Vibration measuring device and vibration measuring method

Abstract

The purpose of the present disclosure is to reduce crosstalk between different optical frequency components, and reduce erroneous detection of vibration and distortion of a vibration waveform.　The present disclosure relates to a vibration measurement device that causes a plurality of light pulses having different optical frequencies to repeatedly enter an optical fiber, thereby performing distributed acoustic sensing-phase (DAS-P). A light pulse having a smaller spectrum side lobe waveform as compared to a rectangular wave is used for each of the plurality of light pulses.

Description

[Technical field]

[0001] The present disclosure relates to a vibration measurement device and a vibration measurement method for performing DAS-P. [Background technology]

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

[0003] DAS detects the change in the optical path length of the optical fiber caused by physical vibration applied to the optical fiber, and senses the vibration. By detecting the vibration, it is possible to detect the movement of objects around the optical fiber under test.

[0004] As a method for detecting backscattered light in DAS, there is a method for measuring the scattered light intensity from each point of the measured optical fiber and observing the time change of the scattered light intensity, which is called DAS-I (DAS-intensity). Although DAS-I has the characteristic that the device configuration can be simplified, it is a qualitative measurement method because it is not possible to quantitatively calculate the change in the optical path length of the fiber due to vibration from the scattered light intensity (Non-Patent Document 2).

[0005] Meanwhile, DAS-P (DAS-phase), a method for determining the phase of scattered light from each point of the measured optical fiber and observing the time change of the phase, is also being researched and developed. Although DAS-P has a more complicated device configuration and signal processing than DAS-I, the phase changes linearly with respect to the change in the optical path length of the fiber due to vibration, and the rate of change is the same in the longitudinal direction of the optical fiber, making it possible to quantitatively measure vibration and faithfully reproduce the vibration applied to the measured optical fiber (for example, Non-Patent Document 2).

[0006] In measurements using the DAS-P, pulsed light is injected into the optical fiber under test, and the phase of the scattered light at time t when the pulsed light is injected is measured in a distributed manner along the longitudinal direction of the optical fiber. In other words, the phase θ(l, t) of the scattered light is measured as distance l from the input end of the optical fiber. By repeatedly injecting pulsed light into the optical fiber under test at time intervals T, the time change θ(l, nT) of the phase of the scattered light at time t=nT, where n is an integer, is measured for each point along the longitudinal direction of the optical fiber under test. However, in reality, the time at which the point at distance l is measured is delayed from the time at which the pulse was injected by the time it takes for the pulsed light to propagate from the input end to distance l. Furthermore, it should be noted that the time measured by the measuring instrument is delayed by the time it takes for the scattered light to return to the input end. It is known that the magnitude of the physical vibration applied to the section from distance l to distance l+δl at each time nT is proportional to the difference δθ(l,nT) between the phase θ(l+δl,nT) at distance l+δl and the phase θ(l,nT) at distance l. In other words, if time zero is taken as the base, the following formula is satisfied.

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[0007] Device configurations for detecting the phase of scattered light include a direct detection configuration in which the backscattered light from the measured optical fiber is directly detected using a photodiode or the like, and a coherent detection configuration in which the backscattered light is combined with a separately prepared reference light and detected (for example, Non-Patent Document 1).

[0008] Mechanisms for performing coherent detection and calculating the phase can be subdivided into two types: software-based processing using the Hilbert transform, and hardware-based processing using a 90-degree optical hybrid. In either method, the in-phase component I(l, nT) and quadrature component Q(l, nT) of the scattered light are obtained, and the phase is calculated using the following equation.

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[0009]

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[0010] In measurements using DAS, there is noise from the measuring instrument, such as thermal noise from the PD (Photo Diode) used to detect light, noise in the subsequent electrical stages, and shot noise from light, etc. Therefore, the intensity and phase of the scattered light being measured are also affected by the noise from the measuring instrument.

[0011] In particular, when measuring the phase of scattered light, if the effect of noise in the measuring instrument becomes large, not only does the uncertainty in the phase increase, but there is also a higher probability that the measured value will differ significantly from the ideal phase value in the absence of noise.

[0012] For example, in the case of coherent detection, the direction of the vector of the measured scattered light when the in-phase component is on the horizontal axis and the quadrature component is on the vertical axis corresponds to the phase to be measured when there is no noise, but when the influence of noise is large, the vector direction is in the opposite direction, and compared with the ideal phase value when there is no noise, the probability that the actually measured phase value will be a value that differs by about π radians increases. At such points, when calculating the magnitude of vibration from equation (1), it can lead to the erroneous recognition that a large physical force has been applied to the optical fiber. Furthermore, when the influence of noise becomes large, the number of points at which the integer q is incorrectly selected increases in the unwrapping process shown in equation (3), and a difference in the phase value that does not actually exist of 2π or more occurs before and after the incorrectly selected point. Such a difference in phase value can also lead to the erroneous recognition that a large physical force has been applied to the optical fiber when calculating the magnitude of vibration from equation (1).

[0013] In order to measure the phase accurately, it is necessary to reduce the effect of the noise of the measuring instrument. The effect of the noise of the measuring instrument becomes large when the intensity of the scattered light itself becomes small when the noise of the measuring instrument can be considered to be the same at each point and each time. Therefore, if the intensity of the scattered light can be increased at each point and each time, the effect of the noise of the measuring instrument can be reduced.

[0014] The cause of the intensity of the scattered light itself becoming smaller is not only the loss due to absorption and scattering that occurs as the pulsed light that serves as the probe propagates through the optical fiber under test. Since pulsed light with a finite time width is input into the optical fiber under test and the scattering of the pulsed light is detected, interference of scattered light from many scatterers that are very finely distributed on the optical fiber under test occurs. As a result of interference, points where the intensity of the scattered light becomes smaller occur depending on the distribution of the scatterers in the longitudinal direction of the optical fiber under test at each time. This phenomenon is called fading (Non-Patent Document 3).

[0015] Therefore, when measuring the phase of scattered light in a DAS-P, there is a challenge in that, in order to reduce the effects of noise in the measuring instrument, it is necessary to prevent the occurrence of points where the intensity of the scattered light becomes low at each time due to fading.

[0016] One method for solving this problem is to simply increase the peak intensity of the incident optical pulse. However, when the peak intensity of the optical pulse is increased, a nonlinear optical effect occurs when the optical pulse propagates through the optical fiber under test, and the characteristics of the pulsed light change as it propagates through the optical fiber under test. This limits the peak intensity of the optical pulse that can be incident, and there are cases in which the problem cannot be solved sufficiently.

[0017] In order to solve the above problems, a phase measurement method and a signal processing device have been proposed that can reduce the influence of noise in the measuring instrument without increasing the peak intensity of the incident light pulse when measuring the phase of scattered light in a DAS-P (Patent Document 1).

[0018] In order to solve the above problem, in Patent Document 1, pulse light in which pulses of different optical frequency components are arranged and wavelength-multiplexed at time intervals where changes in the fiber state due to vibration can be ignored is input into the optical fiber under test, and scattered light at each wavelength from the optical fiber under test is plotted on a two-dimensional plane with the in-phase component on the horizontal axis and the orthogonal component on the vertical axis to create a scattered light vector. The created scattered light vector is rotated for each wavelength at each point on the optical fiber under test to align its direction, and a new vector is generated by averaging the vectors with the aligned directions, and the phase is calculated using the values ​​of the in-phase and orthogonal components of the new vector created.

[0019] In measurements using the DAS-P, there is also the issue of a trade-off between the measurement distance and the upper limit of the measurable vibration frequency. When using a single-frequency optical pulse, as the measurement distance becomes longer, the time at which the scattered light from the far end returns is delayed relative to the time at which the pulse is incident. Therefore, in order to prevent the scattered light from the far end and the scattered light from near the incident end when the next optical pulse is incident from being multiplexed or interfering with each other, there is an upper limit to the repetition frequency at which the optical pulse is incident. Therefore, according to the sampling theorem, there is an issue that vibrations with vibration frequencies higher than the Nyquist frequency, which is 1 / 2 the repetition frequency, cannot be measured correctly due to aliasing.

[0020] Non-Patent Document 4 proposes a method for solving the above problem. In order to solve the above problem, Non-Patent Document 4 proposes that pulsed light, which is wavelength-multiplexed by arranging pulses of different optical frequency components at equal time intervals, is input to the optical fiber under test, and scattered light at each wavelength from the optical fiber under test is plotted on a two-dimensional plane with the in-phase component on the horizontal axis and the orthogonal component on the vertical axis to create a scattered light vector. The phase is calculated using the scattered light vector obtained. In the case of a single optical frequency, the upper limit of the sampling rate determined from the measurement distance is expressed as f s Then, by multiplexing N waves, the upper limit of the sampling rate is N×f s Here, the number of multiplexed wavelengths, “N”, is an arbitrary natural number.

[0021] Here, when performing a frequency multiplexing method as described in Non-Patent Document 4, if the angle of the scattered light vector obtained at each optical frequency is simply concatenated to calculate the phase change without correcting the angle difference between each optical frequency, a problem occurs in that the calculated phase change is distorted relative to the actual phase change, and an accurate vibration waveform cannot be measured. In order to address this problem, Non-Patent Document 4 first calculates the temporal phase difference for each optical frequency, and then concatenates the calculated phase differences for each optical frequency to obtain the Nyquist frequency f v We propose a method to correctly estimate the frequency even if the vibration frequency exceeds N×f. v However, this proposal has a problem in that it is not possible to measure the vibration waveform because it does not obtain the angular difference between the optical frequencies.

[0022] As a countermeasure to this problem, Non-Patent Document 5 proposes that the upper limit of the sampling rate is increased to N×f by correcting the angular difference between the optical frequencies using a correction frequency. s We have proposed a measurement method that can measure vibration waveforms under conditions that are significantly improved. In the proposed method, a compensation optical frequency is used in addition to the main optical frequency to improve the sampling rate, and a probe pulse train is used in which the main frequency component and the compensation optical frequency component are periodically input to the fiber under test at a timing that can be considered to be the same time, thereby correcting the angular difference between the main optical frequency component and the compensation optical frequency component, thereby correcting the angular difference between each of the main optical frequencies.

[0023] In addition, the trade-off between the measurement distance and the upper limit of the measurable vibration frequency requires that phase unwrapping be performed correctly, which imposes even stricter conditions: if the absolute value of the magnitude of the phase change when sampling with adjacent optical pulses changes by more than π, phase unwrapping cannot be performed uniquely, leading to failure of phase unwrapping (Non-Patent Document 6).

[0024] Therefore, the upper limit of the absolute value of the magnitude of the phase change at adjacent sampling points is restricted to π. Therefore, even in the range below the Nyquist frequency, the higher the vibration frequency, the larger the phase change amount at adjacent sampling points becomes, so that if the vibration amplitude becomes large, further conditions arise for the upper limit of the measurable vibration frequency. The proposed method described in Non-Patent Document 5 is effective in relaxing such restrictions because it can measure the vibration waveform.

[0025] In addition, Non-Patent Document 5 also proposes a method of constructing optical frequency pulses and a method of processing received signals in order to simultaneously implement a frequency multiplexing method of injecting pulses of different optical frequencies at different times in order to improve the sampling rate, and the frequency multiplexing method described in Patent Document 1 as a countermeasure against fading.

[0026] The relationship between the magnitude of the phase change and the amount of strain applied to the fiber due to vibration is explained, for example, in Non-Patent Document 7. According to Non-Patent Document 7, when a fiber with a total length l is stretched by Δ1 due to a strain amount ε, the increase in phase change Δφ when light passes through due to the stretching by Δ1 is expressed by the following formula.

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[0027] where k=2πn / λ is the propagation constant, n is the effective refractive index of the fiber, and μ p is the Poisson's ratio, p 11 andp 12 are the strain-optic tensor components. For example, when λ=1555 nm, which is near the usual communication wavelength band, n=1.47, μ p = 0.17, p 11 =0.121, p 12 = 0.271, so

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[0028] In Non-Patent Document 5, pulses of different optical frequencies are input at different times to improve the sampling rate. In this case, if a simple rectangular wave is used as the pulse shape, the band occupied by the scattered light of each optical frequency component has a sinc function shape, so there is a side lobe. The side lobe causes crosstalk in which the bands occupied by the scattered light of each optical frequency component overlap. Due to this crosstalk, the signal of the scattered light of a certain optical frequency component fm is added with the component of the band near the optical frequency fm, which is included in the side lobe of the scattered light of another optical frequency component fn input at a different time, and the two cannot be separated by a digital band pass filter or the like. The two components are added in a vector state on a plane with the in-phase component of the scattered light on the horizontal axis and the orthogonal component on the vertical axis. Since they are added in a vector state, there is a problem that vibration is erroneously detected in sections other than the location where the vibration actually occurred. In addition, the vibration waveform at the vibration location is distorted from the actual waveform, and the angle difference cannot be completely corrected using the compensation optical frequency. [Prior art documents] [Patent documents]

[0029] [Patent Document 1] JP 2020-169904 A [Non-patent literature]

[0030] [Non-Patent Document 1] Ali.Masoudi, TP Newson, “Contributed Rview: Distributed optical fiber dynamic strain sensing.” Review of Scientific Instruments, vol.87, pp011501 (2016) [Non-Patent Document 2] Kenichi Nishiguchi, Cheolhyun Lee, Kuzyk-Arter, Mitsunori Yokoyama, and Kinzo Masuda, "Prototype of distributed acoustic wave sensor using optical fiber and its signal processing," IEICE Technical Report, 115(202), pp29-34 (2015) [Non-Patent Document 3] G. Yang et al., “Long-Range Distributed Vibration Sensing Based on Phase Extraction from Phase-Sensitive OTDR,” IEEE Photonics Journal, vol. 8, no. 3, 2016. [Non-Patent Document 4] D. Iida, K. Toge, T. Manabe, 'Distributed measurement of acoustic vibration location with frequency multiplexed phase-OTDR', Opt. Fiber Technol., 2017, 36, pp 19-25, DOI: 10.1016 / j.yofte.2017.02.005 [Non-Patent Document 5] Y. Wakisaka, D. Iida and H. Oshida, “Distortion-Suppressed Sampling Rate Enhancement in Phase-OTDR Vibration Sensing with Newly Designed FDM Pulse Sequence for Correctly Monitoring Various Waveforms,” 2020 Optical Fiber Communications Conference and Exhibition (OFC), San Diego, CA, USA, 2020, pp. 1-3. [Non-Patent Document 6] Maria Rosario Fernandez-Ruiz, Hugo F. Martins, “Steady-Sensitivity Distributed Acoustic Sensors,” J. Lightwave Technol. 36, 5690-5696 (2018) [Non-Patent Document 7] CD Butter and GB Hocker, “Fiber optics strain gauge,” Appl. Opt. 17, 2867-2869 (1978) [Non-Patent Document 8] AE Alekseev et al., “Fidelity of the dual-pulse phase-OTDR response to spatially distributed external perturbation,” Laser Phys. 29, 055106 (2019) Summary of the Invention [Problem to be solved by the invention]

[0031] The present disclosure aims to reduce crosstalk between different optical frequency components and reduce false detection of vibration and distortion of vibration waveforms. [Means for solving the problem]

[0032] The present disclosure suppresses the degree of overlap between the bands occupied by the scattered light of each optical frequency component by making the contour of the optical pulse incident on the measured optical fiber a waveform with small side lobes in the frequency domain, thereby reducing problems such as erroneous detection of vibrations in sections other than the location where the vibrations caused by the crosstalk actually occur.

[0033] Specifically, in a frequency multiplexed phase OTDR, a waveform with a smaller spectral side lobe than a rectangular wave, such as a raised-cosine wave, is used as the pulse shape of each optical frequency component.

[0034] More specifically, the vibration measuring device and the vibration measuring method of the present disclosure include: In a measuring instrument that performs DAS-P by repeatedly injecting multiple optical pulses with different optical frequencies into an optical fiber, As the plurality of optical pulses, optical pulses having a waveform with smaller spectral side lobes than a square wave are used. Effect of the Invention

[0035] According to the present disclosure, it is possible to reduce crosstalk between different optical frequency components, thereby making it possible to reduce erroneous detection of vibration and distortion of vibration waveforms. [Brief description of the drawings]

[0036] [Figure 1] FIG. 2 is a diagram illustrating a vibration detection system that performs vibration detection using a DAS-P according to the present embodiment. [Diagram 2] 4 shows an example of a pulse pattern configuration. [Diagram 3] 1 shows an example of crosstalk that occurs when the pulse shape of each optical frequency component is a square wave shape. [Figure 4] 3 shows an example of a side lobe included in a scattered light signal. [Diagram 5] 2 shows an example of a main signal component and a crosstalk component. [Figure 6] 3 shows an example of a time waveform of each optical frequency pulse according to the present disclosure. [Figure 7] 1 shows an example of a pulse train used in this disclosure. [Figure 8] An example of generating optical pulses using an SSB modulator is shown below. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Hereinafter, the embodiments of the present disclosure 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 are assumed to indicate the same components.

[0038] (Example 1) 1 is a diagram illustrating a vibration detection system that performs vibration detection using the DAS-P of this embodiment. This vibration detection system includes a light source that inputs a frequency-multiplexed optical pulse train to one end of the optical fiber under test, a light receiver that receives scattered light of each wavelength that has returned to the one end of the optical fiber under test, and a signal processor that observes the vibration of the optical fiber under test as a time change in the phase component of the scattered light.

[0039] The vibration measuring instrument 31 includes a CW light source 1, a coupler 2, an optical modulator 3, a 90-degree optical hybrid 7, and a balanced detector (13, 14). The CW light source 1, the coupler 2, and the optical modulator 3 correspond to the light source. The 90-degree optical hybrid 7 and the balanced detector (13, 14) correspond to the optical receiver. The optical receiver performs coherent detection using the 90-degree optical hybrid 7. The signal processing device 17 corresponds to the signal processing section. However, it is not necessary to use a 90-degree optical hybrid in the receiving system, and other devices and signal processing may be used as long as the in-phase component and quadrature component of the scattered light can be measured. The signal processing device 17 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.

[0040] The vibration measuring device 31 measures the scattered light from the optical fiber 6 under test as follows. 0A continuous light of a single wavelength is emitted and branched into a reference light and a probe light by a coupler 2. The probe light is shaped into a wavelength-multiplexed optical pulse 4 by an optical modulator 3. A multiplexed pulse using a compensation optical frequency as described in Non-Patent Document 5 can be used as the optical pulse 4. An example of the configuration of the optical pulse 4 is shown in FIG.

[0041] The optical frequency component used for the main pulse is f 1 From f NM The N+1 groups of N×M pulses are arranged in order. The entire arrangement is divided into N(N+1) pulse pairs as shown in 202, where M is an arbitrary natural number. The compensation optical frequency f NM+1 is added for every N+1 pulse pairs to generate pulse pair 203. Based on pulse pair 203, the actually incident optical pulse pair train is constructed as shown in 204. In this way, a pulse pattern is generated in which N(N+1) pulse pairs are arranged at a constant time period.

[0042] Here, the compensation optical frequency f NM+1 is added to the pulse pair with pulse pair number 1+k(N+1) (k=0,1,…,(N-1)). For example, when N=3 and M=1, the optical frequency f 1 ,f 2 ,f 3 The pulse pair is repeatedly input to the optical fiber 6 under test. In this case, when k=0, the compensation light frequency f 4 is the optical frequency f 1 When k=1, the optical frequency f 4 is the optical frequency f 2 When k=2, the optical frequency f 4 is the optical frequency f 3 is added to the pulse pair with

[0043] The interval between the pulse pairs is T N Then, T due to the length of the optical fiber 6 to be measured NThe restriction on the minimum value of how small can be relaxed by a factor of N compared to the case of using a single optical frequency pulse. Also, in the configuration of Figure 2, the M pulses present in each pulse pair can be used to reduce fading noise.

[0044] 1, the type of optical modulator 3 is not specifically specified as long as it can generate optical pulse 4, and there may be more than one type. For example, an SSB (Single Side Band) modulator or a frequency-variable AO (Acousto-Optics) modulator may be used, and intensity modulation may be performed using an SOA (Semiconductor Optical Amplifier) ​​or the like to increase the extinction ratio in pulsing. Note that although the pulses of each optical frequency component shown in 204 are rectangular wave shaped, waveforms other than rectangular waves may also be used.

[0045] The optical pulse 4 is input to the optical fiber 6 under test via the circulator 5. The light scattered at each point in the longitudinal direction of the optical fiber 6 under test returns to the circulator 5 as backscattered light and is input to one input port of the 90-degree optical hybrid 7. The reference light branched by the coupler 2 is input to the other input port of the 90-degree optical hybrid 7.

[0046] The internal configuration of the 90-degree optical hybrid 7 may be anything as long as it has the function of a 90-degree optical hybrid. An example configuration is shown in Fig. 1. The backscattered light is incident on a coupler 8 with a branching ratio of 50:50, and the scattered light split into two is incident on the input ports of a coupler 12 with a branching ratio of 50:50 and a coupler 11 with a branching ratio of 50:50. The reference light is incident on a coupler 9 with a branching ratio of 50:50, and one of the split reference light is incident on the input port of coupler 11, and the other is phase-shifted by π / 2 by a phase shifter 10 and is incident on the input port of coupler 12.

[0047] The two outputs of coupler 11 are detected by a balanced detector 13, which outputs the analog in-phase component I analog The two outputs of the coupler 12 are detected by a balanced detector 14, which outputs an analog quadrature component Qanalog An electrical signal 16 is outputted, which is:

[0048] The electric signals 15 and 16 are sent to a signal processing device 17 equipped with AD (Analog to Digital) conversion elements 17a and 17b capable of sampling the frequency band of the signals without aliasing. In the signal processing device 17, the digitized in-phase components I digital and the quadrature component Q digital The signal processor 17c processes each optical frequency f 0 +f i The specific signal processing method is I digital and Q digital From the signal in each band, I i measure (i=1,2,...,NM+1) and Q i measure Any method can be used as long as it can accurately separate (i=1,2,...,NM+1). For example, I digital and Q digital With center frequency f 0 +f i One possible calculation method is to pass the signal through a bandpass filter where W is the pulse width of each optical frequency component, and then compensate for the phase delay. If the pulse width of each optical frequency component is W, then the passband can be set to 2 / W. Alternatively, the in-phase and quadrature components in the form of analog electrical signals may be separated into each frequency component by an analog electrical filter, and then AD converted by the AD conversion elements 17a and 17b.

[0049] The I acquired by the signal processing unit 17c i measure and Q i measure Based on this, the signal processor 17d calculates the phase. First, a complex vector r i Create a.

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[0050] The time when the first pulse of pulse pair k is input is k×T N +n×N×T N (n is an arbitrary integer). The first optical frequency of each pulse pair is taken as the reference wavelength, and the vectors calculated in (1-1) in the band of M different optical frequencies excluding the compensation optical frequencies that make up the pulse pairs are averaged in accordance with the method described in Patent Document 1 to calculate the phase at a position at a distance z from the incident end. The state of the test optical fiber 6 at a position at a distance z from the incident end in the longitudinal direction on the test optical fiber 6 is calculated at time k×T N +n×N×T N +z / ν (n is an arbitrary integer), where ν is the speed of light in the optical fiber 6 under test. Furthermore, when the time it takes for the scattered light to propagate and return to the input end is taken into consideration, the measurement time in the vibration measuring device 31 is k×T N +n×N×T N +2z / ν (n is an arbitrary integer). Therefore, the phase calculated at the point of distance z is expressed explicitly as the measurement time of the vibration measuring device 31, as follows:

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[0051] In this embodiment, the measurement time mT N Phase θ(z,mT N +2z / ν) to mT N +2z / ν=kT N +nNT N Using k and n that satisfy +2z / ν, calculate as follows:

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[0052] And the distance z on the optical fiber 6 to be measured 1 Distance z from 2 The phase change due to the vibration applied to the section is calculated as the difference between formula (1-3a) and formula (1-3b), that is, formula (1-3c).

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[0053] Moreover, since the time at the moment when the state of the measured optical fiber 6 is measured does not include the time required for the scattered light to return to the incident end as described above, at the point of distance z 1 the time is mT N +z 1 / ν, and at the point of distance z 2 the time is mT N +z 2 / ν. Thus, there is a difference of only (z 1 -z 2 ) / ν. However, the difference in distance between z 1 and z 2 is on the same order as the spatial resolution and is usually set to about several meters to several tens of meters. Therefore, the time difference (z 1 -z 2 ) / ν is several tens to several hundreds of nanoseconds, which is very short compared to the time change scale of normal vibrations to be measured. Therefore, the difference in the time when the state of the measured optical fiber 6 is measured can be ignored. Thus, the vibration added to the corresponding section can be accurately measured.

[0054] However, θ(z, mT N +2z / ν) includes a distortion term due to the angular difference between the optical frequencies at the head of each pulse pair. Non-Patent Document 5 proposes a method for correcting the angular difference using a compensation optical frequency. In order to perform the correction of the angular difference between different optical frequencies without omission, it is necessary to correct the angular difference of the optical frequencies at the head of any two pulse pairs. When positive integers i and j satisfying i < j are arbitrarily selected, if the optical frequency at the head of pulse pair j is f j pf and the optical frequency of pulse pair i is f i pf , then the angular difference φ(z, f j pf , f i pf ) can be expanded as follows using f NM+1 .

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[0055] In the example pulse pair optical frequency combination 203, the optical frequency f NM+1 is added to the pulse pair with pulse pair number 1+k(N+1) (k=0,1,…,(N-1)), so the optical frequency f NM+1 and other light frequencies have a period of N(N+1)T N For example, when N=3 and M=1, the number of pulse pairs that make up the pulse pattern is 12. In this case, the first pulse pair has optical frequency f 1 and the optical frequency f 4 and the fifth pulse pair contains optical frequency f 2 and the optical frequency f 4 and the ninth pulse pair contains optical frequency f 3 and the optical frequency f 4 Therefore, the optical frequency f 4 and other frequencies f 1 ,f 2 ,f 3 Each of these is always present once in the same pulse pair. Therefore, each term on the right side of the formula (1-4) can be calculated using the same principle as the method of Patent Document 1. The obtained φ(f j pf ,f i pf ) value, θ(z,mT N +2z / ν) to calculate the final phase.

[0056] First, as described in Non-Patent Document 5, when the pulse shape of each optical frequency component is a rectangular wave shape as shown by 204 in FIG. 2, the problem of not being able to identify the vibration position will be explained. For simplicity, the case where M is 1 will be described, but the same applies when M is any number. The pulse train when M is 1 is 301 in FIG. 3. To explain scattered light, 1 and f 3Only the main signal component is shown in 302. The main signal component can be separated into 303 and 304 by signal processing. However, as shown in FIG. 4, there are side lobes in the band occupied by the scattered light signal, so the optical frequency f 1 The occupied bandwidth 401 and the optical frequency f 3 Even if the main lobes of the occupied band 402 of the optical frequency f1 can be separated from each other, the side lobes of the scattered light due to the optical pulse of the optical frequency f 3 The crosstalk 403 occurs in the main lobe of the scattered light due to the optical pulse of the component f. 3 The signal separated as above is a superposition of the crosstalk component and the main signal component, and the two cannot be separated.

[0057] Now consider the case where vibration occurs at vibration position 306. The section where the crosstalk component overlaps in front of the main signal component is taken as overlap section 307, and consider the phase calculated at an arbitrary position 308 on section 307. Let the distance from the entrance end of position 308 be l. As shown in Fig. 5, the main signal component and the crosstalk component are combined in a vector state on a plane with the horizontal axis as the in-phase component and the vertical axis as the orthogonal component, and become an observation vector. In other words, if the observation vector at time T at distance l is expressed as r 3 (l,T), the vector of the main signal component is R 3 (l,T), the vector of crosstalk components is X 1,3 If (l,T), the following equation is satisfied.

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[0058] Note that in formula (1-5), the value of the distance from the incident end of the crosstalk component is l', which is different from l. The reason for this is that the crosstalk component is received at a different timing from the main signal component, so the vector corresponding to the position 308 of the main signal component overlaps with the vector corresponding to the different position 309 of the crosstalk component. As shown in the positional relationship 310 on the fiber in FIG. 3, position 308 is in front of the vibration position 306, while position 309 is behind the vibration position 306. As shown in FIG. 5, when considering the change from state 501 at time T to state 502 at time T', position 308 is in front of the vibration position 306, so the vector of the main signal component does not change. On the other hand, the crosstalk component has an angle change due to vibration because position 309 is behind the vibration position 306. As a result, the angle of the observation vector also changes. Observation vector r 3 The angle of (l,T) is θ 3 If we use (l,T), the change in angle is expressed by equation (1-6).

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[0059] The amount of change in equation (1-6) depends not only on the magnitude of the vibration occurring at vibration position 306, but also on the lengths of the main signal component vector and the crosstalk component vector, and the angle difference between the two vectors. Considering that the length and angle of the vectors change for each position due to the fading effect, the amount of change in the angle in equation (1-6) changes depending on the distance l. For example, the angle change at a position δl behind position 308 can be considered similarly as long as it is within overlap section 307.

[0060] Assuming a situation in which the length or angle of the vector changes, such as in state 503 or state 504, even if the rotation angle of the crosstalk component itself is the same for l' and l'+δl, the angle change θ 3 (l+δl,T')-θ 1(l+δl,T) is different from the angle change at distance l. Therefore, just like in the background equation (1), when considering the phase change in the section from distance l to l+δl from the input end, the optical frequency f 3 In the above example, the optical frequency f 1 by light frequency f 3 In the above example, we considered crosstalk due to optical frequency f 3 The crosstalk effect of another optical frequency on the optical frequency f 3 It was clarified that vibrations can be observed at locations other than the actual vibration locations for the time sampled at M, but the same holds true for the time sampled at a different optical frequency. Also, although we considered the case where M is 1, the same argument holds true even if M is any number.

[0061] Therefore, in frequency multiplexing technology for improving the sampling rate, a problem generally occurs in which vibration is observed at locations other than the vibration location. This effect is particularly noticeable in situations where a narrow pulse width is used to increase spatial resolution, resulting in a wide band occupied by each optical frequency component in the frequency domain, or when it is necessary to multiplex a large number of optical frequency components in a limited frequency band. In addition, the problem of vibration being observed at locations other than the vibration location due to crosstalk was mentioned in the above example, but considering that the main signal component and the crosstalk component are synthesized in a vector state on a plane with the horizontal axis as the in-phase component and the vertical axis as the orthogonal component to become an observation vector, it can be seen that the phenomenon of the vibration waveform at the vibration location being distorted from the actual waveform and the effect of not being able to completely correct the angle difference using the compensation optical frequency also occur.

[0062] In this embodiment, a method is proposed to solve the problem caused by the crosstalk by making the pulse shape of each optical frequency component a smoothly changing outline. 610 in FIG. 6 shows the time waveform of each optical frequency pulse used in this proposal. The outline of the envelope of the electric field before the carrier frequency is multiplied is shown. Waveform 601 is the conventional square wave shape also used in Non-Patent Document 5, and waveform 602 shows the shape of the waveform used in this disclosure. The waveform shape is not limited to a specific shape as long as it is a waveform that can suppress crosstalk, but for example, a raised cosine waveform can be used. Waveform 602 shows a raised cosine waveform as an example. By making the full width at half maximum 603 of the pulse the same as in the case of a square wave, the spatial resolution of the vibration measuring device can be maintained the same and the deterioration of the signal-to-noise ratio of the signal can be prevented. The power density in the frequency domain is shown in 620. It can be seen that the side lobe is suppressed in 605 in the case of a raised cosine waveform compared to 604 in the case of a square wave, and the magnitude of the crosstalk can be reduced. Therefore, when a raised cosine waveform is used, the pulse train has the outline 701 in Fig. 7. By using the pulse train shown in 701, the vector length due to the crosstalk component can be reduced, and therefore, without changing the signal processing method for phase calculation, it is possible to reduce the problem that vibration is observed at places other than the vibration place due to the crosstalk, the occurrence of the phenomenon that the vibration waveform at the vibration place is distorted from the actual waveform, and the influence that the angle difference cannot be completely corrected using the compensation light frequency.

[0063] An example of a method for generating optical pulses using an SSB modulator will be described with reference to Fig. 8. In this example, a modulation signal 802 is provided to an SSB modulator 801 to modulate continuous light and generate an optical pulse pair 803. If the envelope of each modulation frequency component of the provided modulation signal 802 is set to have a raised cosine waveform, the magnitude of the electric field of each optical frequency component can also have a smoothly changing shape.

[0064] The modulation frequency of the modulating signal 802 is, for example, f 1 and f 2 and f 3In this case, the generated pulse pair 803 has a frequency of f 0 Then, the optical frequency is f 0 +f 1 and f 0 +f 2 and f 0 +f 3 When the scattered light generated by the input of the pulse pair is coherently detected and processed as a signal, the center frequency of the bandpass filter for extracting each optical frequency component in the signal processing unit 17c is also f 1 and f 2 and f 3 It is sufficient to set

[0065] The measurement resolution is also determined by the FWHM ( Full It can be set by considering that the width at half maximum of the pulse pair approximately coincides with the FWHM of each optical frequency component of the electric field magnitude 803 of the pulse pair. For example, the passband width of the bandpass filter in the signal processing unit 17c is determined by the FWHM of the raised cosine waveform. However, it is also possible to determine the resolution by adjusting the FWHM of the raised cosine waveform in consideration of the characteristics of the SSB.

[0066] When selecting each main optical frequency during frequency multiplexing, if possible, the center of the main lobe of the main optical frequency included in the other pulse pairs should be located at the valley of the side lobe of each main optical frequency shown in 606 in Figure 6, thereby reducing the effect of crosstalk due to the side lobes.

[0067] Incidentally, the optical pulse pairs applicable to the present disclosure are not limited to the configuration shown in FIG. 2, but can be used in general for methods that utilize frequency multiplexing to improve the sampling rate.

[0068] (Effects of disclosure) According to the present disclosure, in an optical frequency multiplexed phase OTDR in which different optical frequency components are incident at different times as described in Non-Patent Document 5, a waveform with a small spectral side lobe, such as a raised-cosine wave, is used as the pulse shape of each optical frequency component instead of the conventional rectangular wave, thereby reducing crosstalk between different optical frequency components and suppressing erroneous detection of vibrations other than the vibration point, while reducing the occurrence of the phenomenon in which the vibration waveform at the vibration point is distorted with respect to the actual vibration waveform, and the effect of not being able to completely correct the angle difference using a compensation optical frequency. [Industrial Applicability]

[0069] The present disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0070] 1:CW light source 2: Coupler 3: Optical modulator 4: Light pulse 5: Circulator 6: Optical fiber under test 7: 90 degree optical hybrid 8, 9: Coupler 10: Phase shifter 11, 12: Coupler 13, 14: Balance detector 15: Analog in-phase electrical signal 16: Analog quadrature component electrical signal 17: Signal processing device 17a, 17b: AD conversion element 17c, 17d: signal processing section 31: Vibration measuring instrument

Claims

1. In a vibration measuring instrument that performs distributed acoustic sensing (DAS-P) by repeatedly injecting a plurality of optical pulses with different optical frequencies into an optical fiber, As the plurality of optical pulses, optical pulses having a waveform with a smaller spectral side lobe than a square wave are used, the plurality of optical pulses are composed of a plurality of pulse pairs including a main optical frequency component and a compensation optical frequency component; The centers of the main lobes of the main optical frequencies included in the other pulse pairs are located at the frequency positions that are located at the valleys of the side lobe components of each main optical frequency. Vibration measuring instrument.

2. a single-sideband modulator that generates the plurality of optical pulses by modulating a continuous light with a modulation signal; 2. The vibration measuring instrument according to claim 1.

3. The envelope of each modulation frequency component of the modulation signal has a raised cosine waveform.

3. The vibration measuring instrument according to claim 2.

4. using a modulation frequency component of the modulation signal, the scattered light generated when the plurality of optical pulses are scattered in the optical fiber is separated into each of the plurality of optical pulses.

4. The vibration measuring instrument according to claim 2 or 3.

5. a passband width of a bandpass filter that separates the plurality of optical pulses is determined by a FWHM (Full Width at Half Maximum) of a raised cosine waveform; 5. A vibration measuring instrument according to claim 2.

6. A vibration measurement method in which a plurality of optical pulses having different optical frequencies are repeatedly input to an optical fiber and a distributed acoustic sensing phase (DAS-P) is performed, As the plurality of optical pulses, optical pulses having a waveform with a smaller spectral side lobe than a square wave are used, the plurality of optical pulses are composed of a plurality of pulse pairs including a main optical frequency component and a compensation optical frequency component; The centers of the main lobes of the main optical frequencies included in the other pulse pairs are located at the frequency positions that are located at the valleys of the side lobe components of each main optical frequency. Vibration measurement methods.

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