically, calculation unit 13 calculates the direction vector in the following manner based on the scattered light intensity measured for the (M + 1)-th pulse number

[0156] [Mathematical Formula 7a]

[0157]

[0158] and the scattered light intensities measured after pulse number M + 2

[0159] [Mathematical Formula 7b]

[0160]

[0161] , (step S16).

[0162] [Mathematical Formula 8]

[0163]

[0164] The inner product of the direction vector of Mathematical Formula 8 and the approximate direction vector of Mathematical Formula 6b

[0165] [Mathematical Formula 8a]

[0166] S(z, t m )·d(z) (8a)

[0167] is used as signal s2(z, t m ) in this embodiment for signal detection (step S17).

[0168] The meaning of this step is explained. Assume that the approximate direction vector of Mathematical Formula 6b is correctly estimated and s2(z, t m ) is actually expanded. Among them, using a certain proportionality constant D, the following formula can be obtained.

[0169] [Mathematical Formula 8b]

[0170] d(z) = D(C1(z), C2(z),..., CK (z)) (8b)

[0171] Compare the mathematical expression 9

[0172] [Mathematical expression 9]

[0173]

[0174] with the signal s(z, t of the mathematical expression 7 in Embodiment 1 m ). For example, if it is not

[0175] [Mathematical expression 9a]

[0176] [n k (z, t m ) - n k (z, t M+1 )] (9a)

[0177] itself, but set the magnitude of the noise level in the statistical sense to Δn, then Δn is independent of frequency and time. Assuming that the magnitude of Δn is fixed, the SNR of s2(z, t m ) is as follows.

[0178] [Mathematical expression 10]

[0179]

[0180] On the other hand, if the selection of the optimal frequency f opt (z) = f kopt (z) is error-free, then the SNR of Embodiment 1 is as follows.

[0181] [Mathematical expression 11]

[0182]

[0183] If Mathematical expression 10 and Mathematical expression 11 are compared, the method of Embodiment 2, compared with the method of Embodiment 1, also takes into account the changes at frequencies other than

[0184] [Mathematical expression 11a]

[0185]

[0186] by adding the process of calculating the inner product. Therefore, if the approximate direction vector d(z) can be estimated with sufficient accuracy, the method of Embodiment 2 improves the sensitivity of vibration detection compared with the method of Embodiment 1.

[0187] The method of Embodiment 2 can also be calculated in the same way when processing the change in the vector length on the IQ plane as a signal, and the comparison of the SN ratio can also be explained in the same way.

[0188] In addition, when considering the measurement under the shot noise limit using coherent detection, in the specific formula expansion of the SN ratio, regarding Δn as fixed also holds at each location (the location corresponding to each distance z from the incident end) when processing the change in the vector length on the IQ plane as a signal. On the other hand, when processing the intensity of the scattered light, regarding Δn as fixed is correct as an average evaluation, but it should be noted that for the evaluation of each location individually, the dependence on

[0189] [Mathematical formula 11b]

[0190] needs to be considered.

[0191] Similar to the description of Embodiment 1, at the time of acquiring preliminary data (Step D), the optical frequency of the probe light pulse is deliberately changed from the frequency used in the official measurement. However, when the optical fiber section to be measured is limited and it is known that vibrations naturally occur in this section, or when vibrations can be artificially applied to this section, the optical frequency used in the official measurement can also be used as it is, and preliminary data can be acquired without deliberately applying a change. In addition, when vibration detection is performed based on the acquisition of all measurement data, it is also possible not to distinguish between preliminary data and official measurement data. After all measurement data have been measured, all measurement data can be used to proceed to Step E, and vibration detection can be performed through Step F.

[0192] (Other Embodiments)

[0193] The present invention is not limited to Embodiment 1 and Embodiment 2, and the constituent elements and signal processing methods can be changed within the scope not departing from the gist thereof during the implementation stage. In addition, the storage unit 12 and the calculation unit 13 provided in the vibration detection device of the present invention can also be implemented by a computer and a program. The program can be recorded on a recording medium or provided through a network.

[0194] [Supplementary Note]

[0195] Hereinafter, the vibration detection device and method of the present invention will be described.

[0196] In the vibration measurement using OTDR, as a method for detecting the presence and position of vibrations, a method of monitoring the intensity change at each location of the OTDR waveform is known.

[0197] However, in the case of small vibration, the intensity change is proportional to the amount of deformation. However, this proportionality constant not only differs in sign for each location but also in magnitude for each location. Therefore, the sensitivity of vibration detection deteriorates at locations where the magnitude of the proportionality constant is small.

[0198] Therefore, the device and method of the present invention are not simply a method of averaging the signal intensities of each frequency used for obtaining optical loss in the past, but can be regarded as:

[0199] i) Considering a multi-dimensional space having the dimension of the number of frequencies used, making the change in the scattered light intensity at each optical frequency correspond to each axis,

[0200] ii) Depicting the scattered light signal as a vector in the said space,

[0201] iii) Performing a straight-line approximation passing through the origin on the said space for the multiple vector points depicted as raw data,

[0202] iv) Using the direction vector of the approximate straight line obtained as the result of the approximation for vibration detection.

[0203] The vibration detection device and method of the present invention achieve an improvement in the sensitivity of vibration detection through frequency multiplexing. That is, it is possible to maintain the high speed of calculation which is an advantage as an intensity monitor, and effectively reduce the probability of false detection of vibration through optical frequency multiplexing. Although it is not necessary to perform an accurate waveform measurement of the vibration, it is possible to expect effective application to application targets (intruder detection, equipment position determination based on on-site strikes, etc.) where it is desired to quickly measure the location, time, and approximate frequency of the vibration.

[0204] Description of Reference Numerals

[0205] 1: Laser

[0206] 2: Coupler

[0207] 3: Modulator

[0208] 4: Circulator

[0209] 5: Optical fiber

[0210] 6: Optical 90-degree mixer

[0211] 7, 8: Balanced photodiode

[0212] 9, 10: AD converter

[0213] 11: Calculator

[0214] 12: Storage unit

[0215] 13: Calculation unit

[0216] 21: Optical tester

[0217] 22: Control arithmetic unit

Claims

1. A vibration detection device based on time domain reflectometry, wherein the vibration detection device is characterized by comprising: An optical tester that causes an optical frequency multiplexed pulse containing a plurality of basic optical frequencies to be incident on an optical fiber and obtains the intensity of Rayleigh backscattered light; And A control arithmetic unit that performs: Causing the optical tester to perform a preliminary measurement, the preliminary measurement causing the basic optical frequency to change the optical frequency by moving it for each of the optical frequency multiplexed pulses and be incident on the optical fiber, and obtaining the intensity of Rayleigh backscattered light; Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, at each location of the optical fiber, detecting a direction vector having as an element the time change of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency, and creating an approximate direction vector obtained by statistically processing the direction vectors for each of the moving optical frequencies; Using the created approximate direction vector, selecting, at each location of the optical fiber, an optimal basic optical frequency at which the sensitivity to deformation becomes optimal; And Causing the optical tester to perform a formal measurement, the formal measurement causing the optical frequency multiplexed pulse of the plurality of basic optical frequencies to be incident on the optical fiber, and at each location of the optical fiber, detecting vibration based on the time change of the intensity of the Rayleigh backscattered light with respect to the optimal basic optical frequency.

2. A vibration detection device based on time domain reflectometry, wherein the vibration detection device is characterized by comprising: An optical tester that causes an optical frequency multiplexed pulse containing a plurality of basic optical frequencies to be incident on an optical fiber and obtains the intensity of Rayleigh backscattered light; And A control arithmetic unit that performs: Causing the optical tester to perform a preliminary measurement, the preliminary measurement causing the basic optical frequency to change the optical frequency by moving it for each of the optical frequency multiplexed pulses and be incident on the optical fiber, and obtaining the intensity of Rayleigh backscattered light; Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, at each location of the optical fiber, detecting a direction vector having as an element the time change of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency, and creating an approximate direction vector obtained by statistically processing the direction vectors for each of the moving optical frequencies; And Causing the optical tester to perform a formal measurement, the formal measurement causing the optical frequency multiplexed pulse of the plurality of basic optical frequencies to be incident on the optical fiber, and at each location of the optical fiber, detecting a direction vector having as an element the time change of the intensity of the Rayleigh backscattered light with respect to the basic optical frequency, and detecting vibration based on the inner product of the direction vector and the approximate direction vector.

3. A vibration detection method based on time domain reflectometry, wherein the vibration detection method is characterized by comprising: Performing a preliminary measurement, the preliminary measurement causing a plurality of basic optical frequencies included in an optical frequency multiplexed pulse to change the optical frequency by moving it for each of the optical frequency multiplexed pulses and be incident on an optical fiber, and obtaining the intensity of Rayleigh backscattered light; Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, at each location of the optical fiber, a direction vector having as an element the temporal change of the intensity of the Rayleigh backscattered light with respect to the fundamental optical frequency is detected, and an approximate direction vector obtained by statistically processing the direction vectors for each of the moving optical frequencies is created; Using the created approximate direction vector, at each location of the optical fiber, an optimal fundamental optical frequency at which the sensitivity to deformation becomes optimal is selected; and A formal measurement is performed, in which the optical frequency multiplexed pulses of the plurality of fundamental optical frequencies are incident on the optical fiber, and at each location of the optical fiber, vibration is detected based on the temporal change of the intensity of the Rayleigh backscattered light with respect to the optimal fundamental optical frequency.

4. A vibration detection method based on time domain reflectometry, characterized in that the vibration detection method includes: A preliminary measurement is performed, in which the plurality of fundamental optical frequencies included in the optical frequency multiplexed pulses are changed according to the moving optical frequency of each of the optical frequency multiplexed pulses and are incident on the optical fiber, and the intensity of the Rayleigh backscattered light is obtained; Based on the intensity of the Rayleigh backscattered light obtained in the preliminary measurement, at each location of the optical fiber, a direction vector having as an element the temporal change of the intensity of the Rayleigh backscattered light with respect to the fundamental optical frequency is detected, and an approximate direction vector obtained by statistically processing the direction vectors for each of the moving optical frequencies is created; and A formal measurement is performed, in which the optical frequency multiplexed pulses of the plurality of fundamental optical frequencies are incident on the optical fiber, at each location of the optical fiber, a direction vector having as an element the temporal change of the intensity of the Rayleigh backscattered light with respect to the fundamental optical frequency is detected, and vibration is detected based on the inner product of the direction vector and the approximate direction vector.

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