JP6893137B2 - Optical fiber vibration detection sensor and its method - Google Patents

Description

The present invention relates to an optical fiber vibration detection sensor using a light source and a method thereof.

Various types of interference sensors using optical fiber technology have been researched and developed. The interference type sensor measures the phase change of light and can measure various physical quantities such as temperature and distortion with high sensitivity. For example, a vibration sensor that detects vibration to a structure has been considered for the purpose of maintaining the measured object and detecting an abnormality.

As an interference type sensor, a vibration detection method using a ring type fiber has been studied (see Non-Patent Document 1). In this method, light is incident from both ends of the ring-type fiber, and the vibration position is detected based on the difference in the amount of change at the time of phase change due to vibration or the like in each.

Although the position can be specified by this method, it is essentially not a distribution measurement because it is a method of finding the vibration point because it is necessary to inject light from both ends.

On the other hand, as a method for measuring the reflected light in a distributed manner and measuring the vibration in a distributed manner, a method using an optical time domain reflectometry (OTDR) on an optical fiber is being studied (OTDR). See Non-Patent Document 2). In this method, the reflected light from each point inside the optical fiber is measured, the reflected position is specified by the time when each reflected light is received, and the beat signal generated by interfering with the reference light is received and its phase. That is, vibration is detected from the phase change of scattered light.

As described above, as a vibration sensor using OTDR, phase OTDR for measuring the phase of scattered light is generally studied as a promising method. However, in order to correctly measure the phase change of the scattered light, the phase difference between the scattered light after reciprocating in the optical fiber to be measured and the reference light that interferes to generate the beat signal is a fixed value. That is, the scattered light and the reference light need to be in a coherent state.

If the delay time difference between the test light or its reflected light and the reference light becomes long and the two lights are no longer coherent, the phases of the received interference light become random and it is impossible to detect vibration. Therefore, as a light source having a coherence time exceeding the round trip time of the optical fiber to be measured, it is necessary to use a high-purity laser light source having very small phase noise and a narrow spectral line width. Such a light source is very expensive and difficult to handle, which is a barrier to the widespread use of phase OTDR vibration sensors.

Further, even with such a high coherence light source, there is a limit to the delay time in which the phase can be determined, that is, the time during which coherence can be maintained, and there is a limit to the measurable optical fiber length. Further, even if the scattered light is scattered light from a position that can be measured within the coherence time of the light source and the phase of the interference light is fixed, the phase fluctuation itself due to the phase noise of the light source exists, and the fluctuation causes the phase fluctuation. The accuracy when analyzing the detailed state of vibration deteriorates.

In the phase OTDR, the correction for the frequency drift of the light source has already been studied (see Non-Patent Document 3), but since the entire phase noise of the light source is not corrected, the delay time exceeding the above coherence time is not corrected. The problem of measuring scattered light from the position where it occurs remains.

From the viewpoint of correcting the delay time in the method of measuring reflected light, a method of correcting the phase noise of the light source by adding a reference interferometer to the measurement system and monitoring the output of the interferometer is being studied (non-). See Patent Document 4). In this method, a method of calculating the delay time of an arbitrary multiple of the length of the delay fiber of the reference interferometer has been proposed.

However, this method is a reflection measurement method using light in which the frequency of the light source is swept over time, which is called an optical reflection frequency domain measurement method (OFDR), and corrects the frequency indicating the position at a distant point. It is used for the purpose. Therefore, it is necessary to temporally resample the measurement result using the monitor signal of the reference interferometer. Since this method is a measurement in which the position is uniquely determined by the delay time in the time domain reflected light measurement method called OTDR, the position correction process such as the resampling is meaningless.

Further, in the vibration measurement, the parameter to be measured and the parameter to be corrected are the same, and the parameter is not the delay time or the scattering position but the phase itself. Therefore, the method of Non-Patent Document 4 is used as it is for the vibration sensor. Cannot be used for.

P.R.Hoffman, et al, “Position determination of an acoustic burst along a Sagnac Interferometer,” Journal of Lightwave Technology, vol.22, No.2, February, 2004 Y. Lu, et al, “Distributed vibration sensor based on coherent detection of phase-OTDR,” IEEE Journal of Lightwave Technology, vol. 28, No. 22, November, 2010 F. Zhu, et al, “Active Compensation Method for Light Source Frequency Drifting in Φ-OTDR Sensing System,” IEEE Photonics Technology Letters, vol. 27, No. 24, December 15, 2015 X.Fan, et al., “Phase-Noise-Compensated Optical Frequency-Domain Reflectometry,” IEEE Journal of Quantum Electronics, vol. 45, No. 6, June, 2009

In the existing OTDR vibration sensor, since the phase of light is measured, the phase noise of the light source greatly affects the measurement accuracy. However, a technique for directly correcting the phase noise of scattered light measured by an OTDR has not been devised so far, and since there is no method for compensating for the phase noise, the generation of the phase noise itself can be suppressed to a very small level, which is expensive. There is a problem that it is necessary to use a light source that is difficult to handle. Further, since there is no light source having zero phase noise, there is a limit to the measurement accuracy of the length and vibration of the optical fiber that can be essentially measured.

The present invention has been made in view of such a problem, and an object of the present invention is an optical fiber vibration detection sensor that uses an OTDR to compensate for the limitation of the measurement distance and the deterioration of the measurement accuracy due to the phase noise of the light source. And to provide a method for it.

In order to solve the above problems, the present invention is an optical fiber vibration detection sensor, which is a light source that outputs light having coherence property and a first optical branching portion that branches the light output from the light source into two. And, the light intensity that pulse-modulates the second optical branching portion that branches one of the branched lights branched at the first optical branching portion into two and the one branched light branched at the second optical branching portion. The modulation unit, an optical circulator that incidents pulsed light output from the light intensity modulation unit on the optical fiber to be measured and extracts the backward scattered light of the optical fiber to be measured, and the first optical branching unit are branched. An optical frequency modulator that imparts an optical frequency shift to the other branched light, a third optical branch that branches the frequency-modulated light output from the optical frequency modulator into two, and an output from the third optical branch. A delay circuit that imparts a time delay τ that is smaller than half the coherence time of the light output from the light source to one of the branched lights, and the rear-scattered light and the other branch output from the third optical branch. It receives a first beat signal that combines light and a second beat signal that combines the other branch light output from the second optical branch and the light output from the delay circuit. The signal processing unit that performs digitization processing and the noise component of the phase of the first beat signal I (Nτ) at a time that is N times (N: natural number) of the time delay τ from the reference time are combined with the second beat. Calculated from signal phase X ₁

It is characterized in that it is provided with a phase noise compensating unit for compensating by using.

Further, the present invention is an optical fiber vibration detection method, wherein a step of outputting light having coherence property, a first optical branching step of bifurcating the output light having coherence property, and the first optical branching step. A second optical branching step in which one of the branched lights branched in the optical branching step is branched into two, a light intensity modulation step in which one of the branched lights branched in the second optical branching step is pulse-modulated, and the above. The pulsed light modulated in the light intensity modulation step is incident on the optical fiber to be measured, and the light is applied to the step of extracting the backward scattered light of the optical fiber to be measured and the other branched light branched in the first optical branching step. An optical frequency modulation step for imparting a frequency shift, a third optical branching step for bifurcating the frequency-modulated light modulated in the optical frequency modulation step, and one branched light branched in the third optical branching step. A delay imparting step of imparting a time delay τ smaller than half of the coherence time of the light having coherence property and the rearward scattered light and the other branched light branched in the third optical branching step were combined. A second beat signal obtained by combining the first beat signal, the other branching light branched in the second optical branching step, and the light to which the time delay τ is added in the delay applying step is received, and a numerical value is received. The signal processing step to be processed and the noise component of the phase of the first beat signal I (Nτ) at a time that is N times (N: natural number) of the time delay τ from the reference time are combined with the second beat signal. Calculated from the phase X _{1 of}

It is characterized by having a calculation processing step for compensating using.

By using the technique of the present invention, in the optical fiber vibration sensor by OTDR, the phase noise of the light source, that is, the limitation such as the spectral line width and the drift can be eliminated, and the measurement can be performed more easily, over a long distance and with high sensitivity. This can contribute to the realization of simplification and sophistication of vibration sensors using optical fibers.

It is a figure which shows the structure of the optical fiber vibration detection sensor which concerns on one Example 1 of this invention. (A) is _{a diagram showing the frequency spectrum of the beat signal I (t) calculated by Fourier transforming the beat signal I (t) when t c} is sufficiently smaller than the coherence time, and (b) is a diagram showing the frequency spectrum of the beat signal I (t) when t _c is It is a figure which shows the said frequency spectrum when it is larger than a coherence time. It is a figure which shows the time change of the phase of the discretized beat signal. (A) is a diagram showing the time change of the phase of the beat signal between the scattered light and the local light, and (b) is a diagram showing the time change _{of the phase X 1 of the reference beat signal with the delay time τ, (c).} ) Is a diagram showing the time change _{of the phase X 2} of the reference beat signal having the delay time 2τ.

Hereinafter, embodiments of the present invention will be described in detail.

FIG. 1 shows the configuration of the optical fiber vibration detection sensor according to the first embodiment of the present invention. First, the light emitted from the light source 1 is branched into two at the branch portion 2, one light is incident on the optical frequency modulation unit 3, and the other light is incident on the branch portion 4. The other light guided to the branch portion 4 without frequency modulation is branched into two, and one light is pulse-modulated by the light intensity modulation unit 5 and incident on the optical fiber 7 to be measured through the optical circulator 6. The other light branched at the branch portion 4 is incident on one port of the reference interferometer 8. One of the two branches of the light is frequency-modulated by the optical frequency modulation section 3, then split into two by the branch section 9, and one light is incident on the other port of the reference interferometer 8. The other light branched at the branching portion 9 is used as local light for acquiring a beat signal with the scattered light.

The scattered light from the optical fiber 7 to be measured is combined with the local light branched at the branch portion 9 via the optical circulator 6 and received by the balance photodetector 10. The electric signal generated by the balance photodetector 10 is combined with the signal of the same frequency as the frequency applied by the optical frequency modulator 3 generated from the signal generator 12 by the mixer 11, and the frequency of the signal to be measured is the DC component. And other noise components are cut by the low frequency transmission filter 13. The electric signal from which the noise component is cut is converted into digital data by the digitizing device 18, and the calculation processing unit 19 performs numerical calculation processing.

The reference interferometer 8 includes a delayed optical fiber 14 having a delay time shorter than half the coherence time of the light source 1, and is branched by the frequency-modulated light modulated by the optical frequency modulation unit 3 and the branching unit 4. The test light is combined and the reference beat signal is received by the balance photodetector 15. The electric signal generated by the balance photodetector 15 is combined with the signal from the signal generator 12 by the mixer 16 in the same manner as the scattered light, and unnecessary noise components are cut by the low frequency transmission filter 17. The electric signal from which the noise component is cut is converted into digital data by the digitizing device 18, and the calculation processing unit 19 performs numerical calculation processing.

Here, the scattered light when the pulse test light is incident on the optical fiber 7 to be measured is represented by the following equation (1).

Here, E is the optical amplitude, t _c is the delay time until the pulse test light is incident in the optical fiber 7 to be measured, scattered at a certain position in the optical fiber 7 to be measured, and returned as scattered light, f. Is the frequency of the light of the light source 1, f _A is the modulation frequency applied by the optical frequency modulator 3, t is the elapsed time when the measurement start is 0, and φ is the phase change due to the vibration applied to the optical fiber to be measured. a is a position on the optical fiber 7 to be measured to which vibration is applied, and θ is a phase noise component of light. This scattered light interferes with the local light represented by the following equation (2) and is received as a beat signal.

Since the local light does not pass through the optical fiber 7 to be measured, it is treated as a delay time of 0. The beat signals of the scattered light represented by the formula (1) and the local light represented by the formula (2) are

Will be. In the calculation, 2πft _c has an optical frequency f of the light source 1 that is much larger than that of _{t c , so ft c} = an integer (a value that does not include a value less than 1) and can be ignored. _{Here, if the delay time t c} of the delay in the optical fiber 7 to be measured is not smaller than the coherence time of the light source 1, the term of the difference of the noise component θ in the equation (3) becomes random, and the vibration component. φ cannot be measured correctly.

FIG. 2 shows the frequency spectrum of the beat signal I (t) calculated by Fourier transforming the beat signal I (t). When the delay time t _c is shorter than the coherence time, the beat signal I (t) has a delta-functional peak, and its spread or side peak appears due to the phase change φ due to vibration.

However, when the delay time t _c is longer than the coherence time, the noise component θ becomes random and the frequency spectrum has a spread in the form of a Lorentz function, so that the vibration component φ of the phase change can be measured. It disappears. Even if the delay time t _c is shorter than the coherence time, it has a width of the frequency spectrum due to the phase noise of the light source 1 instead of a strict delta function, and even scattered light with a delay of less than the coherence time makes noise in the phase measurement. Is mixed.

In order to solve this, the reference beat signal generated by the reference interferometer 8 is used. First, the method of phase noise compensation using the received signal in the reference interferometer 8 will be quantitatively described by a mathematical formula. By the same calculation as in the equation (3), the reference beat signal of the reference interferometer 8 is expressed in the form of the following equation (4).

Here, τ is the delay time due to the delayed optical fiber 14. At this time, since τ is very short with respect to the light frequency f of the light source 1, fτ = an integer, and the term of 2πfτ can be ignored. The electric signal represented by this equation (4) is converted into a low frequency near DC by being combined with a signal having the same frequency f as the light source 1 generated by the signal generator 12 in the mixer 11 for signal processing. Will be done. Therefore, as the phase of the reference beat signal output from the reference interferometer 8,

Is measured. Since the above X ₁ is a phase, it is calculated by performing a Hilbert transform on the actually received signal, calculating the sin component, calculating the tan component together with the cos component, and applying the inverse tan function. , The value is represented only by the value from −π to π. Here, the phase when the delay time is multiplied by N is

Therefore, if you try to calculate sequentially from N = 1,

It is expressed as, and at N times

It is expressed as. From these equations (7) and (8), if the phase X _N is expressed only by the phase X ₁ and the delay time τ, it becomes as shown in the following equation (9).

_{According to this equation, the phase X N} of the noise component at the delay time of N times (N is an arbitrary natural number) is obtained only by the time waveform and the delay time τ _{of the phase X 1} of the received signal when one delay optical fiber 14 is used. It is calculable.

The data of the phase change of the light source 1 in the case of various delays calculated from the above reference beat signals will be used thereafter to compensate for the phase noise of the beat signals of the scattered light and the local light.

Next, regarding the vibration measurement as the phase OTDR for the optical fiber 7 to be measured, the limitation due to the coherence time described with respect to the equation (3) will be described in detail. The beat signals of the scattered light and the local light incident on the balance photodetector 10 are measured in the form of the following equation (equation (3)).

This beat signal contains information on scattered light from _{a certain position z 0} of the optical fiber 7 to be measured _{, and the position z 0} has a reciprocating delay time t _c .

There is a relationship of. Here, V is the speed of light propagating in the optical fiber 7 to be measured. When the beat signal represented by the equation (3) is converted into an electric signal by the balance photodetector 10, it is combined with the electric signal of the frequency f generated by the signal generator 12 in the mixer 11. The combined signal is digitized by the digitizing device 18, and at this time, the received signal is combined by the mixer 11 and the low frequency transmission filter 13 is used to obtain a frequency corresponding to the pulse width near direct current. Make it a band signal. The signal input to the digitizing device 18 at this time is

Will be. A large number of electric signals having a waveform represented by the equation (11) are acquired by repeatedly incidenting the test light pulse generated by the light intensity modulation unit 5 on the optical fiber 7 to be measured. The incident frequency of the test light pulse is determined by the length of the optical fiber 7 to be measured so that the waveform of the beat signal represented by the equation (3) does not overlap due to a plurality of pulses, as requested by the operating principle of the OTDR. Take longer than the round trip time. Assuming that the time interval of the test light pulses is T and the number of test light pulses incident on the optical fiber 7 to be measured is n, k = 1, 2, ..., N, the signal represented by the equation (11) From

And, the discretized signal can be acquired. When the phase of this signal is calculated after obtaining the sine component by, for example, the Hilbert transform, it can be drawn as a waveform that changes with time as shown in FIG. 3, and this waveform can be drawn as a phase fluctuation by using a frequency analysis means such as a fast Fourier transform. It should be possible to obtain the vibration component φ of.

However, at this time, if the term of the noise component θ in the equation (12) changes drastically with respect to the vibration components φ and k, the vibration component φ cannot be obtained correctly. In particular as can be seen from equation (12), since the term of the noise component θ has a difference between the value of shift by time delay t _c, if the delay time t _c is longer than the coherence time of the light source 1 , The term of the noise component θ is not determined, and the noise-like behavior causes noise in the vibration measurement.

On the other hand, when the delay time t _c is shorter than the coherence time of the light source 1, the term of the noise component θ becomes a certain value, but it still becomes noise with respect to the vibration component φ, and the waveform in FIG. 3 In, it leads to deterioration of the SN ratio with respect to the phase change φ due to the vibration to be measured.

Generally, in order to make the noise component θ a definite value, it is necessary to make the delay time t _c shorter than the coherence time, in other words, the length of the optical fiber 7 to be measured is smaller than the coherence length of the light source 1. Is synonymous with the need to shorten. In order to exceed this limit, the present invention utilizes the phase of the reference beat signal at an arbitrary delay represented by the following equation (Equation (8)) calculated from the output of the reference interferometer 8.

_{With respect to the delay time t c} calculated by the equation (12), N for which Nτ is closest to the delay time t _{c is obtained, and the phase X N} of the reference beat signal at that time is used. For example, in the case of ideal t _c = Nτ,

Therefore, when added to the phase of equation (12),

Next, it is possible to obtain only the vibration component phi (and 2πf _A t _c). The value of the phase X _N (t _c + (k-1) T) _{can be obtained from the phase X 1} and the delay time τ from the equation (9). Since this calculation is a delay difference that can be calculated even if the delay exceeds the coherence time, it is possible to correctly remove the difference in the noise component θ even when the delay at I (k) exceeds the coherence time. Further, even when the delay is within the coherence time, the difference in the noise component θ can be removed by this calculation, and the phase can be measured with high accuracy by improving the SN ratio.

The specific numerical calculation method of this embodiment will be described below.

First, the parameters related to the measurement are summarized. The sampling rate S (Sa / s) in the digitizing device 18 is determined by the pulse width W, and S ≧ 2 / W. Since the same can be said for the reference beat signal, the reference beat signal is also measured at the same sampling rate S (> 2 / W).

In vibration measurement, a large number of pulses are incident to measure the time change of scattered light. Therefore, the waveform received by the calculation processing unit 19 is as shown in FIG. 4A. For the sake of explanation, the phase on the vertical axis is expressed as power, but it is not actually in such a beautiful shape because it is the phase of the beat signal. _{Further, the waveform of X 1} (t) simultaneously received from the reference interferometer 8 and the waveform of X ₂ (t) that can be calculated from the waveform are also shown in FIGS. 4 (b) and 4 (c).

Here, _{up to X N} (t) will be calculated sequentially. As _{described above, X 1} (t) is sampled at S (≧ 2 / W), but X ₂ (t) separated by the reference τ (delay time by the delayed optical fiber 14) is calculated discretely. ,

Must.

Further, if the delay time τ by the delayed optical fiber 14 in the reference interferometer 8 is made the same as the minimum sampling interval (the reciprocal of the sampling speed S (Sa / s)) of the digitizing device 18, the noise component The phase noise removal of the light source 1 represented by the difference of θ becomes strict at all points where sampling is performed, and more accurate scattered light phase distribution measurement becomes possible.

Here, considering again from FIG. 4, since only one point is extracted for each row of light waveforms from the reference interferometer 8, it is also corrected by _{the phase X N of the reference interferometer 8 for each location.} One point at a time. Therefore, the phase X _{N (t) is calculated based on the phase X 1} which is sampled in large quantities at the sampling speed S as described above, but one scattered light waveform, that is, one time. Of the beat signals sampled at the time interval T in which the pulse test light is incident, only one of them is corrected.

That is, the phase X _N (t) of the reference beat signal is sequentially calculated from the equations (7) and (8). For example, X ₁ (t) uses only t = τ + (k-1) T, and X ₂ (t) uses only the point of t = 2τ + (k-1) T for phase noise compensation. Then, as can be seen from the calculation of the gradual equation in the equation (7), the _{reference data of X N (t) used for compensation is X 1} (τ + (k-1) in the case of _{X 2} (t), for example. Calculated from two points, T) and X ₁ (2τ + (k-1) T), X ₃ (t) is X ₂ (2τ + (k-1) T) and X ₁ (3τ + (k-1) T). _{The values of X N} (Nτ + (k-1) T) at all N are calculated from the _{two points of X N-1} ((N-1) τ + (k-1) T). ) T) and X ₁ (Nτ + (k-1) T). This can be expressed by equation (8)

Is.

Therefore, for the signal sampled during the time interval T in which one pulse test light is incident, the data point of the reference beat signal used for phase noise compensation is X for X _{N at N with a natural number of 2 or more. There are only two points, N-1} ((N-1) τ + (k-1) T) and X ₁ (Nτ + (k-1) T). The number of sampling points of one scattered light waveform is 2LS / v (L: fiber length, v: speed of light), and if the sampling rate is equivalent to the delay τ as described above, S = 2 / W = 1 / τ. _{Therefore, when the number of X N} data handled by the equation (7) is calculated for all t, the X _N score calculated by the calculation is a matrix of 2 L / τv × N, which is a very large number. If only the points actually used are calculated, the amount of X ₁ is 2 (N-1) + 2L / τv, which is very small, considering that all 2L / τv is used.

Further, from the above, in the calculation of the phase of an arbitrary delay from the reference interferometer in the equations (7) and (8), even if the delay time τ due to the reference delay optical fiber is reduced to the minimum sampling time, the calculation is performed. The amount of data used in is very small, and it is an amount that can be calculated realistically with a general processor, and it is very effective in performing signal processing with a small calculation load in a vibration sensor that requires performance close to real time.

By the above method, the phase information superimposed on the beat signal of the scattered light and the local light can be compensated by using the phase information at an arbitrary delay time calculated from the reference beat signal, and as a result, the phase state of the scattered light can be highly accurate. It becomes possible to measure. Furthermore, by repeating the above measurement, it is measured how the phase of the scattered light at an arbitrary position on the optical fiber to be measured fluctuates with time in a state where the phase noise of the light source 1 that fluctuates with time is compensated. In other words, it is possible to analyze in detail the vibration applied to an arbitrary position on the optical fiber to be measured.

Further, although the frequency of the light source 1 was fixed at f in the above calculation, when this frequency drifts as a characteristic of the laser, it can be written as f (t) = f + δ (t). The drift amount δ (t) of this optical frequency means that the frequency fluctuates with the elapsed time of measurement. Even in this case, since the drift amount δ (t) can be included in the phase noise in the equation (3) representing the scattered light, as shown in FIG. 4, the reference beat signal acquired at the same time as the signal waveform It can be compensated by phase. That is, in the present invention, not only the phase noise of the light source 1 but also the drift phenomenon of the optical frequency of the light source 1 can be compensated at the same time.

The present invention is not limited to the above embodiment, and at the implementation stage, the components can be modified and embodied within a range that does not deviate from the gist thereof. For example, in this embodiment, the frequency modulation is transferred to direct current by the mixer 11 and the signal generator 12, but it may be processed by digital signal processing by a high-precision digitizer. When the mixer 11 is not used, equations (6) to (8) are

Then, the component of the _{modulation frequency f A} added by the optical frequency modulation unit 3 is included. However, since the modulation frequency f _A is a parameter known in advance, it can be easily removed by signal processing, and the result is the same as when the mixer 11 is used.

Further, when the mixer 11 is not used, the modulation frequency f _A remains in the signal received by the digitizing device 18, so the sampling rate may be set to _{2 f A} instead of 2 / W.

Further, in this embodiment, the heterodyne detection configuration in which the optical frequency modulation unit 3 is arranged in front of the optical branch 9 is used, but it may be in front of the optical branch 4, and in this case, the optical frequency on the test light side is shifted. However, the same result as in this embodiment can be obtained.

Further, in order to obtain the phase from the equation (5) and the equation (12), the coherent detection using the optical 90-degree hybrid is used in the optical receiving unit to receive the cos component and the sin component instead of performing the Hilbert transform. It may be shaped. In this case, since both cos and sin are confirmed before the digitizing device 18 receives the data, the Hilbert transform is unnecessary and the calculation process becomes lighter. However, in this case, noise due to optical control in coherent detection is added.

Since the phase noise compensation means in OFDR of Non-Patent Document 4 uses frequency sweep light, it is necessary to acquire a beat signal and a reference beat signal in a time range over the sweep time in order to calculate an arbitrary delay time by Fourier transform. Therefore, in order to compensate for the phase noise of the light source, it is necessary to perform the calculation process for compensation after the above measurement is completed, which is significantly different from the present embodiment. In other words, in the means of Non-Patent Document 4, all the reference beat signals sampled in the time range T over the sweep time are used for compensation, and this is temporally resampled for all the beat signals sampled in the same time range. In contrast, the present invention assumes that for a signal sampled during the time range T, the data point of the reference beat signal used for phase noise compensation has the measurement time of the beat signal I as Nτ. _{The processing itself is also different in that the value of the phase noise component X N} of the delay time Nτ (N: natural number) at that time Nτ is only one point, and the temporal resampling processing itself is unnecessary. Further, the means of Non-Patent Document 4 provides phase noise in an arbitrary delay time exceeding the coherence time of the light source (corresponding to a distant point of the optical fiber to be measured and smaller than the time range T over the wavelength sweep time). While the object of the present invention is to compensate and achieve a high distance resolution, the present invention is a vibration sensor, and an object of the present invention is to stably extract a phase state in the measurement elapsed time direction in which repeated measurements are performed. The purpose and means are also non-existent.

In addition, various inventions can be formed by an appropriate combination of a plurality of components disclosed in the above-described embodiment. For example, some components may be deleted from all the components shown in the embodiment. Further, the components of different embodiments may be combined as appropriate. The delay τ of the reference interferometer 8 may be wider than the resolution due to the pulse width, and the real-time property and simultaneity control can be freely set depending on when the numerical calculation is performed within the measurement time.

1 Light source 2, 4, 9 Branch 3 Optical frequency modulator 5 Optical intensity modulator 6 Optical circulator 7 Optical fiber to be measured 8 Reference interferometer 10, 15 Balanced photodetector 11, 16 Mixer 12 Signal generator 13, 17 Low frequency transmission Filter 14 Delayed optical fiber 18 Quantifier 19 Calculation processing unit

Claims (6)

A light source that outputs light with coherence,
A first optical branch that splits the light output from the light source into two,
A second optical branching portion that branches one of the branched lights branched at the first optical branching portion into two, and a second optical branching portion.
An optical intensity modulation unit that pulse-modulates one of the branched lights branched by the second optical branching unit,
An optical circulator that injects pulsed light output from the light intensity modulator into the optical fiber to be measured and extracts backscattered light from the optical fiber to be measured.
An optical frequency modulation unit that imparts an optical frequency shift to the other branched light branched by the first optical branching unit,
A third optical branching section that splits the frequency-modulated light output from the optical frequency modulation section into two, and a third optical branching section.
A delay circuit that imparts a time delay τ smaller than half of the coherence time of the light output from the light source to one of the branched lights output from the third optical branching portion.
The delay circuit and the first beat signal obtained by combining the backward scattered light and the other branch light output from the third optical branch portion, the other branch light output from the second optical branch portion, and the delay circuit. A signal processing unit that receives and digitizes the second beat signal that combines the light output from
The noise component of the phase of the first beat signal I (Nτ) at a time that is N times (N: natural number) of the time delay τ from the reference time is calculated from _{the phase X 1 of the second beat signal.}

And the phase noise compensator that compensates using
An optical fiber vibration detection sensor characterized by being equipped with. The optical fiber vibration detection sensor according to claim 1, wherein the Nτ is incident on the optical fiber to be measured and is substantially equal to a delay time until the backscattered light of the optical fiber to be measured is taken out. The optical fiber vibration detection sensor according to claim 1 or 2 , wherein the minimum sampling time in the digitization process is equal to the time delay τ of the delay circuit. Steps to output light with coherence and
The first optical branching step of bifurcating the output light having coherence, and
A second optical branching step in which one of the branched lights branched in the first optical branching step is branched into two, and a second optical branching step.
An optical intensity modulation step that pulse-modulates one of the branched lights branched in the second optical branching step,
A step of incidenting the pulsed light modulated in the light intensity modulation step into the optical fiber to be measured and extracting the backscattered light of the optical fiber to be measured.
An optical frequency modulation step that imparts an optical frequency shift to the other branched light branched in the first optical branching step,
A third optical branching step of bifurcating the frequency-modulated light modulated in the optical frequency modulation step,
A delay imparting step of imparting a time delay τ smaller than half of the coherence time of the light having coherence property to one of the branched lights branched in the third optical branching step,
The delay imparting the first beat signal obtained by combining the backscattered light and the other branched light branched in the third optical branching step, the other branched light branched in the second optical branching step, and the delay imparting. A signal processing step that receives a second beat signal that is combined with light to which a time delay τ is added in the step and digitizes it.
The noise component of the phase of the first beat signal I (Nτ) at a time that is N times (N: natural number) of the time delay τ from the reference time is calculated from _{the phase X 1 of the second beat signal.}

Compensation processing steps and compensation using
An optical fiber vibration detection method characterized by having. The optical fiber vibration detection method according to claim 4 , wherein the Nτ is incident on the optical fiber to be measured and is substantially equal to a delay time until the backscattered light of the optical fiber to be measured is taken out. The optical fiber vibration detection method according to claim 4 or 5 , wherein the minimum sampling time in the digitization process is equal to the time delay τ.

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