Signal processing method for phase otdr

Abstract

The present invention is a device comprising a signal processing device that calculates an oscillation waveform using the phase of scattered light from a frequency-multiplexed optical pulse being reflected or scattered by optical fiber. The signal processing device includes a functional unit that: selects a reference optical frequency from a plurality of frequency-multiplexed optical frequencies; rotates the phase of a scattered light vector of another optical frequency among the scattered light vectors of the plurality of optical frequencies, using the phase of the reference optical frequency as a reference; and, thereafter, averages the scattered light vectors of the plurality of optical frequencies. To select the reference optical frequency, the functional unit temporally tracks and dynamically selects an optical frequency having a greater scattered light intensity.

Description

Signal processing method in phase OTDR

[0001] The present invention relates to a phase OTDR (Optical Time Domain Reflectometer) that measures the phase of scattered light signals from each point in an optical fiber under test.

[0002] As a means for measuring physical vibrations applied to an optical fiber in a distributed manner along the length of the optical fiber, a method called Distributed Acoustic Sensing (DAS) is known, in which pulsed test light is incident on the optical fiber under test and backscattered light due to Rayleigh scattering is detected (Non-Patent Document 1).

[0003] DAS-P (DAS-phase) is a method for measuring the phase of scattered light signals from each point in an optical fiber under test and measuring the time change of the phase. In DAS-P, the phase changes linearly with respect to the change in the optical path length of the optical fiber due to vibration, and the rate of change can be considered to be approximately the same at each point in the longitudinal direction of the optical fiber. This enables quantitative measurement of vibration and faithfully reproduces the vibration waveform applied to the optical fiber under test (for example, Non-Patent Document 2).

[0004] In DAS-P using a single-frequency pulse, interference between scattered light beams within the pulse width creates points where the scattered light intensity is low, resulting in a deterioration of sensitivity. One method for preventing this deterioration of sensitivity is to perform frequency multiplexing and average signals at different frequencies (Patent Document 1). Hereinafter, the method disclosed in Patent Document 1 will be referred to as Prior Art 1. Prior Art 1 utilizes the fact that the points where the scattered light intensity is low change with different frequencies.

[0005] In Prior Art 1, an efficient averaging method specifically involves averaging in the following scattered light vector states: Step S01: Using measured data from multiple times, the difference in phase offset values ​​between the scattered light vector of a frequency component selected as an internal reference of the multiplexed frequency components (hereinafter referred to as the "reference optical frequency") and the scattered light vectors of the other frequency components is calculated. The difference in the calculated phase offset values ​​is used to calculate a phase rotation angle for correcting the phase offset of each optical frequency. Step S02: The scattered light vector of each frequency component at each time is rotated by the phase rotation angle, and the scattered light vectors after rotation of the different optical frequencies are averaged to calculate an optical frequency average vector, and the angular change of the optical frequency average vector is calculated. Step S03: Using the angle change, the phase difference between two points separated by the gauge length is calculated, and phase unwrapping and other processes are performed to calculate the vibration waveform.

[0006] In particular, the details of step S01 are as follows: where, assuming that the sampling interval of vibration is t and measurement data of M points from measurement time 0 to (M-1)t (M is a natural number) is used, the reference optical frequency (f １ The scattered light vector at １ (mt, z) (depending on the distance z from the input end because it depends on the optical fiber position), and each optical frequency f ｉ (i is an integer between 2 and N, and N represents the number of frequency multiplexing) is expressed as r ｉ Let mt, z be the reference optical frequency. In prior art 1, the reference optical frequency can be set arbitrarily.

[0007] Step S01-1: Scattered light vector r of the reference optical frequency at each time and each optical fiber point １ Angle θ of (mt, z) １ (mt, z) is calculated. The scattered light vector r １ If we consider (mt, z) as a complex vector on the complex plane, that is, a complex number, then the angle θ １ (mt, z) is arg[r １ (mt, z)] where arg is an operator that gives the argument of a complex number.

[0008] Step S01-2: Scattered light vector r of each optical frequency at each time and each optical fiber point ｉ (mt, z) is the angle -θ １ Rotate by (mt, z) and ｉ＿ｒｏｔ (mz, t). The scattered light vector r ｉ If (mt, z) is a complex vector on the complex plane, the vector r after rotation is ｉ＿ｒｏｔ (mz, t) is exp[-θ １ (mt, z)] × r ｉ It can be calculated as (mt, z).

[0009] Step S01-3: Rotate the scattered light vector of each optical frequency at each optical fiber point to obtain the vector r ｉ＿ｒｏｔ (mt, z) is averaged over M points, where m is the measurement data used, from 0 to (M-1), to obtain the time-averaged vector r ｉ＿ａｖｅｔ In actual calculations, vector sums may be used instead of vector averages for M points. Regardless of which method is used, the same results will be obtained in subsequent processing.

[0010] Step S01-4: The phase rotation angle α when rotating the scattered light vector of each optical frequency at each optical fiber point in step S02 ｉ (z) is expressed as -arg[r ｉ＿ａｖｅｔ (z)] + arg[r ｉ＿ａｖｅｔ (z)] is the difference in the phase offset value, so -arg[r ｉ＿ａｖｅｔ (z)] to correct for the difference in phase offset.

[0011] The phase rotation angle α given in step S01-4 ｉ Since time averaging is performed in the steps up to step S01-3, (z) is a value in which noise has been effectively reduced, and its phase rotation angle α ｉ By calculating the optical frequency mean vector in step S02 using (z), the signal-to-noise ratio of the final vibration waveform can be improved and waveform distortion can be suppressed.

[0012] In step S01 of prior art 1, the phase rotation angle of each optical frequency is calculated using measurement data at M points from measurement time 0 to (M-1)t (M is a natural number), and in step S02, the calculated phase rotation angle is used to perform frequency averaging on the measurement data at M points from measurement time 0 to (M-1)t (M is a natural number) and on measurement data at other times.

[0013] The optimal value of the phase rotation angle calculated by the method of step S01 in Prior Art 1 varies over time due to changes in optical characteristics such as the laser oscillation frequency, temperature changes in the optical fiber under test itself, and the application of large dynamic strain to the optical fiber under test. Therefore, in long-term distributed vibration measurement, it is necessary to continuously update the optimal value of the phase rotation angle and perform the processes from step S02 onwards. However, Prior Art 1 does not disclose a method for performing the processes from step S02 onwards while continuously updating the optimal value of the phase rotation angle.

[0014] A solution to this problem has been proposed in Prior Application 1 (PCT / JP2022 / 34477). In the method described in Prior Application 1, the processes from step S02 onwards are performed while continuously updating the optimum value of the phase rotation angle. Hereinafter, the method described in Prior Application 1 will be referred to as Prior Invention 1. By using Prior Invention 1, it is possible to deal with changes in the optimum value of the phase rotation angle.

[0015] However, in the prior invention 1, the phase rotation angle is calculated and updated for each block with a time length of Mt (M is a natural number corresponding to the number of data points), so the value of the phase rotation angle changes discontinuously each time the block moves to the next. Therefore, a phenomenon is observed in which the final vibration waveform also changes discontinuously between the measurement data points at the time of switching to the next block. For example, a phenomenon occurs in which the vibration waveform changes discontinuously between time (MK-1)t, which is the last measurement data point in block K, and time MKt, which is the first measurement data point in block K+1.

[0016] As a countermeasure to this problem, the method (Prior Application Invention 2) described in Prior Application 2 (PCT / JP2023 / 4382) removes discontinuous changes in the phase rotation angle used to average the scattered light vectors of different frequency components. A time window with a window length Mt (M is a natural number corresponding to the number of data points) used in signal processing is then set in advance. The time window is then moved to calculate the phase rotation angle using the measurement data contained within the time window. This phase rotation angle is then used to average the scattered light vectors of the frequency components at the center of the time window or at a time a predetermined number of measurement data points behind. Calculating the phase rotation angle while moving the time window is a broad-sense moving average, and it is possible to obtain a smoothly changing phase rotation angle that does not include the discontinuous changes. Using a smoothly changing phase rotation angle, discontinuous changes in the vibration waveform can be removed.

[0017] In all of Prior Art 1, Prior Invention 1, and Prior Invention 2, when averaging scattered light vectors of different optical frequencies, an arbitrary optical frequency is used as a reference, and the scattered light vectors of the different optical frequencies are rotated and then averaged. Prior Invention 1 and Prior Invention 2 in particular focus on the fact that the difference between the phase offset of the scattered light vector of an optical frequency different from the reference optical frequency and the phase offset of the scattered light vector of the reference optical frequency changes over time when large vibrations, temperature changes, etc. occur, and by updating the phase rotation angle in accordance with these changes, highly sensitive phase calculations are possible throughout the entire measurement time.

[0018] In other words, in the prior application invention 1 and the prior application invention 2, the oscillatory waveform finally obtained through updating the phase rotation angle has reduced phase noise compared to an oscillatory waveform obtained using a signal of only one of the selected reference optical frequency or any other optical frequency, even when large vibrations or temperature changes occur, and the influence of noise-induced phase uncoupling errors is also reduced.

[0019] However, since the final vibration waveform obtained follows the vibration waveform of the reference optical frequency when the influence of noise is removed, vibration waveform distortion not caused by noise remains as the vibration waveform distortion of the reference optical frequency, and therefore cannot be effectively suppressed by Prior Application Invention 1 and Prior Application Invention 2. Vibration waveform distortion not caused by noise mainly includes vibration waveform distortion caused by the interference effect between scattered light from Rayleigh scatterers randomly distributed in the optical fiber, and is difficult to remove by improving the device configuration, etc. Therefore, Prior Application Invention 1 and Prior Application Invention 2 have the problem of not being able to faithfully measure vibrations occurring in the optical fiber.

[0020] As a countermeasure to this, in the method (Prior Invention 3) described in Prior Application 3 (PCT / JP2023 / 11104), each of the multiplexed optical frequencies is selected as a reference optical frequency, an oscillatory waveform is calculated by the method of Prior Invention 1 or Prior Invention 2, and the waveforms obtained by the calculation for the number of multiplexed frequencies are averaged to obtain a final oscillatory waveform. The oscillatory waveform distortion of the oscillatory waveform obtained when each of the multiplexed optical frequencies is selected as a reference optical frequency corresponds to the waveform distortion when the optical frequency selected as the reference optical frequency is used alone, and therefore the oscillatory waveform distortion can be reduced by switching the reference optical frequencies and averaging them.

[0021] However, unlike Prior Art 1, the methods of Prior Application Invention 1, Prior Application Invention 2, and Prior Application Invention 3 update the phase rotation angle used in step S02 and subsequent steps, and are therefore subject to the influence of temporal fluctuations in the phase rotation angle due to measurement noise. Prior Application Invention 1 and Prior Application Invention 2 select a single reference optical frequency at each point on the optical fiber, but if the optical intensity of the selected reference optical frequency is low, the estimation accuracy of the phase rotation angle deteriorates. Prior Application Invention 1 and Prior Application Invention 2 also perform time averaging within the window length Mt to improve the estimation accuracy of the phase rotation angle, but cannot completely eliminate the deterioration of the estimation accuracy of the phase rotation angle. Therefore, assuming that the optical intensities of optical frequencies other than the reference optical frequency are the same, the smaller the optical intensity of the selected reference optical frequency, the greater the noise-induced fluctuations contained in the temporal change in the phase rotation angle.

[0022] When a scattered light vector of an optical frequency other than the reference optical frequency is rotated using a phase rotation angle that includes large fluctuations, the direction of the scattered light vector after rotation also fluctuates significantly relative to the direction of an ideal scattered light vector that is free from the influence of noise. The scattered light vector with a fluctuating direction is added to the scattered light vector of the reference optical frequency, which has a low optical intensity (i.e., a short vector length), to calculate the optical frequency average vector, so the direction of the optical frequency average vector also fluctuates significantly. Therefore, the influence of this fluctuation is reflected in the oscillatory waveform calculated based on the angle of the optical frequency average vector, resulting in oscillatory waveform distortion. In the prior invention 3, the oscillatory waveform when each of the multiplexed optical frequencies is selected as the reference optical frequency is calculated and averaged, so the oscillatory waveform when a reference optical frequency with a low optical intensity is selected is also included in the average and is similarly affected by the temporal fluctuations of the phase rotation angle.

[0023] Patent Publication No. 2020-169904 (NTT, registered)

[0024] Ali. Masoudi, T. P. Newson, "Contributed Review: Distributed optical fiber dynamic strain sensing." Review of Scientific Instruments, vol. 87, p. 011501 (2016); Kenichi Nishiguchi, Cheolhyun Lee, Kuzik-Ata, Mitsunori Yokoyama, and Yoshimasu Masuda, "Prototype of a distributed acoustic wave sensor using optical fiber and its signal processing," IEICE Technical Report, 115 (202), pp. 29-34 (2015) Yoshifumi Wakisaka, Daisuke Iida, Hiroyuki Oshida, and Nazuki Honda, “Fading Suppression of Φ-OTDR With the New Signal Processing Methodology of Complex Vectors Across Time and Frequency Domains,” J. Lightwave Technology. 39, 4279-4293 (2021)

[0025] An object of the present invention is to reduce distortion of the vibration waveform due to the influence of noise-induced fluctuations in the phase rotation angle used to average frequency-multiplexed scattered light signals.

[0026] The phase OTDR system of the present invention comprises a measurement device that measures scattered light signals generated by frequency-multiplexed optical pulses reflected or scattered by an optical fiber, and a signal processing device that calculates a vibration waveform in the optical fiber using the scattered light signals measured by the measurement device. The signal processing device of the present invention executes a signal processing method for generating a final vibration waveform from the scattered light signals of multiple optical frequencies with reduced vibration waveform distortion due to fluctuations in the phase rotation angle.

[0027] The signal processing device of the present invention comprises a functional unit that selects a reference optical frequency from among a plurality of frequency-multiplexed optical frequencies, rotates the scattered light vectors of other optical frequencies among the scattered light vectors obtained from the measurement data of the plurality of optical frequencies using the reference optical frequency as a reference, and then averages the scattered light vectors of the plurality of optical frequencies, and in selecting the reference optical frequency, the functional unit dynamically tracks over time and selects an optical frequency with a greater intensity of the scattered light.

[0028] The functional unit also averages the scattered light vectors of the plurality of optical frequencies at different times using the scattered light vector of the selected reference optical frequency, and performs signal processing based on the change between the optical frequency average vectors at different times obtained by the averaging.Therefore, the signal processing device of the present invention includes the functional unit and acquires a vibration waveform based on the angular change of the optical frequency average vector obtained by the averaging.

[0029] Here, the signal processing device of the present invention provides a selection rule in which, when selecting the reference optical frequency, optical frequencies with low scattered light intensity and short vector lengths are excluded from the selection of the reference optical frequency, and only optical frequencies with high scattered light intensity and long vector lengths are included in the selection of the reference optical frequency. Because the scattered light intensity varies over time, the selection of whether to exclude or include an optical frequency in the selection of the reference optical frequency is dynamically updated as needed along the time axis at each point on the fiber.

[0030] Specifically, when the present invention is applied to a method of selecting one reference optical frequency at each point on a fiber, the optical frequency with the greatest measured scattered light intensity is dynamically selected as the reference optical frequency. For example, this corresponds to the case where the present invention is applied to Prior Application Invention 1 or Prior Application Invention 2. When the present invention is applied to a method of selecting and processing multiple reference optical frequencies at each point on a fiber, a selection rule is established, such as setting a threshold based on the magnitude of the noise in the measuring instrument, and including only optical frequencies equal to or greater than that threshold in the dynamic selection of the reference optical frequency. For example, this corresponds to the case where the present invention is applied to Prior Application Invention 3.

[0031] For example, as in Prior Invention 1 and Prior Invention 2, the functional unit may average the scattered light vectors of the plurality of optical frequencies using scattered light vectors included in a predetermined time window. In this case, the reference optical frequency may be selected using scattered light vectors included in the time window.

[0032] For example, as in Prior Invention 3, the functional unit may average scattered light vectors of the plurality of optical frequencies using two or more optical frequencies among the plurality of optical frequencies included in a predetermined time window as a reference. In this case, the reference optical frequency may be selected from scattered light vectors included in the time window.

[0033] In the present invention, when selecting the reference optical frequency, an optical frequency at which the scattered light intensity is greatest is selected, and therefore optical frequencies at which the scattered light intensity is small can be excluded from the selection of the reference optical frequency, thereby making it possible to reduce oscillatory waveform distortion due to the influence of noise-induced fluctuations in the phase rotation angle used to average the frequency-multiplexed scattered light signal.

[0034] 1 is a configuration example of a measurement system; FIG. 2 is an explanatory diagram of an optical pulse output from an optical modulator; FIG. 3 is a configuration example of a signal processing device; FIG. 4 is an example of processing by the signal processing device; FIG. 5 is an example of processing by the signal processing device; FIG. 6 is an example of processing by the signal processing device; FIG. 7 is an example of processing by the signal processing device;

[0035] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0036] (System Configuration) Fig. 1 shows an example configuration of a measurement system according to the present invention. The measurement system according to the present invention comprises a measurement device 31 and a signal processing device 17. The measurement device 31 is a phase OTDR that measures the phase of scattered light from each point in the optical fiber 6 under test. The phase OTDR of this embodiment uses frequency multiplexing to measure the phase at a plurality of different optical frequencies. In this embodiment, the phase OTDR outputs an in-phase component and a quadrature component, which are used by the signal processing device 17 to calculate a scattered light vector signal representing the phase, and the scattered light vector signal is used to calculate a vibration waveform. Specifically, the system has the following configuration:

[0037] The optical frequency from the CW light source 1 is f ０ A continuous light beam with a single wavelength of f is emitted and split into a reference light beam and a probe light beam by a coupler 2. The probe light beam is shaped into an optical pulse 4 shown in FIG. 2 by an optical modulator 3. The optical pulse 4 has an optical frequency of f ｉ = f ０ +Δf ｉ (i is an integer) and pulses whose pulse width is set to a value P corresponding to the spatial resolution of measurement in the longitudinal direction of the optical fiber are arranged in a sequence of i=1, 2, ..., N (N is an integer representing frequency multiplexing). ｉ is selected so that the scattered light intensities at each time and point are sufficiently separated so that they can be considered uncorrelated between different i. The pulse width P corresponds to the spatial resolution.

[0038] There is no specific designation for the type of optical modulator 3, and there may be more than one type as long as it can generate the optical pulse 4. For example, a single side band amplitude modulation (SSB) modulator or a frequency-tunable acousto-optics (AO) modulator may be used, and intensity modulation may be further performed using a semiconductor optical amplifier (SOA) or the like to increase the extinction ratio in pulsing.

[0039] The pulsed probe light 4 is incident on the optical fiber 6 under test via the circulator 5. Light scattered or reflected at each point in the longitudinal direction of the optical fiber 6 under test returns to the circulator 5 as backscattered light and is incident on one input terminal of the optical 90-degree hybrid 7. The reference light branched by the coupler 2 is incident on the other input terminal of the optical 90-degree hybrid 7.

[0040] The internal configuration of the optical 90-degree hybrid may be any configuration as long as it has the function of an optical 90-degree hybrid. Two of the four outputs of the optical 90-degree hybrid are detected by the balance detector 13, and the analog in-phase component I ａｎａｌｏｇ The remaining two outputs of the optical 90-degree hybrid are detected by a balanced detector 14, and an analog quadrature component Q ａｎａｌｏｇ The electric signals 15 and 16 are sent to a signal processing device 17 that includes AD conversion function elements 17a and 17b that are capable of sampling the frequency band of the signals without aliasing.

[0041] 3 shows an example of the configuration of the signal processing device 17. The signal processing device 17 is a device that calculates a vibration waveform using measurement data of scattered light that is generated when frequency-multiplexed optical pulses are reflected or scattered by an optical fiber. Specifically, the signal processing device 17 includes AD conversion function elements 17a and 17b, and signal processing units 17c, 17d, 17e, 17f, and 17g.

[0042] In the signal processing device 17, the digitized in-phase component signal I output from the AD conversion function element 17a and the AD conversion function element 17b isｄｉｇｉｔａｌ and the quadrature component signal Q ｄｉｇｉｔａｌ The signal processing unit 17c processes the signal of each optical frequency f constituting the optical pulse 4. ｉ Signal I due to scattered light from pulses (i = 1, 2, ..., N) ｉ and signal Q ｉ There are no specific restrictions on the signal processing method for separation, but for example, ｄｉｇｉｔａｌ and Q ｄｉｇｉｔａｌ The center frequency is Δf ｉ One possible approach is to pass the signal through a digital band-pass filter with a passband of 2 / P and then compensate for the phase delay. If the filter characteristics of the digital band-pass filter are set to specifications that allow streaming of the measurement data, taking into account the memory size of the computer being used, the measurement data can be streamed up to the signal processing unit 17c.

[0043] Signal processing units 17d, 17e, 17f, and 17g then perform phase calculations. Signal processing unit 17d is a functional unit that executes step S01, signal processing unit 17e is a functional unit that executes step S02, and signal processing units 17f and 17g are functional units that execute step S03. Specifically, signal processing unit 17d is a functional unit that selects a reference optical frequency from among the multiple frequency-multiplexed optical frequencies and calculates the phase rotation angles of scattered light vectors of other optical frequencies based on the reference optical frequency among the scattered light vectors obtained from the measurement data of the multiple optical frequencies. Signal processing unit 17e is a functional unit that rotates the phase rotation angles and then averages the scattered light vectors of the multiple optical frequencies to calculate an optical frequency average vector and calculates an angular change in the optical frequency average vector. Signal processing units 17f and 17g are functional units that use the angle change to calculate the phase difference between two points separated by the gauge length, perform phase unwrapping processing, and calculate a vibration waveform.

[0044] In the present disclosure, in order to exclude optical frequencies with low scattered light intensity from the selection of the reference optical frequency, optical frequencies with higher scattered light intensity are dynamically selected over time by tracking the optical frequencies with higher scattered light intensity. By using the present invention, while maintaining the feature obtained by updating the phase rotation angle that enables high-sensitivity measurement of large vibrations, etc., it is possible to reduce vibration waveform distortion due to the influence of angular fluctuations in the phase rotation angle resulting from measurement instrument noise, and to measure the vibration waveform more accurately.

[0045] Hereinafter, the pulse incidence interval, i.e., the vibration sampling rate, is defined as t, and the signal processing device 17 acquires measurement data at time kt using an integer k. Also, the distance from the incident end of the optical fiber 6 to be measured, at which the probe light 4 is incident, is defined as z. In other words, the signal processing unit 17d receives the signal I ｉ (kt, z) and signal Q ｉ (kt, z) is input.

[0046] (First embodiment) In this embodiment, an example in which the present invention is applied to prior invention 1 will be described. In prior invention 1, the processing from step S02 onwards is performed while continuously updating the optimum value of the phase rotation angle. An example of an embodiment in which the present invention is applied to prior invention 1 will be described.

[0047] In the prior art 1, the signal processing unit 17d also calculates the scattered light vector r ｉ (kt, z) to each optical frequency f ｉ The phase rotation angle α ｉ However, unlike Prior Art 1, the signal processing unit 17d of Prior Art 1 divides the measurement data into blocks of the number of time points, and calculates and updates the phase rotation angle for each block of measurement time Mt.

[0048] That is, if k=0 is set as the first point of the measurement data and the calculation of the phase rotation angle is started from the first point, for example, the scattered light vector r １The measurement data for (kt, z) is the first block, and k from M to 2M-1 is the second block. K is used as a number symbol to distinguish the blocks. For example, in the above example, k from 0 to M-1 is the block K=1, and k from M to 2M-1 is the block K=2. The Kth block is the measurement data for k from M(K-1) to MK-1. To calculate the phase rotation angle for each block, the value changes depending on the block K, and the phase rotation angle is expressed as α ｉ (z, K). The phase rotation angle α in each block ｉ The calculation method for (z, K) in the present invention is the same as step S01 in prior art 1, but the present invention adds a process for selecting a reference optical frequency.

[0049] The main points regarding the signal processing of the measurement data of the Kth block are explained below. A flowchart of the signal processing unit 17d is shown in Figure 4. When measurement starts, the measurement data obtained by the signal processing unit 17c is sent to the signal processing unit 17d.

[0050] Step S101-1: Signal I at time kt ｉ (kt, z) and signal Q ｉ (kt, z) to the scattered light vector r ｉ Calculate (kt, z). For example, if the imaginary unit is j, it can be calculated as follows: ｉ (kt, z) = I ｉ (kt, z)+j・Q ｉ (kt, z) (111) Calculated scattered light vector r ｉ The (kt, z) data is sequentially streamed to the signal processing unit 17e.

[0051] Step S101-2: Scattered light vector r in one time block ｉ The scattered light vector r ｉ Vector length |r of (kt, z) ｉ (kt, z) in the block. That is, in the case of the Kth block, ｉ (kt, z)| is calculated for k from M(K-1) to MK-1. The optical frequency with the largest calculated sum is selected as the reference optical frequency. Here, the reference optical frequency is f ｒｅｆLet the reference optical frequency f ｒｅｆ depends on the distance z from the entrance end and the time block K. When these dependencies need to be taken into account, ｒｅｆ (z, K) and f ｒｅｆ (z) and f ｒｅｆ This will be referred to as (K) below.

[0052] Step S101-3: For the data in the same block, the scattered light vector r of the reference optical frequency at each optical fiber point at time kt is calculated. ｒｅｆ Angle θ of (kt, z) ｒｅｆ Calculate (kt, z). For example, calculate θ ｒｅｆ (kt,z)=arg[r ｒｅｆ (kt, z)] (113)

[0053] Step S101-4: Scattered light vector r of each optical frequency at each optical fiber point at time kt within the time block ｉ (kt, z) is the angle -θ ｒｅｆ Rotate by (kt, z) and ｉ＿ｒｏｔ Let (kt, z). For example, calculate the following: r ｉ＿ｒｏｔ (kt, z)=exp[-jθ ｒｅｆ (kt, z)] × r ｉ (kt, z) (114) where j is the imaginary unit.

[0054] Step S101-5: Calculated r ｉ＿ｒｏｔ Calculate the sum vector within the time block of (kt, z). For example, calculate: r ｉ＿ａｖｅｔ (z, K) = Σr ｉ＿ｒｏｔ (kt, z) (115) Step S101-6: Obtained r ｉ＿ａｖｅｔ (z, K) is used to calculate the phase rotation angle α of each optical frequency using the measurement data of the Kth block. ｉ Calculate (z, K). The calculation formula can be exemplified as follows: α ｉ (z, K)=-arg[r ｉ＿ａｖｅｔ (z, K)] (116)

[0055] When step S101-6 is completed, α ｉ(z, K) is passed to the signal processing unit 17e. The signal processing unit 17e then passes the phase rotation angle α calculated by the signal processing unit 17d. ｉ Using each optical frequency f ｉ Scattered light vector r ｉ is rotated and averaged to calculate the light frequency average vector. The phase rotation angle α ｉ is the phase rotation angle α calculated by the signal processing unit 17d in the previous block K-1. ｉ That is, the detailed processing of the signal processing unit 17e for the Kth block is as follows:

[0056] Step S102-1: The signal processing unit 17e calculates the scattered light vector r of each optical frequency at time kt. ｉ (kt, z) is the phase rotation angle α calculated in the (K-1)th block. ｉ Rotate by (z, K-1) and the rotated vector R ｉ Calculate (kt, z). The calculation method can be exemplified as follows: R ｉ (kt, z)=exp(j・α ｉ (z, K-1))・r ｉ (kt,z) (121)

[0057] Step S102-2: The signal processing unit 17e calculates the rotated vector R ｉ (kt, z) is frequency-averaged to calculate a vector. Note that the final result is the same whether it is vector averaging or vector synthesis (simple vector sum), so in the actual calculation procedure, vector synthesis is performed to obtain the optical frequency average vector R ａｖｅｆ Let (kt, z) be the coordinate system. The calculation method is as follows.

[0058] Step S102-3: Optical frequency average vector R ａｖｅｆ Angle θ of (kt, z) ａｖｅｆ (kt, z) is calculated and passed to the signal processing unit 17f at the subsequent stage. The calculation method is as follows. ａｖｅｆ (kt,z)=arg[R ａｖｅｆ (kt, z)] (123)

[0059] In the first block of K=1, since there is no data of the previous block, the phase rotation angle is not yet acquired, and the procedure of the signal processing unit 17e cannot be performed. Therefore, the measurement time of the first Mt is a preliminary measurement, and the processing after the signal processing unit 17e is not performed. Alternatively, ｉ It is also possible to separately store (kt, z) in a computer and calculate the vibration waveform using prior art 1.

[0060] The signal processing unit 17f at the subsequent stage calculates the angle θ of the obtained optical frequency average vector. ａｖｅｆ Using the change in (kt, z), the phase difference between two points separated by the gauge length is calculated, and a phase unwrapping process is performed to calculate the vibration waveform. The specific calculation method can be the same as the method for calculating the vibration waveform from the phase in a general phase OTDR.

[0061] However, the angle θ between adjacent blocks ａｖｅｆ If the reference optical frequency used to calculate (kt, z) is different, the calculated value of the vibration change from the last time of the earliest block among the blocks to the first time of the next block will include a difference in the phase offset of the reference optical frequency, and the correct value will not be reflected. For example, the optical frequency that is the reference for the phase rotation angle used in the signal processing unit 17e of block K is f selected in step S101-2 using the data of block K-1. ｒｅｆ (K-1), and the optical frequency that is the reference for the phase rotation angle used in the signal processing unit 17e of block K+1 is f selected in step S101-2 using the data of block K. ｒｅｆ (K). That is, the angle θ of block K ａｖｅｆ The phase offset of (kt, z) is the reference optical frequency f ｒｅｆ (K-1) is the reference, and the angle θ of block K+1 ａｖｅｆ The phase offset of (kt, z) is the reference optical frequency f ｒｅｆ Since (K) is used as the reference, the reference optical frequency f ｒｅｆ (K-1) and f ｒｅｆ If (K) is different, the time kt is the angle θ from MK-1 to MK ａｖｅｆThe time variation of includes the effect of the difference in phase offset between different optical frequencies.

[0062] To avoid this effect, a conceivable method is to set the time length of each block to (M+1)t, share data points between adjacent blocks, and perform correction at those data points. For example, block K is set to include (M+1)t pieces of data where k ranges from M(K-1) to MK, and block K+1 is set to include (M+1)t pieces of data where k ranges from MK to (M+1)K. By setting block boundaries in this way, the data point kt=MKt is included in both the Kth block and the K+1th block. Therefore, if the signal processing units 17d and 17e are set in the same way except for setting the block length, the angle θ at time kt=MKt can be calculated. ａｖｅｆ (MKt, z) to obtain two types of data: the last data of the Kth block and the first data of the K+1th block. ａｖｅｆ (MKt, z, K), and the latter is defined as θ ａｖｅｆ (MKt, z, K+1) to distinguish it. ａｖｅｆ (MKt,z,K+1)-θ ａｖｅｆ (MKt, z, K) is a value that reflects the difference in the offset of the reference optical frequency. Therefore, all angles θ included in the K+1th block ａｖｅｆ (kt, z) as θ ａｖｅｆ (kt, z)-[θ ａｖｅｆ (MKt,z,K+1)-θ ａｖｅｆ (MKt, z, K)], the influence of the difference in phase offset due to the difference in reference optical frequency can be reduced.

[0063] In this embodiment, the processing of the K+1th block has been described, but by performing the same processing starting with the smallest block, it is possible to reduce the above-mentioned effect at all times. Note that this method of setting the time length of each block to (M+1)t, sharing a data point with adjacent blocks, and performing correction at that data point also has the effect of compensating for the drawback of Prior Invention 1, namely, phase discontinuity caused by discontinuity in the phase rotation angle, even if the reference optical frequency is the same in adjacent blocks.

[0064] In this embodiment, the signal processing unit 17e calculates the angle θ ａｖｅｆ Although the value of (kt, z) itself is transferred to the signal processing unit 17f, the signal processing unit 17e may transfer the time difference of the angle expressed by the following equation, and the time difference of the angle may be processed by the signal processing unit 17f. ａｖｅｆ ((k+1)t,z)-θ ａｖｅｆ (kt, z)

[0065] In this case, in order to reduce the influence of the difference in phase offset between different optical frequencies, the signal processing unit 17f calculates the angle change of the optical frequency mean vector caused by the change in time from (MK-1)t to MKt using the following equation: θ ａｖｅｆ (MKt,z,K)-θ ａｖｅｆ ((MK-1)t,z,K)

[0066] Furthermore, while the procedure of Prior Invention 1 allows for streaming processing of data, the addition of procedure S101-2 according to the present invention creates a new need to store all scattered light data within one block. If it is desired to maintain the streaming processing characteristics of Prior Invention 1, it is possible to devise a method such as limiting the data points used to add up the vector lengths in procedure S101-2 to the first few points of each block that can be stored in the memory of the computer.

[0067] The present invention is characterized by step S101-2. In the prior invention 1, signal processing unit 17d divides the measurement data into blocks of M time points and calculates and updates the phase rotation angle for each block of measurement time Mt, thereby coping with temporal changes in the optimal value due to changes in optical characteristics such as the laser oscillation frequency, changes in the temperature of the optical fiber under test itself, and the application of large dynamic strain to the optical fiber under test. By making the measurement time Mt per block short relative to the time scale of changes in optical characteristics such as the laser oscillation frequency, changes in the temperature of the optical fiber under test itself, and the application of large dynamic strain to the optical fiber under test, it is possible to update the phase rotation angle with sufficient precision, and there is no problem in using the phase rotation angle calculated by signal processing unit 17d in block K-1 as the phase rotation angle to be used when signal processing unit 17e calculates the frequency average of the scattered light vectors belonging to block K.

[0068] According to Prior Invention 1, setting M to approximately 100 ensures sufficient accuracy of the phase rotation angle, and if the pulse transmission period t is 1 ms, Mt is 100 ms or less, which is sufficient to handle cases where the time scale of the optimal phase rotation angle change is 1 s or more. The order of this set value for Mt is approximately the same as the order of the time width over which the magnitude relationship of the scattered light intensity of each optical frequency can be considered constant within each block. Therefore, by applying the present invention in step S101-2, selecting a reference optical frequency from the (K-1)th block, calculating the phase rotation angle, and using it in processing the frequency averaging of the Kth block, it is possible to reduce vibration waveform distortion due to the influence of angular fluctuations in the phase rotation angle resulting from measuring instrument noise.

[0069] However, even if the length of the time block length used to select the reference optical frequency and the length of the time block length used to update the optimal phase rotation angle used for frequency averaging are not the same, it is possible to implement the present invention in the same manner as this embodiment, as long as the intent of the present invention is not changed.

[0070] (Second Embodiment) In this embodiment, an example will be described in which the present invention is applied to Prior Invention 2. In Prior Invention 2, a time window of window length Mt (M is a natural number corresponding to the number of data points) used in signal processing is set in advance, and then the time window is moved while calculating a phase rotation angle using the measurement data included in the time window, and this phase rotation angle is used to average the scattered light vectors of the frequency components at the center of the time window or at a time positioned a predetermined number of measurement data points later. The window length Mt may be set in the same way as the block length in the first embodiment.

[0071] The outline of the method of applying the present invention to Prior Invention 2 is the same as the method of applying the present invention to Prior Invention 1, but in Prior Invention 2, the phase rotation angle is continuously updated at each time, and by applying the present invention, the reference optical frequency may also be switched at any time. Therefore, it is necessary to continuously correct the difference in phase offset due to the difference in reference optical frequency, which is different from the case of applying to Prior Invention 1.

[0072] In the signal processing unit 17d, the scattered light vector r ｉ (kt, z) is calculated and continuously streamed to the signal processing unit 17e. Also, using the scattered light vector from time (k-W)t to (k+W)t, each optical frequency f ｉ The phase rotation angle α ｉ (kt, z) is calculated. That is, the window length Mt satisfies Mt = (2W + 1)t. The calculated phase rotation angle α ｉ (kt,z) is continuously streamed to the signal processing unit 17e.

[0073] In the present invention, a process for dynamically and continuously updating the selection of the reference optical frequency is added. In addition, in order to continuously correct the difference in phase offset caused by dynamically updating the reference optical frequency, the phase rotation angle α ｉ (kt, z) is used by the signal processing unit 17e not only for the time kt but also for the frequency averaging at the time (k+1)t, and the time change of the phase is transmitted to the signal processing unit 17f.

[0074] First, a specific example of the calculation procedure of the signal processing unit 17d will be described below. Assume that measurement starts from k=0. The flowchart of the signal processing unit 17d is shown in FIG. 5. Step S201-1: Signal I at time kt ｉ (kt, z) and signal Q ｉ (kt, z) to the scattered light vector r ｉ Calculate (kt, z). For example, if the imaginary unit is j, it can be calculated as follows: ｉ (kt, z) = I ｉ (kt, z)+j・Q ｉ (kt, z) (211) The calculated scattered light vector data is sequentially streamed to the signal processing unit 17e.

[0075] Step S201-2: Scattered light vector length |r from time (k−2W)t to time kt ｉ (kt, z)| is summed, and the optical frequency with the largest sum is the reference optical frequency f ｒｅｆ The reference optical frequency f ｒｅｆ depends on the distance z from the incident end and the time kt. If these dependencies need to be taken into account, ｒｅｆ (kt, z) and fｒｅｆ (z) and f ｒｅｆ (kt) below.

[0076] Step S201-3: The reference optical frequency f at each optical fiber point at time kt ｒｅｆ Scattered light vector r of (kt, z) ｒｅｆ Angle θ of (kt, z) ｒｅｆ Calculate (kt, z). For example, it can be calculated as follows: θ ｒｅｆ (kt,z)=arg[r ｒｅｆ (kt, z)] (213)

[0077] Step S201-4: Scattered light vector r of each optical frequency at each optical fiber point at time kt ｉ (kt, z) is the angle -θ ｒｅｆ Rotate by (kt, z) and ｉ＿ｒｏｔ (kt, z). For example, it can be calculated as follows: r ｉ＿ｒｏｔ (kt, z)=exp[-jθ ｒｅｆ (kt, z)] × r ｉ (kt,z) (214)

[0078] Step S201-5: r from time (k-2W)t to time kt ｉ＿ｒｏｔ (kt, z) and calculate the time-averaged vector r ｉ＿ａｖｅｔ Calculate ((k-W)t,z). The argument time of the time average vector is the center time of the time window. Also, since the final result is the same whether it is an average or a simple addition, calculate the time average vector as an addition as follows: However, by measuring from kt = 0 to kt = 2Wt, data of a window length of (2W + 1)t or more can be obtained, so r ｉ＿ａｖｅｔ If kt in (kt, z) is less than Wt, calculation is not possible. Therefore, measurement data where the time kt is less than Wt can be used as a preliminary measurement, or the time average vector can be aligned with k=W. In the latter case, for k<W, the following equation is used: ｉ＿ａｖｅｔ (kt, z) = r ｉ＿ａｖｅｔ (Wt, z) (215-2)

[0079] Step S201-6: Calculated time-averaged vector r ｉ＿ａｖｅｔUsing the angle ((k−W)t,z), the phase rotation angle α ｉ Calculate ((k-W)t,z). The calculation formula can be exemplified as follows: α ｉ ((kW)t,z)=-arg[r ｉ＿ａｖｅｔ ((k−W)t,z)] (216) Calculated phase rotation angle α ｉ ((kW)t,z) are sequentially streamed to the signal processing unit 17e.

[0080] The signal processing unit 17e converts the phase rotation angle α calculated by the signal processing unit 17d into ｉ Using each optical frequency f ｉ Scattered light vector r ｉ is rotated and averaged to calculate the optical frequency mean vector. In the prior invention 2, the angle of the optical frequency mean vector is sent directly to the signal processing unit 17f, but in the present invention, as a measure to deal with the dynamic replacement of the reference optical frequency, the change in angle (phase) at adjacent times over time is calculated and then sent to the signal processing unit 17f. A specific example of the calculation procedure is described below. The calculation procedure is shown in Figure 6.

[0081] Step S202-1: The signal processing unit 17e calculates the scattered light vector r streamed in step S201-1. ｉ (kt, z) and r ｉ ((k+1)t,z) and the phase rotation angle α streamed in step S201-6. ｉ Rotate by (kt, z) and the rotated vector R ｉ (kt, z) and R ｉ Calculate ((k+1)t,z). The calculation method can be exemplified as follows: R ｉ (kt, z)=exp(j・α ｉ (kt, z)) r ｉ (kt, z) (221-1) R ｉ ((k+1)t,z)=exp(j・α ｉ (kt, z)) r ｉ ((k+1)t,z) (221-2)

[0082] Incidentally, the optical frequency mean vector R ｉ ((k+1)t,z) and R ｉ The feature of this method is that the phase rotation angle calculated using the same reference optical frequency is used to calculate (kt, z). However, it is not always necessary to use the same phase rotation angle. Therefore, the reference optical frequency f ｒｅｆ Using the measurement data from time (k-W+1)t to time (k+W+1)t based on (kt, z), the scattered light vector r ｉ The phase rotation angle for rotating ((k+1)t,z) is calculated separately, and the α of (221-2) is calculated. ｉ (kt, z) may be used instead. In that case, f ｒｅｆ Using the measurement data from time (k-W+1)t to time (k+W+1)t based on (kt, z), ｉ The calculation method for the phase rotation angle for rotating ((k+1)t,z) is from step S201-2 to step S201-6, and the reference optical frequency is f ｒｅｆ This method keeps the data (kt, z) and replaces the measurement data to be calculated with the measurement data from time (k-W+1)t to time (k+W+1)t, rather than the measurement data from time (k-W)t to time (k+W)t.

[0083] Step S202-2: The signal processing unit 17e calculates the rotated vector R ｉ (kt, z) is frequency-averaged to calculate a vector. Note that the final result is the same whether it is vector averaging or vector synthesis (simple vector sum), so in the actual calculation procedure, vector synthesis is performed to obtain the optical frequency average vector R ａｖｅｆ Let (kt, z). The calculation method can be exemplified as follows.

[0084] Step S202-3: The signal processing unit 17e calculates the optical frequency average vector R ａｖｅｆ Angle θ of (kt, z) ａｖｅｆ (kt, z), and the optical frequency mean vector R ａｖｅｆ Angle θ of ((k+1)t,z) ａｖｅｆ ((k+1)t,z) is calculated, and the change θ ａｖｅｆ ((k+1)t,z)-θ ａｖｅｆ(kt, z) is passed to the signal processing unit 17f at the subsequent stage. ａｖｅｆ The calculation method of θ can be exemplified as follows: ａｖｅｆ (kt,z)=arg[R ａｖｅｆ (kt, z)] (223-1) θ ａｖｅｆ ((k+1)t,z)=arg[R ａｖｅｆ ((k+1)t,z)] (223-2)

[0085] Furthermore, the signal processing unit 17f calculates the time-dependent change θ of the phase obtained by the signal processing unit 17e. ａｖｅｆ ((k+1)t,z)-θ ａｖｅｆ Using (kt, z), the spatial difference in phase between two points separated by the gauge length is calculated, and then phase unwrapping processing is performed to calculate the vibration waveform.Specific calculation methods that can be used are the same as those used in general DAS-P.

[0086] In this embodiment, the length of the time window used to select the reference optical frequency is the same as the length of the time window used to update the optimal phase rotation angle used for frequency averaging. However, even if they are not the same, the present invention can be implemented in the same manner as this embodiment, as long as the gist of the present invention is not changed.

[0087] As in the prior invention 2, the phase rotation angle α ｉ ((k-W)t, z) can also be used for frequency averaging at times (k-W+Delay)t and (k-W+Delay+1)t after time (k-W)t (the variable Delay is an integer that reflects the amount of delay). In other words, a phase rotation angle calculated using measurement data within a time window with a window length of 2W+1 can be used for frequency averaging at times after the center time of the time window. The greater the temporal difference amount Delay, the greater the optimal phase rotation angle α at time (k-W+Delay)t. ｉ ((k-W+Delay)t,z) and the phase rotation angle α ｉ Since the difference in ((k−W)t, z) also becomes large, it is desirable to keep it to the minimum value determined by memory capacity, etc.

[0088] Third Embodiment In this embodiment, an example will be described in which the present invention is applied to the prior invention 3. The roles of the signal processing units 17d to 17g are as follows.

[0089] (Signal Processing Unit 17d) A specific example of the calculation procedure is described below. Assume that measurement starts from k=0. The flowchart of the signal processing unit 17d is also shown in FIG. 7. Step S301-1: Signal I ｉ (kt, z) and signal Q ｉ (kt, z) to the scattered light vector r ｉ Calculate (kt, z). The calculation formula can be exemplified as follows: r ｉ (kt, z) = I ｉ (kt, z)+j・Q ｉ (kt, z) (311) Calculated scattered light vector r ｉ The (kt,z) data is continuously streamed to the signal processing unit 17e.

[0090] Step S301-2: The vector length |r of the scattered light vector from time (k−2W)t to time kt ｉ (kt, z) | ｉ Calculate the index (kt, z), where the window length Mt satisfies Mt = (2W + 1)t. L ｉ (kt, z) is the threshold L Ｔ The larger the optical frequency f ｉ Let X be the set of X. Note that X depends on the position of the time window, so sets X calculated using time windows centered at different times are not necessarily the same. In the following procedure, an element of X is selected as the reference optical frequency, which is a feature of the present invention. Ｔ The set value of depends on the required specifications of the measurement. For example, the magnitude of the noise of the measuring instrument is set to the threshold L Ｔ You can set it to.

[0091] Step S301-3: For scattered light vectors from time (k-2W)t to kt, each light frequency f included in set X is calculated. ｉ Scattered light vector r ｉ Angle θ of (kt, z) ｉ Calculate (kt, z). For example, it can be calculated as follows: θ ｉ(kt,z)=arg[r ｉ (kt, z)] (313)

[0092] Step S301-4: For the scattered light vector from time (k-2W)t to kt, ｉ Scattered light vector r ｉ (kt, z) for each optical frequency f ｘ is the reference optical frequency, the phase rotation angle -θ ｘ Rotate by (kt, z) ｉ＿ｒｏｔ，ｘ Calculate (kt, z). For example, it can be calculated as follows: r ｉ＿ｒｏｔ，ｘ (kt, z)=exp[-jθ ｘ (kt, z)] × r ｉ (kt, z) (314) In the above equation, the combination (i, x) of i and x calculates only the optical frequencies belonging to the set X for x, and all possible cases for i.

[0093] Step S301-5: For data from time (k-2W)t to kt, the reference optical frequency is f ｘ When ｉ＿ｒｏｔ，ｘ By adding ｉ＿ａｖｅｔ，ｘ Calculate ((k−W)t, z). The calculation formula can be exemplified as follows:

[0094] However, by measuring from kt = 0 to kt = 2Wt, data of a window length of (2W + 1)t or more can be obtained, so r ｉ＿ａｖｅｔ，ｘ If kt in (kt, z) is less than Wt, calculation is not possible. Therefore, measurement data where the time kt is less than Wt can be used as a preliminary measurement, or the time average vector can be aligned with k=W. In the latter case, for k<W, the following equation is used: ｉ＿ａｖｅｔ，ｘ (kt, z) = r ｉ＿ａｖｅｔ，ｘ (Wt, z) (315-2) In the above equation, the combination of i and x (i, x) calculates only the optical frequencies that belong to the set X for x, and all possible cases for i.

[0095] Step S301-6: Time average vector r ｉ＿ａｖｅｔ，ｘ ((kW)t,z) to each optical frequency fｘ When the reference optical frequency is f ｉ The phase rotation angle α ｉ，ｘ ((k−W)t,z) is calculated and streamed to the signal processing unit 17e. The calculation formula can be exemplified as follows: α ｉ，ｘ ((kW)t,z)=-arg[r ｉ＿ａｖｅｔ，ｘ ((k−W)t,z)] (316) In the above equation, for the combination of i and x (i, x), only optical frequencies belonging to the set X are calculated for x, and all possible cases for i are calculated.

[0096] 8 shows a flowchart of the process executed by the signal processors 17e, 17f, and 17g. (Signal processor 17e) The signal processor 17e calculates the phase rotation angle α calculated by the signal processor 17d. ｉ，ｘ Using each optical frequency f ｉ Scattered light vector r ｉ is rotated and averaged to calculate the optical frequency mean vector. ｘ Specifically, the calculation is performed as follows: Step S302-1: The scattered light vector r streamed in Step S301-1 is calculated. ｉ (kt, z) and r ｉ ((k+1)t,z) and the phase rotation angle α streamed in step S301-6. ｉ，ｘ (kt, z) and the reference optical frequency becomes f ｘ The vector R after rotation ｉ，ｘ Calculate (kt, z). The calculation method can be exemplified as follows: R ｉ，ｘ (kt, z)=exp(j・α ｉ，ｘ (kt, z)) r ｉ (kt, z) (321-1) R ｉ，ｘ ((k+1)t,z)=exp(j・α ｉ，ｘ (kt, z)) r ｉ ((k+1)t,z) (321-2)

[0097] Furthermore, R ｉ，ｘ((k+1)t,z) and R ｉ，ｘ The same reference optical frequency f is used to calculate (kt, z). ｘ However, it is not necessary to use the same phase rotation angle. ｘ Based on this, the measurement data from time (k-W+1)t to time (k+W+1)t is used to calculate r ｉ，ｘ The phase rotation angle for rotating ((k+1)t, z) is calculated separately and then α in equation (321-2) is used. ｉ，ｘ (kt, z) may be used instead. In that case, f ｘ Using the measurement data from time (k-W+1)t to time (k+W+1)t based on ｉ，ｘ The phase rotation angle for rotating ((k+1)t, z) can be calculated using the same method as that described in the prior application 3.

[0098] Step S302-2: Vector R after rotation ｉ，ｘ (kt, z) is frequency-averaged to calculate a vector. Note that the final result is the same whether it is vector averaging or vector synthesis (simple vector sum), so in the actual calculation procedure, vector synthesis is performed to obtain the optical frequency average vector R ａｖｅｆ，ｘ Let (kt, z). The calculation method can be exemplified as follows.

[0099] Step S302-3: The reference optical frequency is f ｘ When ａｖｅｆ，ｘ Angle θ of (kt, z) ａｖｅｆ，ｘ (kt, z), and the optical frequency mean vector R ａｖｅｆ，ｘ Angle θ of ((k+1)t,z) ａｖｅｆ，ｘ ((k+1)t,z) is calculated, and the change over time θ ａｖｅｆ，ｘ ((k+1)t,z)-θ ａｖｅｆ，ｘ (kt, z) is passed to the signal processing unit 17f. ａｖｅｆ The calculation method of θ can be exemplified as follows: ａｖｅｆ，ｘ (kt,z)=arg[R ａｖｅｆ，ｘ (kt, z)] (323-1) θ ａｖｅｆ，ｘ ((k+1)t,z)=arg[R ａｖｅｆ，ｘ((k+1)t,z)] (323-2) Each optical frequency belonging to the set X is set to the reference optical frequency f ｘ Calculate as follows.

[0100] (Signal Processing Unit 17f) Step S303: The signal processing unit 17f receives the reference optical frequency f ｘ Angle θ ａｖｅｆ，ｘ The change in θ over time ａｖｅｆ，ｘ ((k+1)t,z)-θ ａｖｅｆ，ｘ Using (kt, z), the phase difference between two points separated by the gauge length is calculated. If the magnitude exceeds π, an integer multiple of 2π is added to correct it so that it is less than π. This operation corresponds to the phase unwrapping process. The final value obtained is calculated by multiplying the reference optical frequency by f ｘ The vibration waveform ψ when ｘ The change in (kt, z) over time ψ ｘ ((k+1)t,z)−ψ ｘ (kt, z). Note that the specific calculation method can be the same as the method for calculating the vibration waveform from the phase in a general phase OTDR. ｘ The calculation is performed as follows: Vibration waveform ψ ｘ The change in (kt, z) over time ψ ｘ ((k+1)t,z)−ψ ｘ (kt,z) is streamed to the signal processing unit 17g.

[0101] (Signal Processing Unit 17g) Step S304: The signal processing unit 17g calculates the time-dependent change ψ ｘ ((k+1)t,z)−ψ ｘ (kt, z) is averaged with respect to x, and the change over time of the final vibration waveform ψ(kt, z) is calculated as ψ((k+1)t, z) - ψ(kt, z). The change over time is cumulatively added to obtain the final vibration waveform ψ(kt, z). The offset value at the start time of cumulative addition can be set in the same way as the offset setting for vibration waveforms in a general phase OTDR.

[0102] In the present invention, the optical frequency that serves as the reference optical frequency is selected based on the scattered light intensity in step S301-2, and the phase rotation angle αｉ This can reduce vibration waveform distortion due to the influence of angle fluctuations caused by noise. Also, in the case of this embodiment, the combination of optical frequencies selected as the reference optical frequency can change dynamically, so there is a possibility that a difference in phase offset will have an effect when the reference optical frequency is different. To address this problem, in this embodiment, the signal processing unit 17f and the signal processing unit 17g adjust the angle θ ａｖｅｆ，ｘ and vibration waveform ψ ｘ In order to perform such a calculation based on the change over time, in this embodiment, the phase rotation angle α ｉ，ｘ Using (kt, z), the angle θ ａｖｅｆ，ｘ (kt, z) and angle θ ａｖｅｆ，ｘ ((k+1)t, z) is calculated. This is the angle θ ａｖｅｆ，ｘ is the phase rotation angle α ｉ，ｘ (kt, z) and phase rotation angle α ｉ，ｘ This means that the calculation is performed using two of ((k-1)t, z), and the fact that the calculation is performed twice is a feature of this method.

[0103] Fourth Embodiment In this embodiment, referring to FIG. 9, in step S302-1 of the third embodiment, the signal processing unit 17d calculates the reference optical frequency f ｘ Based on this, the phase rotation angle α for all possible (i, x) ｉ，ｘ The calculation method for calculating is explained below.

[0104] Step S401-1: Signal I ｉ (kt, z) and signal Q ｉ (kt, z) to the scattered light vector r ｉ Calculate (kt, z). For example, if the imaginary unit is j, it can be calculated as follows: ｉ (kt, z) = I ｉ (kt, z)+j・Q ｉ (kt, z) (411) The calculated scattered light vector data is sequentially streamed to the signal processing unit 17e.

[0105] Step S401-2: Each optical frequency f at each optical fiber point at time kt ｉ Scattered light vector r ｉAngle θ of (kt, z) ｉ Calculate (kt, z). For example, it can be calculated as follows: θ ｉ (kt,z)=arg[r ｉ (kt, z)] (412)

[0106] Step S401-3: Each optical frequency f at each optical fiber point at time kt ｉ Scattered light vector r ｉ (kt, z) for each optical frequency f ｘ is the reference optical frequency, the phase rotation angle -θ ｘ Rotate by (kt, z) ｉ＿ｒｏｔ，ｘ Calculate (kt, z). For example, it can be calculated as follows: r ｉ＿ｒｏｔ，ｘ (kt, z)=exp[-jθ ｘ (kt, z)] × r ｉ (kt, z) (413) Calculate all possible cases for the combination (i, x).

[0107] Step S401-4: The reference optical frequency calculated using the data from time (k-2W-1)t to time (k-1)t is f ｘ The time average vector r ｉ＿ｒｏｔ，ｘ (kt, z) and r at time (k-2W-1)t ｉ＿ｒｏｔ，ｘ ((k-2W-1)t,z) and r at time kt ｉ＿ｒｏｔ，ｘ (kt, z) and the reference optical frequency is f ｘ The time average vector r ｉ＿ａｖｅｔ，ｘ Calculate ((k-W)t,z). The calculation formula can be exemplified as follows: r ｉ＿ａｖｅｔ，ｘ ((kW)t,z)=r ｉ＿ａｖｅｔ，ｘ ((kW-1)t,z) -r ｉ＿ｒｏｔ，ｘ ((k-2W-1)t,z)+r ｉ＿ｒｏｔ，ｘ (kt,z) (414)

[0108] However, by measuring from k = 0 to k = 2W, data of a time window of a window length (2W + 1)t or more can be obtained. Therefore, in the case of k = 2W, the time average vector is calculated as r up to k ≦ 2W. ｉ＿ｒｏｔ，ｘIt is obtained as the addition of (kt, z). The calculation formula can be exemplified as follows:

[0109] For data of k<2W, a preliminary measurement is performed or the time average vector is calculated using the conventional technique 1. When the conventional technique 1 is used, the time average vectors for all times of k<2W can be aligned with the time average vector for k=2W. In other words, the following equation is established. r ｉ＿ａｖｅｔ，ｘ (kt, z) = r ｉ＿ａｖｅｔ，ｘ (Wt, z) (416) Calculate all possible cases for the combination (i, x).

[0110] Steps S401-5 and S401-6: Time average vector r ｉ＿ａｖｅｔ，ｘ ((kW)t,z) to each optical frequency f ｘ When the reference optical frequency is f ｉ The phase rotation angle α ｉ，ｘ ((kW)t, z) is calculated for all possible (i, x) and streamed to the signal processing unit 17e.

[0111] (Other Embodiments) In the above embodiment, the vibration waveforms that have been subjected to the phase unwrapping process are averaged in the processing by the signal processing units 17f and 17g. However, the order of the specific calculation steps can be changed within the scope that does not impair the essence of the present invention, for example, by performing averaging before performing the phase unwrapping process and then performing the phase unwrapping process.

[0112] In this embodiment, the length of the time window used to select the reference optical frequency is the same as the length of the time window used to update the optimal phase rotation angle used for frequency averaging. However, even if they are not the same, the present invention can be implemented in the same manner as this embodiment, as long as the gist of the present invention is not changed.

[0113] Note that this embodiment describes a specific implementation procedure for further applying the present invention when Prior Invention 3 is implemented based on Prior Invention 2. However, Prior Invention 3 is not limited to those based on Prior Invention 2, and can be widely applied to cases based on Prior Invention 1 or other methods of averaging frequency-multiplexed signals in a vector state, and the present invention can be applied to each of these calculation methods.

[0114] The signal processing device of the present disclosure 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. The program of the present disclosure is a program for causing a computer to realize each functional unit of the signal processing device according to the present disclosure, and is a program for causing a computer to execute each step of a method executed by the signal processing device according to the present disclosure.

[0115] 1: CW light source 2: Coupler 3: Optical modulator 4: Optical pulse 5: Circulator 6: Optical fiber to be measured 7: 90-degree optical hybrid 13, 14: Balanced detector 15, 16: Electrical signal 17: Signal processing device 17a, 17b: AD conversion function element 17c, 17d, 17e, 17f, 17g: Signal processing unit 31: Measuring device

Claims

1. An apparatus comprising a signal processing device which calculates an oscillatory waveform using measurement data of scattered light resulting from reflection or scattering of frequency-multiplexed optical pulses in an optical fiber, said signal processing device comprising a functional unit which selects a reference optical frequency from among a plurality of frequency-multiplexed optical frequencies, rotates scattered light vectors of other optical frequencies among the scattered light vectors obtained from the measurement data of the plurality of optical frequencies using the reference optical frequency as a reference, and then averages the scattered light vectors of the plurality of optical frequencies, and in selecting the reference optical frequency, dynamically selects an optical frequency having the greatest intensity of the scattered light by tracking it over time.

2. The device described in claim 1, wherein the functional unit averages the scattered light vectors of the multiple optical frequencies at different times using the scattered light vector of the selected reference optical frequency, and performs signal processing based on the change between the optical frequency average vectors at different times obtained by the averaging.

3. The device according to claim 1, wherein the functional unit averages the scattered light vectors of the multiple optical frequencies with scattered light vectors that fall within a predetermined time window, and the selection of the reference optical frequency is performed with respect to scattered light vectors that fall within the time window.

4. The device according to claim 1, wherein the functional unit averages the scattered light vectors of the multiple optical frequencies using two or more of the multiple optical frequencies included in a predetermined time window as a reference, and the reference optical frequency is selected from the scattered light vectors included in the time window.

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