JP2020051941A - Optical fiber strain and temperature measuring device, and optical fiber strain and temperature measuring method - Google Patents

Abstract

To extend a measurement range without degrading detection sensitivity in a BOTDR using a self-delay heterodyne interferometer.SOLUTION: A measurement signal is bifurcated into a first and a second measurement signal, each sent to a first and a second BFS calculation unit 170, 270. A local oscillation signal is bifurcated into a first and a second local oscillation signal. The first local oscillation signal is sent to the first BFS calculation unit, and the second local oscillation signal is sent to the second BFS calculation unit after being phase shifted 90 degrees. The first BFS calculation unit calculates a first phase shift amount φ1 and a third phase shift amount φ3. The second BFS calculation unit calculates a second phase shift amount φ2 and a fourth phase shift amount φ4. A determination unit 80 determines whether φ3 and φ4 are larger than 3π/2, and adopts φ3 when larger, otherwise, determines whether φ2 is larger π and adopts φ2 when φ2 is larger, otherwise, adopts φ1.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an optical fiber strain and temperature measuring device and an optical fiber strain and temperature measuring method using Brillouin scattered light.

  With the development of optical fiber communication, distributed optical fiber sensing using the optical fiber itself as a sensing medium has been actively studied. In particular, optical fiber sensing using scattered light, unlike an electric sensor that measures point by point, can perform sensing as a long-distance distribution, and thus can measure the physical quantity of the entire measurement target.

  In distributed optical fiber sensing, a typical example is an optical time domain reflectometry (OTDR) in which an optical pulse is incident from one end of an optical fiber and light backscattered in the optical fiber is measured with respect to time. is there. Backscattering in optical fibers includes Rayleigh scattering, Brillouin scattering and Raman scattering. Among them, the one that measures natural Brillouin scattering is called BOTDR (Brillouin OTDR) (for example, see Non-Patent Document 1).

  Brillouin scattering is observed at a frequency shifted by about GHz to the Stokes side and the anti-Stokes side with respect to the center frequency of an optical pulse incident on an optical fiber, and the spectrum is called a Brillouin gain spectrum (BGS). be called. The BGS frequency shift and the spectral line width are called Brillouin frequency shift (BFS) and Brillouin line width, respectively. The BFS and Brillouin line width differ depending on the material of the optical fiber and the wavelength of the incident light. For example, in the case of a silica-based single mode optical fiber, it has been reported that the BFS size and Brillouin line width at a wavelength of 1.55 μm are about 11 GHz and about 30 MHz, respectively. In addition, according to Non-Patent Document 1, the magnitude of strain in a single mode fiber and the BFS due to a change in temperature are 0.049 MHz / με and 1.0 MHz / ° C. at a wavelength of 1.55 μm, respectively.

  Thus, BFS is dependent on strain and temperature. For this reason, BOTDR can be used for monitoring large buildings, such as bridges and tunnels, and places where landslides may occur, and has attracted attention.

  In BOTDR, in order to measure the spectrum waveform of natural Brillouin scattered light generated in an optical fiber, it is general to perform heterodyne detection with separately prepared reference light. The intensity of natural Brillouin scattered light is 2-3 orders of magnitude lower than the intensity of Rayleigh scattered light. For this reason, heterodyne detection is also useful for improving the minimum light receiving sensitivity.

  Here, the natural Brillouin scattered light is very weak, so that even if heterodyne detection is applied, a sufficient signal-to-noise ratio (S / N) cannot be secured. As a result, an averaging process for improving the S / N is required. In a conventional optical fiber distortion measuring device that performs BOTDR, three-dimensional information of time, amplitude, and frequency is acquired. However, it is difficult to reduce the measurement time because of the averaging process and the acquisition of this three-dimensional information.

  On the other hand, an optical fiber distortion measuring apparatus using natural Brillouin scattered light, which obtains two-dimensional information of time and phase by measuring a frequency change of light as a phase difference of a beat signal given by coherent detection An optical fiber strain measurement method has been proposed (for example, see Patent Document 1).

  With reference to FIG. 7, a description will be given of a basic configuration of a conventional optical fiber distortion measuring device that measures a frequency change of light as a phase difference of a beat signal given by coherent detection. FIG. 7 is a schematic block diagram of a conventional optical fiber strain measuring device.

  The optical fiber distortion measuring device includes a light source unit 10, a circulator 20, an optical amplifier 30, an optical bandpass filter 32, a self-delay heterodyne interferometer 41, and a timing controller 90.

  The light source unit 10 generates probe light. The light source unit 10 includes a light source 12 that generates continuous light and an optical pulse generator 14 that generates an optical pulse from the continuous light.

  The optical pulse generator 14 generates an optical pulse from the continuous light according to the electric pulse generated by the timing controller 90. This light pulse is output from the light source unit 10 as probe light.

  The probe light output from the light source unit 10 enters the measured optical fiber 100 via the circulator 20.

Backscattered light from the measured optical fiber 100 is sent to, for example, the optical amplifier 30 via the circulator 20. The backscattered light amplified by the optical amplifier 30 is sent to the optical bandpass filter 32. The optical bandpass filter 32 transmits only natural Brillouin scattered light. This natural Brillouin scattered light is sent to the self-delay heterodyne interferometer 41. The signal E ₀ (t) at time t of the natural Brillouin scattered light emitted from the optical bandpass filter 32 is represented by the following equation (1).

Here, A ₀ is the amplitude, η _B (t) is the Brillouin scattering coefficient, f _B (t) is the optical frequency of the Brillouin scattered light, and φ ₀ is the initial phase. Note that the Brillouin scattering coefficient η _B (t) and the optical frequency f _B (t) of the Brillouin scattered light are functions of time t because they change due to local strain or temperature change in the optical fiber.

  The self-delay heterodyne interferometer 41 includes a branching unit 42, an optical frequency shifter unit 43, a delay unit 48, a multiplexing unit 50, a coherent detection unit 60, a local electric signal source 83, and a BFS acquisition unit 70.

The local electric signal source 83 generates an electric signal of the frequency _fAOM .

  The branching unit 42 receives the backward Brillouin scattered light generated in the optical fiber 100 to be measured by the probe light through the optical bandpass filter 32, and bifurcates the light into the first optical path and the second optical path.

The optical frequency shifter 43 is provided on the first optical path. The optical frequency shifter unit 43 uses the electric signal of the frequency _fAOM generated by the local electric signal source 83 to give a frequency shift of the frequency _fAOM to the light propagating through the first optical path. Further, a delay unit 48 is provided in the second optical path. The delay unit 48 delays the light propagating through the second optical path by a time τ.

The multiplexing unit 50 multiplexes light propagating through the first optical path and the second optical path to generate multiplexed light. The optical signal E ₁ (t) propagating through the first optical path and the optical signal E ₂ (t−τ) propagating through the second optical path, which are incident on the multiplexing unit 50, are represented by the following equations (2) and (2), respectively. It is represented by 3).

Here, _{A 1} and _{A 2} is the amplitude of each _E 1 (t) and _{E 2 (t-τ),} φ 1 and phi ₂ are each _E 1 (t) and _E 2 _(t-τ) Is the initial phase.

The coherent detection unit 60 heterodyne-detects the combined light to generate a beat signal. The coherent detection unit 60 includes, for example, a balanced photodiode (PD) 62, an FET amplifier 64, and an analog-digital converter (A / D) 66. Beat signal I ₁₂ provided by heterodyne detection is expressed by the following equation (4).

Beat signal _{I 12} generated by the coherent detection unit 60 is sent to the BFS acquisition unit 70 as the measurement signal. The electric signal generated by the local electric signal source 83 is sent to the BFS acquisition unit 70 as a local signal. The BFS acquisition unit 70 includes a mixer 72, a low-pass filter (LPF) 74, and a BFS calculation unit 76.

The mixer 72 performs homodyne detection on the measurement signal and the local oscillation signal to generate a homodyne signal. The electric signal _IAOM generated by the local electric signal source 83 is represented by the following equation (5).

  The homodyne signal generated by the mixer 72 is represented by the following equation (6) obtained by multiplying the above equations (4) and (5).

  When the sum frequency component in the above equation (6) is removed by the LPF 74, a signal represented by the following equation (7) is obtained.

Since the φ ₁ −φ ₂ −φ _{AOM in the} above equation (7) and the delay time τ are constant, only a change in the Brillouin frequency f _B (t) is output as a difference in output intensity.

The Brillouin frequency f _B (t) changes due to two factors: fluctuation of the oscillation frequency of the light source 12 and distortion of the optical fiber 100 to be measured. However, by using a frequency-stabilized narrow line light source as the light source 12, the influence of distortion of the measured optical fiber 100 becomes dominant.

  FIGS. 8A and 8B are schematic diagrams showing the Brillouin shift and the phase change of the beat signal. FIG. 8A shows time t on the horizontal axis and frequency on the vertical axis. FIG. 8B shows time t on the horizontal axis and voltage on the vertical axis.

  The time on the horizontal axis indicates the location where Brillouin scattering has occurred. That is, when the backward Brillouin scattered light is incident after the elapse of the time t with respect to the time when the probe light is emitted, and the propagation speed of the light in the measured optical fiber is v, from the incident end of the measured optical fiber. This means that backward Brillouin scattering has occurred at the position of vt / 2.

In FIGS. 8 (A) and 8 (B), in the section corresponding to the time T from the time t ₁ to t _2, shows an example in which frequency shift occurs. At this time, since the delay time τ is given by the self-delay heterodyne interferometer, the phase change changes from t ₁ to t ₁ + τ, and returns to the original state from time t ₂ to t ₂ + τ. Return. That is, in order to measure the phase difference with the optical fiber distortion measuring device, it is necessary to satisfy the relationship of T ≧ τ, and the measurable time resolution (that is, the spatial resolution) is determined by τ.

  On the other hand, in the technique described in Patent Document 1, as can be seen from the above equation (7), the detection range is given by 0 to π in order to detect the phase shift amount. The conversion from the phase shift amount to the BFS depends on the delay time τ. Thus, the measurable frequency range (detection range) is also determined by the magnitude of τ.

  That is, as τ increases, the detection range decreases, but the spatial resolution increases. On the other hand, when τ decreases, the spatial resolution decreases, but the detection range increases. Thus, there is a trade-off between the delay time and the detection range.

  FIG. 9 is a diagram illustrating the relationship between the delay time and the measurable frequency. Here, the minimum detection sensitivity of the phase change is π / 1000, and the maximum value is π. Assuming that the delay time τ is 1 ns, the frequency measurement range is 1 MHz to 500 MHz. A 1 ns delay time [tau] corresponds to a spatial resolution of 20 cm, and a detection range of 1 MHz to 500 MHz corresponds to an optical fiber distortion of 0.002 to 1%. These values satisfy sufficient spatial resolution and measurement accuracy for measuring the strain of the optical fiber.

  According to the optical fiber distortion measuring device and the optical fiber distortion measuring method disclosed in Patent Document 1, the frequency of light is measured using a self-delay heterodyne type BOTDR (SDH-BOTDR: Self-Delayed Heterodyne BOTDR) technology. By measuring the change as a phase difference between beat signals given by coherent detection, two-dimensional information of time and phase is obtained. Since the SDH-BOTDR does not require a frequency sweep, the measurement time is shortened as compared with the related art which requires acquisition of three-dimensional information.

JP-A-2006-191659

T. Kurashima et al. , "Brillouin Optical-fiber time domain reflectometry", IEICE Trans. Commun. , Vol. E76-B, no. 4, pp. 382-390 (1993)

  Here, in the above-described conventional example, there is a relationship shown in FIG. 10 between the output intensity of the LPF 74 and the BFS. FIG. 10 is a diagram illustrating a relationship between the output intensity of the LPF 74 and the BFS. In FIG. 10, the horizontal axis indicates BFS [unit: MHz], and the vertical axis indicates the LPF output intensity in an arbitrary unit (au) normalized. As can be seen from FIG. 10, the behavior of the output intensity of the LPF 74 is inverted at the BFS of 500 MHz corresponding to the phase shift amount π. That is, the output intensity of the LPF 74 is line-symmetric at a BFS of 500 MHz. Therefore, even if BFS of 600 MHz is actually added, the measurement result is calculated to be 400 MHz.

  On the other hand, as described above, there is a trade-off relationship between the delay time τ and the measurable frequency (the amount of phase shift). Therefore, when the upper limit of the measurable frequency is increased, that is, when the measurement range is expanded, the minimum detectable frequency is also increased, and the detection sensitivity is deteriorated.

  The present invention has been made in view of the above problems. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical fiber distortion and temperature measuring apparatus and an optical fiber distortion and temperature measuring method for expanding a measurement range without deteriorating detection sensitivity in a BOTDR using a self-delay heterodyne interferometer. It is in.

  In order to achieve the above-described object, an optical fiber distortion and temperature measuring device according to the present invention includes a light source unit, a branch unit, an optical frequency shifter unit, a delay unit, a multiplexing unit, a coherent detection unit, It comprises a signal generation unit and a Brillouin frequency shift (BFS) acquisition unit.

  The light source generates probe light. The branching unit bifurcates the backward Brillouin scattered light generated in the optical fiber to be measured by the probe light into a first optical path and a second optical path. The optical frequency shifter is provided on one of the first optical path and the second optical path, and gives a frequency shift of the beat frequency. The delay unit is provided on one of the first optical path and the second optical path. The multiplexing unit multiplexes light propagating through the first optical path and the second optical path to generate multiplexed light. The coherent detection unit performs heterodyne detection of the combined light and outputs a difference frequency as a measurement signal. The electrical signal generation unit generates a local oscillation signal having the same frequency as the measurement signal. The BFS acquisition unit performs homodyne detection on the measurement signal and the local oscillation signal to acquire a frequency shift amount.

  The BFS acquisition unit includes a first BFS calculation unit, a second BFS calculation unit, a determination unit, and a 90 ° phase shift unit.

  The measurement signal sent to the BFS acquisition unit is split into two, and one of the two split first measurement signals is sent to the first BFS calculation unit, and the other second measurement signal is sent to the second BFS calculation unit.

  The local oscillation signal sent to the BFS acquisition unit is branched into two, and one of the two branched first local oscillation signals is sent to the first BFS calculation unit. The other second local oscillation signal is sent to the second BFS calculation section after being subjected to a 90 ° phase shift in the 90 ° phase shift section.

  The first BFS calculation unit is configured to determine, based on the first measurement signal and the first local oscillation signal, a first BFS corresponding to a first phase shift amount φ1 in a range of 0 to π, and a third BFS given by φ3 = 2π−φ1. A third BFS corresponding to the phase shift amount φ3 is obtained.

  The second BFS calculation unit calculates the second BFS corresponding to the second phase shift amount φ2 in the range of π / 2 to 3π / 2 and the second phase shift amount φ2 based on the second measurement signal and the second local oscillation signal. When it is not more than π, the fourth BFS corresponding to the fourth phase shift amount φ4 given by 3π-φ2 is obtained when the second phase shift amount φ2 is larger than π.

  The determining unit determines whether both the third phase shift amount φ3 and the fourth phase shift amount φ4 are greater than 3π / 2. As a result of the determination, when both the third phase shift amount φ3 and the fourth phase shift amount φ4 are larger than 3π / 2, the third BFS corresponding to the third phase shift amount φ3 is adopted.

  On the other hand, when either the third phase shift amount φ3 or the fourth phase shift amount φ4 is 3π / 2 or less, it is determined whether the second phase shift amount φ2 is larger than π. As a result of the determination, when the second phase shift amount φ2 is equal to or more than π, the second BFS corresponding to the second phase shift amount φ2 or the third BFS corresponding to the third phase shift amount φ3 is adopted. On the other hand, when the second phase shift amount φ2 is smaller than π, the first BFS corresponding to the first phase shift amount φ1 is adopted. Further, a BFS waveform is synthesized based on the first to third BFSs.

  According to another embodiment of the optical fiber distortion and temperature measurement device described above, the first BFS calculation unit corresponds to the phase shift amount φ1 in the range of 0 to π based on the first measurement signal and the first local oscillation signal. The second BFS calculation unit obtains a second BFS corresponding to a second phase shift amount φ2 in the range of π / 2 to 3π / 2 based on the second measurement signal and the second local oscillation signal. I do.

  The determination unit determines whether the second phase shift amount φ2 is greater than π, and when the second phase shift amount φ2 is equal to or greater than π, employs a second BFS corresponding to the second phase shift amount φ2, When the second phase shift amount φ2 is smaller than π, the first BFS corresponding to the first phase shift amount φ1 is adopted, and a BFS waveform is synthesized based on the first and second BFSs.

  An optical fiber strain and temperature measuring method according to the present invention includes the following steps. First, probe light is generated. Next, backward Brillouin scattered light generated in the optical fiber to be measured by the probe light is split into two light paths, a first optical path and a second optical path. Next, a frequency shift of a beat frequency is given to light propagating in one of the first optical path and the second optical path. In addition, a delay is given to light propagating in one of the first optical path and the second optical path. Next, the light propagating through the first optical path and the second optical path are multiplexed to generate multiplexed light. Next, the combined light is heterodyne-detected and the difference frequency is output as a measurement signal. Next, a local oscillation signal having the same frequency as the measurement signal is generated. Next, the measurement signal and the local oscillation signal are subjected to homodyne detection to obtain a frequency shift amount.

  The step of acquiring the frequency shift amount further includes the following steps. First, the measurement signal is branched into a first measurement signal and a second measurement signal. Further, the local oscillator signal is branched into a first local oscillator signal and a second local oscillator signal.

  Next, based on the first measurement signal and the first local oscillation signal, a first BFS corresponding to a first phase shift φ1 in the range of 0 to π, and a third phase shift φ3 given by φ3 = 2π−φ1 To obtain a third BFS corresponding to. After giving the phase shift of 90 ° to the second local signal, the second BFS and the fourth BFS are acquired based on the second measurement signal and the second local signal. The second BFS corresponds to a second phase shift amount φ2 in the range of π / 2 to 3π / 2. The fourth BFS corresponds to the fourth phase shift amount φ4. The fourth phase shift amount φ4 is given by π−φ2 when the second phase shift amount φ2 is equal to or smaller than π, and is given by 3π−φ2 when the second phase shift amount φ2 is larger than π.

  Next, it is determined whether both the third phase shift amount φ3 and the fourth phase shift amount φ4 are greater than 3π / 2, and both the third phase shift amount φ3 and the fourth phase shift amount φ4 are greater than 3π / 2. If larger, the third BFS corresponding to the third phase shift amount φ3 is adopted.

  On the other hand, when either the third phase shift amount φ3 or the fourth phase shift amount φ4 is 3π / 2 or less, it is further determined whether or not the second phase shift amount φ2 is larger than π. When the second phase shift amount φ2 is equal to or more than π, the second BFS corresponding to the second phase shift amount φ2 or the third BFS corresponding to the third phase shift amount φ3 is adopted, and the second phase shift amount φ2 is larger than π. If smaller, the first BFS corresponding to the first phase shift amount φ1 is adopted. Thereafter, a BFS waveform is synthesized based on the first to third BFSs.

  According to another embodiment of the optical fiber strain and temperature measurement method described above, the step of obtaining the frequency shift amount includes the following steps.

  First, the measurement signal is branched into a first measurement signal and a second measurement signal. Further, the local oscillator signal is branched into a first local oscillator signal and a second local oscillator signal.

  Next, a first BFS corresponding to a phase shift amount φ1 in the range of 0 to π is obtained based on the first measurement signal and the first local oscillation signal. Further, after giving a phase shift of 90 ° to the second local oscillation signal, the second local oscillation signal corresponds to the second phase shift amount φ2 in the range of π / 2 to 3π / 2 based on the second measurement signal and the second local oscillation signal. To obtain a second BFS.

  Next, it is determined whether the second phase shift amount φ2 is greater than π. If the second phase shift amount φ2 is greater than or equal to π, a second BFS corresponding to the second phase shift amount φ2 is adopted, When the two phase shift amounts φ2 are smaller than π, the first BFS corresponding to the first phase shift amount φ1 is adopted. Thereafter, a BFS waveform is synthesized based on the first and second BFSs.

  According to the optical fiber strain and temperature measuring apparatus and the optical fiber strain and temperature measuring method of the present invention, the measuring range can be expanded 1.5 to 2 times as compared with the conventional technology.

FIG. 3 is a schematic block diagram illustrating a configuration example of a measurement device. It is a typical block diagram of a BFS acquisition part with which a measuring device is provided. FIG. 6 is a diagram illustrating a relationship between output intensity of a first LPF and a second LPF and BFS. FIG. 3 is a diagram (1) illustrating a processing flow of a determination unit; It is a figure showing an example of the BFS waveform of the 1st-the 4th BFS. FIG. 6 is a diagram (2) illustrating a processing flow of a determination unit; It is a typical block diagram of the conventional optical fiber strain measuring device. FIG. 4 is a schematic diagram showing a Brillouin shift and a phase change of a beat signal. FIG. 3 is a diagram illustrating a relationship between a delay time and a measurable frequency. FIG. 6 is a diagram illustrating a relationship between the output intensity of the LPF and the BFS.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the drawings are merely schematic diagrams to the extent that the present invention can be understood. Hereinafter, a preferred configuration example of the present invention will be described, but it is merely a preferred example. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the present invention.

(Configuration example)
An optical fiber strain and temperature measuring device according to the present invention will be described with reference to FIGS. FIG. 1 is a schematic block diagram showing a configuration example of an optical fiber strain and temperature measuring device (hereinafter, also simply referred to as a measuring device) of the present invention. FIG. 2 is a schematic block diagram of a BFS acquisition unit provided in the measurement device.

  The measuring device includes a light source unit 10, a circulator 20, an optical amplifier 30, an optical bandpass filter 32, a self-delay heterodyne interferometer 41, and a timing controller 90.

  The light source unit 10 generates probe light. The light source unit 10 includes a light source 12 that generates continuous light and an optical pulse generator 14 that generates an optical pulse from the continuous light.

  Here, the measuring device measures a phase difference according to a frequency change. For this reason, the frequency fluctuation and the frequency spectrum line width (hereinafter, also simply referred to as line width) of the light source 12 must be sufficiently smaller than the Brillouin shift. Therefore, a frequency-stabilized narrow linewidth light source is used as the light source 12. For example, when the distortion of an optical fiber to be measured (hereinafter, also referred to as an optical fiber to be measured) 100 is 0.008%, the Brillouin shift corresponds to 4 MHz. Therefore, in order to measure a distortion of about 0.008%, it is desirable that the frequency fluctuation and the line width of the light source 12 be sufficiently smaller than 4 MHz and not more than several tens kHz. Note that a narrow line width laser having a frequency fluctuation and a line width of about 10 kHz or less is generally available as an off-the-shelf product.

  The optical pulse generator 14 is configured using any suitable conventionally known acoustic optical (AO) modulator or electric optical (EO) modulator. The optical pulse generator 14 generates an optical pulse from the continuous light according to the electric pulse generated by the timing controller 90. The repetition period of the optical pulse is set longer than the time required for the optical pulse to reciprocate in the optical fiber under test 100. This light pulse is output from the light source unit 10 as probe light.

  The probe light output from the light source unit 10 enters the measured optical fiber 100 via the circulator 20. Note that an optical coupler may be used instead of the circulator 20.

The backscattered light from the measured optical fiber 100 is sent through the circulator 20 to an optical amplifier 30 composed of, for example, an erbium-doped optical fiber amplifier (EDFA). The backscattered light amplified by the optical amplifier 30 is sent to the optical bandpass filter 32. The optical bandpass filter 32 has a transmission band of about 10 GHz and transmits only natural Brillouin scattered light. This natural Brillouin scattered light is sent to the self-delay heterodyne interferometer 41. The signal E ₀ (t) at time t of the natural Brillouin scattered light emitted from the optical bandpass filter 32 is represented by the above equation (1).

Here, A ₀ is the amplitude, η _B (t) is the Brillouin scattering coefficient, f _B (t) is the optical frequency of the Brillouin scattered light, and φ ₀ is the initial phase. The Brillouin scattering coefficient η _B (t) and the optical frequency f _B (t) of the Brillouin scattered light are functions of time t because they change due to local strain or temperature change in the optical fiber.

  The self-delay heterodyne interferometer 41 includes a branching unit 42, an optical frequency shifter unit 43, a delay unit 48, a multiplexing unit 50, a coherent detection unit 60, a local electric signal source 83 as an electric signal generation unit, and a BFS acquisition unit 71. It is configured with.

The local electric signal source 83 generates an electric signal of the frequency _fAOM .

  The branching unit 42 receives the backward Brillouin scattered light generated in the optical fiber 100 to be measured by the probe light through the optical bandpass filter 32, and bifurcates the light into the first optical path and the second optical path.

In this configuration example, the optical frequency shifter unit 43 is provided on the first optical path. The optical frequency shifter unit 43 uses the electric signal of the frequency _fAOM generated by the local electric signal source 83 to give a frequency shift of the frequency _fAOM to the light propagating through the first optical path.

  Further, in this configuration example, a delay unit 48 is provided in the second optical path. The delay unit 48 delays the light propagating through the second optical path by a time τ.

The multiplexing unit 50 multiplexes light propagating through the first optical path and the second optical path to generate multiplexed light. The optical signal E ₁ (t) propagating through the first optical path and the optical signal E ₂ (t−τ) propagating through the second optical path, which are incident on the multiplexing unit 50, are expressed by the above equations (2) and (3), respectively. ).

The coherent detection unit 60 heterodyne-detects the combined light to generate a beat signal. The coherent detector 60 includes, for example, a balanced photodiode (PD) 62, an FET amplifier 64, and an A / D 66. Beat signal I ₁₂ provided by heterodyne detection is represented by the above formula (4).

Beat signal _{I 12} generated by the coherent detection unit 60 is sent to the BFS acquisition unit 71 as the measurement signal. The electric signal generated by the local electric signal source 83 is sent to the BFS acquisition unit 71 as a local signal.

  The BFS acquisition unit 71 includes a first BFS calculation unit 170, a second BFS calculation unit 270, a determination unit 80, and a 90 ° phase shift unit 78.

  The measurement signal sent to the BFS acquisition unit 71 is branched into two. One of the two first branched measurement signals is sent to first BFS calculation section 170. The other second measurement signal is sent to second BFS calculation section 270.

  The local oscillation signal sent to the BFS acquisition unit 71 is branched into two. One of the two first branched local oscillation signals is sent to the first BFS calculation section 170. The other second local oscillation signal is sent to the second BFS calculation section 270 after having undergone a 90 ° phase shift in the 90 ° phase shift section 78. When the local oscillation signal is a cosine wave, the first local oscillation signal is sent to the first BFS calculation section 170 as a cos wave, and the second local oscillation signal is sent to the second BFS calculation section 270 as a sin wave.

  The first BFS calculation section 170 includes a first mixer 172, a first LPF 174, and a first BFS calculation means 176. Similarly, the second BFS calculator 270 includes a second mixer 272, a second LPF 274, and a second BFS calculator 276.

The first mixer 172 performs homodyne detection on the first measurement signal and the first local oscillation signal to generate a homodyne signal. A station power generation gas source 83 in the generated local oscillation signal I _AOM represented by the above formula (5).

  The homodyne signal generated by the first mixer 172 is represented by the above equation (6) obtained by multiplying the above equations (4) and (5).

  When the sum frequency component in the above equation (6) is removed by the first LPF 174, a signal represented by the above equation (7) is obtained.

  The second mixer 272 and the second LPF 274 also operate similarly to the first mixer 172 and the first LPF 174 except that when the first local oscillation signal is a cos wave, the second local oscillation signal is a sine wave. Therefore, the description is omitted.

  The first BFS calculation unit 176 acquires two BFSs from the output intensity of the first LPF 174. Further, the second BFS calculation means 276 acquires two BFSs from the output intensity of the second LPF 274. In this example, the frequency measurement range of the measurement device is 1 to 1000 [MHz]. This frequency measurement range corresponds to a phase shift amount of 0 to 2π. For example, frequencies of 250 [MHz], 500 [MHz], 750 [MHz] and 1000 [MHz] correspond to π / 2, π, 3π / 2 and 2π, respectively.

  The relationship between the output intensities of the first LPF 174 and the second LPF 274 and the BFS is shown in FIG. FIG. 3 is a diagram illustrating the relationship between the output intensity of the first LPF 174 and the second LPF 274 and the BFS. FIG. 3A is a diagram illustrating a relationship between the output intensity of the first LPF 174 and the BFS, in which BFS [unit: MHz] is shown on the horizontal axis, and the output intensity of the first LPF 174 is normalized on the vertical axis. It is shown in arbitrary units (au). FIG. 3B is a diagram showing the relationship between the output intensity of the second LPF 274 and the BFS, in which BFS [unit: MHz] is shown on the horizontal axis, and the output intensity of the second LPF 274 is standardized on the vertical axis. It is shown in arbitrary units (au).

  The first BFS calculating means 176 calculates a first BFS corresponding to the first phase shift amount φ1 in the range of 0 or more and π or less. Next, the first BFS calculating means 176 calculates a third BFS corresponding to the third phase shift amount φ3 (= 2π−φ1).

  The second BFS calculation means 276 acquires a second BFS corresponding to the second phase shift amount φ2 in a range from π / 2 to 3π / 2. Next, when the second phase shift amount is equal to or less than π, the second BFS calculation unit 276 acquires a fourth BFS corresponding to the fourth phase shift amount φ4 (= π−φ2). On the other hand, when the second phase shift amount φ2 is larger than π, the second BFS calculation means 276 acquires the fourth BFS corresponding to the fourth phase shift amount φ4 (= 3π−φ2).

  The first to fourth BFSs are sent to the determination unit 80. The processing of the determination unit 80 will be described with reference to FIG. FIG. 4 is a diagram illustrating a processing flow of the determination unit 80. The determination unit 80 can be configured using, for example, a commercially available personal computer on which software for executing the processing described below is installed.

  In step (hereinafter, step is referred to as S) 102, determination section 80 acquires first to fourth BFS from first BFS calculation section 170 and second BFS calculation section 270.

  Next, the determination unit 80 determines in S112 whether the third phase shift amount φ3 corresponding to the third BFS and the fourth phase shift amount φ4 corresponding to the fourth BFS are both greater than 3π / 2. It is determined whether the third BFS and the fourth BFS are larger than 750 MHz. When both the third BFS and the fourth BFS are larger than 750 MHz (Yes), the third BFS is adopted as the BFS in S114.

  If either the third phase shift amount φ3 or the fourth phase shift amount φ4 is 3π / 2 or less, and in this example, the third BFS and the fourth BFS are 750 MHz or less (No), then in S122, the second It is determined whether or not the phase shift amount φ2 is greater than π, in this example, whether or not the second BFS is greater than 500 MHz. If the second BFS is larger than 500 MHz, the second BFS is adopted as the BFS in S124. On the other hand, when the second BFS is equal to or lower than 500 MHz, the first BFS is adopted as the BFS in S126. In S124, the third BFS may be adopted as the BFS.

  Thereafter, in S130, a BFS waveform is synthesized from the first to third BFSs adopted in S114, S124 and S126. After obtaining the BFS waveform, strain or temperature is obtained by a conventionally known method.

  In the conventional measuring device, one value obtained from one BFS calculation unit is used as BFS. For this reason, the measuring range of the conventional measuring device is 1 to 500 MHz corresponding to a phase shift of 0 to π. On the other hand, in this measuring device, BFS is obtained from the four values obtained from the first and second BFS calculators. As a result, the measurement range of this measuring device is 1 to 1000 MHz corresponding to a phase shift of 0 to 2π, which is twice that of the conventional measuring device.

  With reference to FIG. 5, an example in which a 1000 MHz BFS occurs at a certain position of the optical fiber 100 will be described. FIG. 5 is a diagram illustrating an example of the BFS waveforms of the first to fourth BFSs. 5A to 5D respectively show the BFS waveforms of the first to fourth BFSs by solid lines. 5A to 5D, the horizontal axis represents the position [m] of the optical fiber, and the vertical axis represents the BFS size [MHz]. 5A to 5D, the BFS waveforms to be combined are indicated by dotted lines.

  Until the value of the generated BFS reaches 500 MHz, the first BFS is adopted by the processes of S112, S122 and S126. When the generated BFS exceeds 500 MHz, the second BFS is adopted through steps S112, S122, and S124 until it reaches 750 MHz. Further, when the generated BFS exceeds 750 MHz, the third BFS is adopted in steps S112 and S114. When the generated BFS becomes smaller than 1000 MHz, the third BFS is adopted in steps S112 and S114 until the frequency reaches 750 MHz. When the BFS becomes 750 MHz or less, the second BFS is adopted in steps S112, S122 and S124, and further, 500 MHz. , The first BFS is adopted by the processes of S112, S122 and S126.

(Other configuration examples)
In the above-described measuring device, the first BFS calculator 170 and the second BFS calculator 270 calculate two BFSs, respectively. On the other hand, in the measurement device of another configuration example, the first BFS calculation unit 170 obtains one BFS, that is, only the first BFS, and the second BFS calculation unit 270 obtains one BFS, that is, only the second BFS. .

  The first and second BFSs are sent to the determination unit 80. The processing of the determination unit 80 will be described with reference to FIG. FIG. 6 is a diagram illustrating a processing flow of the determination unit.

  In S102, the determination unit 80 acquires the first and second BFSs from the first BFS calculation unit 170 and the second BFS calculation unit 270.

  Next, in S122, the determination unit 80 determines whether the second phase shift amount φ2 is larger than π, in this example, whether the second BFS is larger than 500 MHz. If the second BFS is larger than 500 MHz, the second BFS is adopted as the BFS in S124.

  On the other hand, when the second BFS is equal to or smaller than π, the first BFS is adopted as the BFS in S126. In S124, the third BFS may be adopted as the BFS.

  Thereafter, in S131, a BFS waveform is synthesized from the first and second BFSs adopted in S122 and S124.

  As in the first measuring device, the first BFS is a value in the range of 0 to π, and the second BFS is a value in the range of π / 2 to 3π / 2. Since both the first BFS and the second BFS are used, in this configuration example, the measurement range is 0 to 3π / 2, which is 1.5 times that of the related art.

  In this configuration example, the measurement range is smaller than the measurement range in the processing described with reference to FIG. 4, but is wider than the measurement range of the related art. Also, the number of BFSs output from the first BFS calculation unit and the second BFS calculation unit is half that of the process described with reference to FIG. Further, the number of times of determination by the determination unit is one compared to two times of the processing described with reference to FIG. From these points, the load on the BFS acquisition unit is lighter than the processing described with reference to FIG.

Reference Signs List 10 light source unit 20 circulator 30 optical amplifier 32 optical bandpass filter 41 self-delay heterodyne interferometer 42 branch unit 43 optical frequency shifter unit 48 delay unit 50 multiplexing unit 60 coherent detection unit 62 balanced PD
64 FET amplifier 66 A / D
70, 71 BFS acquisition unit 72, 172, 272 Mixer 74, 174, 274 Low-pass filter (LPF)
76, 176, 276 BFS calculation means 78 90 ° phase shift unit 80 determination unit 83 local electric signal source 90 timing controller 170, 270 BFS calculation unit

Claims (4)

A light source unit for generating probe light,
A branching unit that branches back Brillouin scattered light generated in an optical fiber to be measured by the probe light into a first optical path and a second optical path,
An optical frequency shifter unit that is provided in one of the first optical path and the second optical path and that gives a frequency shift of a beat frequency;
A delay unit provided in one of the first optical path and the second optical path;
A multiplexing unit that multiplexes light propagating through the first optical path and the second optical path to generate multiplexed light;
A coherent detection unit that heterodyne-detects the combined light and outputs a difference frequency as a measurement signal;
An electric signal generation unit that generates a local oscillation signal having the same frequency as the measurement signal,
The homodyne detection of the measurement signal and the local oscillation signal, comprising a Brillouin frequency shift acquisition unit for acquiring a frequency shift amount,
The Brillouin frequency shift acquisition unit includes a first Brillouin frequency shift calculation unit, a second Brillouin frequency shift calculation unit, a determination unit and a 90 ° phase shift unit,
The measurement signal sent to the Brillouin frequency shift acquisition unit is split into two, one of the two branched first measurement signals is sent to the first Brillouin frequency shift calculation unit, and the other second measurement signal is the second measurement signal. Sent to the Brillouin frequency shift calculator,
The local oscillation signal sent to the Brillouin frequency shift obtaining unit is branched into two, and one of the two branched first local oscillation signals is sent to the first Brillouin frequency shift calculating unit. The other second local oscillation signal is sent to the second Brillouin frequency shift calculator after being subjected to a 90 ° phase shift by the 90 ° phase shifter,
The first Brillouin frequency shift calculator calculates a first Brillouin frequency shift corresponding to a first phase shift amount φ1 in a range of 0 to π based on the first measurement signal and the first local oscillation signal, and φ3 = Obtain a third Brillouin frequency shift corresponding to a third phase shift amount φ3 given by 2π−φ1,
The second Brillouin frequency shift calculator calculates a second Brillouin frequency shift corresponding to a second phase shift amount φ2 in a range of π / 2 to 3π / 2 based on the second measurement signal and the second local oscillation signal. When the second phase shift amount φ2 is equal to or less than π, it corresponds to π−φ2, and when the second phase shift amount φ2 is larger than π, it corresponds to the fourth phase shift amount φ4 given by 3π−φ2. To obtain the fourth Brillouin frequency shift
The determination unit includes:
It is determined whether both the third phase shift amount φ3 and the fourth phase shift amount φ4 are greater than 3π / 2, and if the third phase shift amount φ3 and the fourth phase shift amount φ4 are both greater than 3π / 2, , A third Brillouin frequency shift corresponding to the third phase shift amount φ3,
When either the third phase shift amount φ3 or the fourth phase shift amount φ4 is 3π / 2 or less, it is determined whether the second phase shift amount φ2 is larger than π, and the second phase shift amount φ2 is set to π. In the above case, the second Brillouin frequency shift corresponding to the second phase shift amount φ2 or the third Brillouin frequency shift corresponding to the third phase shift amount φ3 is adopted, and the second phase shift amount φ2 is smaller than π. Adopts a first Brillouin frequency shift corresponding to the first phase shift amount φ1,
An optical fiber distortion and temperature measuring device, wherein a Brillouin frequency shift waveform is synthesized based on first to third Brillouin frequency shifts. A light source unit for generating probe light,
A branching unit that branches back Brillouin scattered light generated in an optical fiber to be measured by the probe light into a first optical path and a second optical path,
An optical frequency shifter unit that is provided in one of the first optical path and the second optical path and that gives a frequency shift of a beat frequency;
A delay unit provided in one of the first optical path and the second optical path;
A multiplexing unit that multiplexes light propagating through the first optical path and the second optical path to generate multiplexed light;
A coherent detection unit that heterodyne-detects the combined light and outputs a difference frequency as a measurement signal;
An electric signal generation unit that generates a local oscillation signal having the same frequency as the measurement signal,
The homodyne detection of the measurement signal and the local oscillation signal, comprising a Brillouin frequency shift acquisition unit for acquiring a frequency shift amount,
The Brillouin frequency shift acquisition unit includes a first Brillouin frequency shift calculation unit, a second Brillouin frequency shift calculation unit, a determination unit and a 90 ° phase shift unit,
The measurement signal sent to the Brillouin frequency shift acquisition unit is split into two, one of the two branched first measurement signals is sent to the first Brillouin frequency shift calculation unit, and the other second measurement signal is the second measurement signal. Sent to the Brillouin frequency shift calculator,
The local oscillation signal sent to the Brillouin frequency shift obtaining unit is branched into two, and one of the two branched first local oscillation signals is sent to the first Brillouin frequency shift calculating unit. The other second local oscillation signal is sent to the second Brillouin frequency shift calculator after being subjected to a 90 ° phase shift by the 90 ° phase shifter,
The first Brillouin frequency shift calculation unit acquires a first Brillouin frequency shift corresponding to a phase shift amount φ1 in a range of 0 to π based on the first measurement signal and the first local oscillation signal,
The second Brillouin frequency shift calculator calculates a second Brillouin frequency shift corresponding to a second phase shift amount φ2 in a range of π / 2 to 3π / 2 based on the second measurement signal and the second local oscillation signal. And get
The determination unit includes:
It is determined whether the second phase shift amount φ2 is greater than π, and if the second phase shift amount φ2 is equal to or greater than π, a second Brillouin frequency shift corresponding to the second phase shift amount φ2 is adopted. When the two phase shift amount φ2 is smaller than π, the first Brillouin frequency shift corresponding to the first phase shift amount φ1 is adopted,
An optical fiber distortion and temperature measuring device, wherein a Brillouin frequency shift waveform is synthesized based on first and second Brillouin frequency shifts. Generating a probe light;
A process of splitting backward Brillouin scattered light generated in the optical fiber to be measured by the probe light into a first optical path and a second optical path,
Providing a frequency shift of a beat frequency to light propagating in one of the first optical path and the second optical path;
Delaying light propagating in one of the first optical path and the second optical path;
Combining the light propagating through the first optical path and the second optical path to generate a combined light;
A step of heterodyne detecting the combined light and outputting a difference frequency as a measurement signal;
Generating a local oscillator signal having the same frequency as the measurement signal;
A step of homodyne detection of the measurement signal and the local oscillation signal to obtain a frequency shift amount,
The step of obtaining the frequency shift amount includes:
Bifurcating the measurement signal into a first measurement signal and a second measurement signal;
Bifurcating the local oscillation signal into a first local oscillation signal and a second local oscillation signal;
A first Brillouin frequency shift corresponding to a first phase shift amount φ1 in a range of 0 to π based on the first measurement signal and the first local oscillation signal, and a third phase shift given by φ3 = 2π−φ1 Obtaining a third Brillouin frequency shift corresponding to the quantity φ3;
After giving a phase shift of 90 ° to the second local oscillation signal, a second phase shift amount φ2 in the range of π / 2 to 3π / 2 is determined based on the second measurement signal and the second local oscillation signal. The corresponding second Brillouin frequency shift and, when the second phase shift amount φ2 is equal to or less than π, are given by π−φ2, and when the second phase shift amount φ2 is greater than π, the third is given by 3π−φ2. Obtaining a fourth Brillouin frequency shift corresponding to the four phase shift amount φ4;
It is determined whether both the third phase shift amount φ3 and the fourth phase shift amount φ4 are greater than 3π / 2, and if the third phase shift amount φ3 and the fourth phase shift amount φ4 are both greater than 3π / 2, , A third Brillouin frequency shift corresponding to the third phase shift amount φ3,
When either the third phase shift amount φ3 or the fourth phase shift amount φ4 is 3π / 2 or less, it is determined whether the second phase shift amount φ2 is larger than π, and the second phase shift amount φ2 is set to π. In the above case, the second Brillouin frequency shift corresponding to the second phase shift amount φ2 or the third Brillouin frequency shift corresponding to the third phase shift amount φ3 is adopted,
When the second phase shift amount φ2 is smaller than π, a process of employing the first Brillouin frequency shift corresponding to the first phase shift amount φ1 and a process of synthesizing a Brillouin frequency shift waveform based on the first to third Brillouin frequency shifts An optical fiber strain and temperature measurement method, comprising: Generating a probe light;
A process of splitting backward Brillouin scattered light generated in the optical fiber to be measured by the probe light into a first optical path and a second optical path,
Providing a frequency shift of a beat frequency to light propagating in one of the first optical path and the second optical path;
Delaying light propagating in one of the first optical path and the second optical path;
Combining the light propagating through the first optical path and the second optical path to generate a combined light;
A step of heterodyne detecting the combined light and outputting a difference frequency as a measurement signal;
Generating a local oscillator signal having the same frequency as the first electrical signal;
Homodyne detection of the measurement signal and the second electric signal to obtain a frequency shift amount,
The step of obtaining the frequency shift amount includes:
Bifurcating the measurement signal into a first measurement signal and a second measurement signal;
Bifurcating the local oscillation signal into a first local oscillation signal and a second local oscillation signal;
A step of obtaining a first Brillouin frequency shift corresponding to a phase shift amount φ1 in a range of 0 to π based on the first measurement signal and the first local oscillation signal;
After giving a 90 ° phase shift to the second local signal, the second local signal corresponds to the second phase shift amount φ2 in the range of π / 2 to 3π / 2 based on the second measurement signal and the second local signal. Obtaining a second Brillouin frequency shift,
It is determined whether the second phase shift amount φ2 is greater than π. If the second phase shift amount φ2 is greater than or equal to π, a second Brillouin frequency shift corresponding to the second phase shift amount φ2 is adopted. When the two phase shift amount φ2 is smaller than π, a process of employing the first Brillouin frequency shift corresponding to the first phase shift amount φ1 and a process of synthesizing a Brillouin frequency shift waveform based on the first and second Brillouin frequency shifts An optical fiber strain and temperature measuring method, comprising:

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