JP7003807B2 - Optical fiber strain and temperature measuring device and optical fiber strain and temperature measuring method - Google Patents

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, optical fiber sensing using the optical fiber itself as a sensing medium is being actively researched. In particular, optical fiber sensing using scattered light enables long-distance distributed sensing, unlike the case of using an electric sensor that measures point by point. Therefore, the physical quantity of the entire measurement target can be measured.

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

Brillouin scattering is observed at a position shifted by about GHz to the Stokes side and the anti-Stokes side with respect to the center frequency of the optical pulse incident on the optical fiber, and the spectrum is the Brillouin Gain Spectrum (BGS). Called. The BGS frequency shift and spectral line width are referred to as Brillouin Frequency Shift (BFS) and Brillouin line width, respectively. The BFS and Brillouin line widths vary depending on the material of the optical fiber and the incident light wavelength. For example, in the case of a quartz-based single-mode optical fiber, it is known that the size of BFS and the Brillouen line width at a wavelength of 1.55 μm are about 11 GHz and about 30 MHz, respectively. Further, from Non-Patent Document 1, the magnitudes of BFS due to distortion and temperature change in the single mode fiber are 0.049 MHz / με and 1.0 MHz / ° C., respectively, at a wavelength of 1.55 μm.

In this way, since BFS is dependent on strain and temperature, BOTDR is attracting attention as it can be used for monitoring large buildings such as bridges and tunnels, and places where landslides may occur. Has been done.

As a BOTDR, a self-delayed heterodyne type BOTDR (SDH-BOTDR: Self-Delayed Heterodyne BOTDR) has been proposed (see, for example, Patent Document 1). In SDH-BOTDR, two-dimensional information of time and phase is acquired by measuring the change of BFS as the phase difference of the beat signal given by coherent detection. Therefore, SDH-BOTDR does not require frequency sweeping, and can realize an inexpensive measuring instrument capable of high-speed measurement.

Here, not only in BOTDR but also in distributed optical fiber sensing using Brillouin scattering, BFS occurs for both strain and temperature as described above. Therefore, it is an essential task to distinguish between strain and temperature. To solve this problem, a method using a strain-dependent coefficient and a temperature-dependent coefficient of rear Brillouin scattering in an optical fiber has been proposed (see, for example, Non-Patent Document 2 or 3).

It has been reported that in backward Brillouin scattering, not only the frequency shift but also the scattering coefficient has temperature and strain dependence. If the strain dependence coefficient and temperature dependence coefficient of BFS are C _νε and C _νT , respectively, and the strain dependence coefficient and temperature dependence coefficient of _Brilluan scattering coefficient are CPε and _CPT , respectively, these coefficients are measured in advance. By solving the binary simultaneous equations (1) and (2) shown in the equation, strain and temperature can be separated.

Here, δν _B and δP _B are the rate of change of BFS and Brillouin scattering intensity, respectively. These δν _B and δP _B are values measured by BOTDR. Further, δε and δT are strains and temperature changes, respectively.

By solving the above binary simultaneous equations (1) and (2), the following equations (3) and (4) are obtained.

The waveform measured by the BOTDR device is BGS. Therefore, the magnitude of BFS is calculated from the peak frequency of BGS, and the intensity change rate is calculated from the peak frequency intensity of BGS. The strain and temperature changes _δε and δT can be obtained separately from these two measurement results and the four coefficients C _νε , C _νT , _CPε and CPT measured in advance.

With reference to FIG. 8, the separation of the strain change amount δε and the temperature change amount δT in the conventional BOTDR apparatus will be described. FIG. 8 is a schematic diagram for explaining the separation of the strain change amount δε and the temperature change amount δT in the conventional BOTDR apparatus. In FIGS. 8A to 8D, the horizontal axis shows the distance [unit: m] from one end of the optical fiber. Further, in FIGS. 8A and 8D, the temperature change amount δT [unit: ° C.] and the strain change amount δε [unit: με] are shown on the left axis and the right axis, respectively, and FIG. In FIG. 8C, BFS [unit: MHz] is shown on the vertical axis, and the intensity change rate δP _B [unit:%] is shown on the vertical axis.

FIG. 8A is a diagram showing strain changes and temperature changes applied to the optical fiber. In FIG. 8A, the temperature change is shown by the solid line I, and the strain change is shown by the dotted line II. Here, in a 1 km optical fiber, a temperature change of 100 ° C. in a section of 200 to 250 m from one end where an optical pulse is incident (I in FIG. 8 (A)) and a strain change of 1000 με in a section of 400 to 450 m (FIG. It is assumed that a temperature change of 50 ° C. and a strain change of 800 με are applied to the section of II) of 8 (A), 600 to 650 m.

FIG. 8B is a diagram showing the measurement result of BFS, and FIG. 8C is a diagram showing the measurement result of the intensity change rate. Further, FIG. 8D shows strain and temperature changes δε and δT separated from the measurement results shown in FIGS. 8B and 8C using the above equations (3) and (4). It is a figure. In this way, the strain change (II in FIG. 8D) and the temperature change (I in FIG. 8D) can be clearly separated and obtained from the BGS measured by the BOTDR apparatus.

Japanese Unexamined Patent Publication No. 2001-165808

T. Kurashima et al. , "Brillouin Optical-fiber time domain reflectometry", IEICE Trans. Commun. , Vol. E76-B, no. 4, pp. 382-390 (1993) T. R. Parker et al. , "Simultaneus distributed measurement of stripe and temperature from noise-initiated Brillouin scattering in optical fiber", IEEE J. Quantum Electron. , Vol. 34, No. 4, pp. 645-659 (1998) Y. Sakairi et al. , "Asystem for measuring temperature and stripe separation by BOTDR and OTDR", Proceeding of SPIE, vol. 4920, pp. 274-284 (2002)

In the separation method described above, only strain changes and temperature changes in the optical fiber are considered. However, in an actual measurement environment, the strength changes due to the connection of an optical fiber connector or the connection of another type of optical fiber.

With reference to FIG. 9, the separation of the strain change amount δε and the temperature change amount δT when a loss occurs due to the connector connection will be described. FIG. 9 is a schematic diagram for explaining the separation of the strain change amount δε and the temperature change amount δT when a loss occurs due to the connector connection. FIG. 9A is a diagram showing the measurement result of the intensity change rate. Further, FIG. 9B shows strain and temperature changes δε and δT separated from the measurement results shown in FIGS. 8B and 9A using the above equations (3) and (4). It is a figure which shows.

In FIGS. 9A and 9B, the horizontal axis shows the distance [unit: m] from one end of the optical fiber. In FIG. 9A, the vertical axis shows the intensity change rate δP _B [unit:%]. Further, in FIG. 9B, the temperature change amount δT [unit: ° C.] and the strain change amount δε [unit: με] are shown on the left axis and the right axis, respectively.

Here, in addition to the strain change amount and the temperature change amount given to the optical fiber described with reference to FIG. 8A, a loss of about 0.2 dB due to the connector connection at a point of 300 m in the optical fiber is lost. It shall have occurred.

When the strength loss occurs due to the connector connection, as shown in FIG. 9A, the influence of the strength loss is superimposed on the strength change rate after 300 m as an offset. As a result, as shown in FIG. 9B, it can be seen that the temperature (I) after separation deviates from the true value of about 15 ° C. and the strain (II) deviates from the true value of about 260 με.

The present invention has been made in view of the above-mentioned problems. An object of the present invention is an optical fiber strain and temperature measuring device and an optical fiber capable of eliminating the influence of an optical fiber connector connection, a strength loss due to a connection of another type of optical fiber, etc. on the measured intensity change rate. To provide a strain and temperature measuring method.

In order to achieve the above-mentioned object, the optical fiber strain and temperature measuring device of the present invention includes a light source unit, a branch unit, an optical frequency shifter unit, a delay unit, a combiner unit, a coherent detection unit, and electricity. It is configured to include a signal generation unit, a frequency shift amount acquisition unit, a signal strength acquisition unit, and a signal processing unit.

The light source unit generates probe light. The probe light is incident on the optical fiber to be measured (measured optical fiber). The branching portion bifurcates the rear Brillouin scattered light generated in the optical fiber to be measured by the probe light into the first optical path and the second optical path. The optical frequency shifter portion is provided in either the first optical path or the second optical path, and gives a frequency shift of the beat frequency. The delay portion is provided in either one of the first optical path and the second optical path, and gives a delay time difference between the light propagating in the first optical path and the second optical path. The combined wave unit generates combined wave light by combining the light propagating in the first optical path and the second optical path. The coherent detection unit performs heterodyne detection of the combined light and outputs the difference frequency as the first electric signal. The electric signal generation unit generates a second electric signal having the same frequency as the first electric signal. The frequency shift amount acquisition unit acquires the frequency shift amount by homodyne detection of one of the two branches of the first electric signal and the second electric signal. This first electric signal is a so-called beat signal. The signal strength acquisition unit generates information on the strength of the first electric signal as a strength signal.

The signal processing unit includes a reference intensity change rate acquisition unit, an intensity change rate acquisition unit, a subtraction unit, and a strain / temperature separation unit. The reference intensity change rate acquisition means acquires information on the intensity change rate in the initial state as the reference intensity change rate from the intensity information obtained by the signal intensity acquisition unit. The intensity change rate acquisition means acquires the intensity change rate information from the intensity information obtained by the signal intensity acquisition unit. The subtracting means subtracts the reference intensity change rate from the intensity change rate acquired by the intensity change rate acquisition means to acquire the offset removal intensity change rate. The strain / temperature separation means separates and acquires the strain change amount δε and the temperature change amount δT from the frequency shift amount acquired by the frequency shift amount acquisition unit and the offset removal intensity change rate.

Further, according to another preferred embodiment of the optical fiber strain and temperature measuring device, the first optical frequency shifter portion and the second optical frequency shifter portion are provided in place of the optical frequency shifter portion.

The first optical frequency shifter portion is provided in the first optical path and provides a frequency shift of the first frequency. The second optical frequency shifter portion is provided in the second optical path and provides a frequency shift of the second frequency. In this case, the second electric signal is generated as a difference frequency between the first frequency and the second frequency, so that it is a so-called beat signal.

Further, according to another preferred embodiment of the optical fiber strain and temperature measuring device, the light source unit, the branch unit, the self-delayed homodyne interferometer, the signal strength acquisition unit, and the signal processing unit are provided. ..

The light source unit generates probe light. The branching portion bifurcates the rear Brillouin scattered light generated in the optical fiber to be measured by the probe light. In the self-delayed homodyne interferometer, one of the scattered lights branched into two at the branch portion is input, and an interference signal is generated by self-delayed homodyne interference. The signal intensity acquisition unit acquires the intensity of the scattered light by inputting the other scattered light branched into two at the branching unit. The signal processing unit separately acquires the strain change amount δε and the temperature change amount δT from the frequency shift amount indicated by the intensity of the interference signal and the offset removal intensity change rate. Here, in the self-delayed homodyne interferometer, the phase of one of the two-branched scattered light can be changed.

Further, the optical fiber strain and temperature measuring method of the present invention is configured to include the following processes.

First, probe light is generated. The probe light is incident on the optical fiber to be measured. Next, the rear Brillouin scattered light generated in the optical fiber to be measured by the probe light is bifurcated into the first optical path and the second optical path. Next, a frequency shift of the beat frequency is given to the light propagating in either the first optical path or the second optical path. Next, a delay time difference is given between the light propagating in the first optical path and the second optical path. Next, the light propagating in the first optical path and the second optical path is combined to generate combined light. Next, the combined wave light is heterodyne-detected and the difference frequency is output as the first electric signal. Next, a second electric signal having the same frequency as the first electric signal is generated. Next, one of the two branches of the first electric signal and the second electric signal are homodyne-detected to obtain the frequency shift amount. In addition, information on the strength of the first electric signal is generated as a strength signal.

Next, the information on the intensity change rate is acquired from the intensity information, and the reference intensity change rate acquired in advance is subtracted from the intensity change rate to acquire the offset removal intensity change rate. The strain change amount δε and the temperature change amount δT are separately obtained from the frequency shift amount and the offset removal intensity change rate.

Further, according to another preferred embodiment of the optical fiber strain and temperature measuring method, the light propagating in the first optical path is given a frequency shift of the first frequency, and the light propagating in the second optical path is subjected to the frequency shift. The frequency shift of the second frequency is given.

In carrying out this optical fiber strain and temperature measuring method, preferably, the strain change amount δε and the temperature change amount δT are obtained as a frequency shift amount δν _B and an intensity change rate δP _B in the optical fiber. From the distortion-dependent coefficient C _{ν ε} of the frequency shift of the rear Brillouin scattering, the temperature-dependent coefficient C _{ν T} , and the distortion-dependent coefficient C P ε of the scattering coefficient of the rear _Brillouin scattering, and the temperature-dependent coefficient C _PT , the above binary simultaneous equation (1) and Obtained by solving (2).

According to the optical fiber strain and temperature measuring device and the optical fiber strain and temperature measuring method of the present invention, the optical fiber connector connection is performed by subtracting the reference intensity change rate acquired in advance from the measured intensity change rate. Alternatively, the effect of strength loss not caused by strain change or temperature change due to the connection of another type of optical fiber can be offset, and the strain change amount and temperature change amount can be obtained separately.

It is a schematic block diagram of the 1st measuring device. It is a schematic block diagram of the signal processing unit included in the 1st measuring device. It is a schematic diagram explaining the operation of the signal processing unit included in the 1st measuring device. It is a schematic block diagram of the 2nd measuring device. It is a schematic block diagram of the 3rd measuring device. It is a schematic diagram of a self-delayed homodyne interferometer. It is a schematic diagram explaining the standardization performed in the optical fiber strain and the temperature measuring apparatus. It is a schematic diagram for demonstrating the separation of the strain change amount δε and the temperature change amount δT in the conventional BOTDR apparatus. It is a schematic diagram for demonstrating the separation of a strain change amount δε and a temperature change amount δT when a loss occurs by a connector connection.

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

(First Embodiment)
The optical fiber strain and temperature measuring device (hereinafter, also referred to as the first measuring device) of the first embodiment will be described with reference to FIGS. 1 to 3. FIG. 1 is a schematic block diagram of the first measuring device. FIG. 2 is a schematic block diagram of a signal processing unit included in the first measuring device. FIG. 3 is a schematic diagram illustrating the operation of the signal processing unit included in the first measuring device.

The first measuring device includes a light source unit 10, a circulator 20, an optical amplifier 30, an optical bandpass filter 32, a self-delayed heterodyne interferometer 40, a signal strength acquisition unit 80, a timing controller 88, and a signal processing unit 90. To.

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 light pulses from continuous light.

Here, the first measuring device measures the phase difference according to the frequency change. Therefore, 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 Brillouen shift. Therefore, a frequency-stabilized narrow line width light source is used as the light source 12. For example, when the strain of the optical fiber to be measured (hereinafter, also referred to as the optical fiber to be measured) 100 is 0.008%, the Brillouin shift corresponds to 4 MHz. Therefore, in order to measure the distortion of about 0.008%, it is desirable that the frequency fluctuation and the line width of the light source 12 are sufficiently smaller than 4 MHz and not more than several tens of kHz. 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: Acoustic Optical) modulator or electrooptic (EO: Electrical Optical) modulator. The optical pulse generator 14 generates an optical pulse from continuous light in response to an electric pulse generated by the timing controller 88. The repetition period of this optical pulse is set longer than the time required for the optical pulse to reciprocate in the optical fiber 100 to be measured. This optical pulse is output from the light source unit 10 as probe light.

The probe light output from the light source unit 10 passes through the circulator 20 and is incident on the optical fiber 100 to be measured. An optical coupler may be used instead of the circulator 20.

The backscattered light from the optical fiber 100 to be measured is sent to an optical amplifier 30 composed of, for example, an erbium-added optical fiber amplifier (EDFA) 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 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-delayed heterodyne interferometer 40.

The self-delayed heterodyne interferometer 40 includes a branching section 42, an optical frequency shifter section 43, a delay section 46, a combiner section 48, a coherent detection section 50, and an electrical signal generation section 70.

The local oscillator electric signal source 72 of the electric signal generation unit 70 generates an electric signal of frequency f _AOM .

The branching portion 42 receives the rear Brillouin scattered light generated in the optical fiber 100 to be measured by the probe light through the optical bandpass filter 32, and branches into two optical paths, the first optical path and the second optical path.

The optical frequency shifter portion 43 is provided in the first optical path. The optical frequency shifter unit 43 uses the electric signal of the frequency f _AOM generated by the local electric signal source 72 to give a frequency shift of the frequency f _AOM to the light propagating in the first optical path.

Further, in this configuration example, the delay portion 46 is provided in the second optical path. The delay unit 46 gives a delay of time τ to the light propagating in the second optical path.

The combined wave unit 48 combines the light propagating in the first optical path and the second optical path to generate combined wave light.

The coherent detection unit 50 performs heterodyne detection of the combined wave light to generate a beat signal. The coherent detection unit 50 includes, for example, a balanced photodiode (PD) 52 and a FET amplifier 54.

The beat signal generated by the coherent detection unit 50 is branched into two, one is sent to the mixer unit 56 as a first electric signal, and the other is sent to the signal strength acquisition unit 80. Further, the electric signal generated by the local oscillator electric signal source 72 is sent to the mixer unit 56 as a second electric signal.

The mixer unit 56 homodyne detects the first electric signal and the second electric signal to generate a homodyne signal. The homodyne signal generated by the mixer unit 56 is sent to the signal processing unit 90 via the low-pass filter (LPF) 58. Here, the mixer unit 56 and the LPF 58 form a frequency shift amount acquisition unit. From the frequency shift amount acquisition unit, δν _B in the above equation (1) can be obtained.

The signal strength acquisition unit 80 includes a square circuit 82, an LPF 84, and a 1/2 square circuit 86. The signal strength acquisition unit 80 realizes the function of envelope detection of the beat signal, which is the output of the coherent detection unit 50. As a result, when the beat signal passes through the squared circuit 82, the LPF84, and the 1/2 squared circuit 86 in order, the strength information of the beat signal is obtained. That is, δP _B in the above equation (2) can be obtained from the squared circuit 82, the LPF84, and the 1/2 squared circuit 86. The strength information of the beat signal is sent to the signal processing unit 90.

The signal processing unit 90 includes, for example, a CPU (Central Processing Unit) 190, a ROM (Read Only Memory) 92, a RAM (Random Access Memory) 94, and a storage unit 96. By executing the program stored in the ROM 92 by the CPU 190, the reference intensity change rate acquisition unit 192, the intensity change rate acquisition unit 194, the subtraction unit 196, and the strain / temperature separation unit 198 are realized. The processing result of each functional means is stored in the RAM 94.

The reference intensity change rate acquisition unit 192 acquires information on the intensity change rate in the initial state from the beat signal intensity information obtained by the signal intensity acquisition unit 80 as the reference intensity change rate 98 and stores it in the storage unit 96. (Dotted line II in FIG. 3 (A)). Here, the initial state is, for example, a state in which no strain or temperature change is given at the time of laying the optical fiber.

Similar to the reference intensity change rate acquisition means 192, the intensity change rate acquisition means 194 acquires information on the measured intensity change rate in the used state from the beat signal intensity information obtained by the signal intensity acquisition unit 80 (FIG. 3 (FIG. 3). A) Solid line in I). Here, the usage state is, for example, a state in which a strain change or a temperature change can be given to the optical fiber.

The subtracting means 196 subtracts the reference intensity change rate from the measured intensity change rate acquired by the intensity change rate acquiring means 194, and acquires the offset removal intensity change rate. As a result, it is possible to offset the influence of the strength loss that is not caused by the strain change or the temperature change due to the connection of the optical fiber connector or the connection of another type of optical fiber, which is superimposed on the measured intensity change rate as an offset. As a result, only the intensity change rate derived from the strain change and the temperature change is extracted (FIG. 3 (B)).

The strain / temperature separating means 198 separates and acquires the strain change amount and the temperature change amount from the frequency shift amount acquired by the frequency shift amount acquisition unit and the offset removal intensity change rate obtained by the subtraction means 196. The process of the strain / temperature separating means 198 corresponds to the process of solving the above two simultaneous equations (1) and (2) to obtain the above equations (3) and (4).

In the above-mentioned first measuring device, the influence of the strength loss not caused by the strain change or the temperature change due to the connection of the optical fiber connector or the connection of another type of optical fiber is offset, and the strain change amount and the temperature change amount are separated. Can be obtained.

(Second Embodiment)
The optical fiber strain and temperature measuring device (hereinafter, also referred to as a second measuring device) of the second embodiment will be described with reference to FIG. In the second measuring device, the first optical frequency shifter portion 44 is provided in the first optical path of the self-delayed heterodyne interferometer 41. Further, a second optical frequency shifter portion 45 and a delay portion 46 are provided in the second optical path.

The electric signal generation unit 71 includes a first local oscillator electric signal source 73, a second local oscillator electric signal source 74, a mixer unit 75, and a low-pass filter (LPF) 76. The first local oscillator electric signal source 73 and the second local oscillator electric signal source 74 may be outside the electric signal generation unit 71. The first local oscillator electric signal source 73 generates an electric signal having a _first frequency f1. The second local oscillator electric signal source 74 generates an electric signal having a _second frequency f2. The mixer unit 75 generates a sum frequency component and a difference frequency component of the first frequency f ₁ and the second frequency f ₂ from the electric signal of the first frequency f ₁ and the electric signal of the second frequency f ₂ . The LPF 76 outputs a beat signal of the difference frequency component f _AOM (= f ₁ − f ₂ ) from the signal generated by the mixer unit 75.

The first optical frequency shifter portion 44 is provided in the first optical path. The first optical frequency shifter unit 44 uses the electric signal of the _first frequency f1 generated by the first local oscillator electric signal source 73, and has the first frequency f1 with respect to the light propagating in the _first optical path. Gives a frequency shift.

The second optical frequency shifter portion 45 is provided in the second optical path. The second optical frequency shifter unit 45 uses the electric signal of the _second frequency f2 generated by the second local oscillator electric signal source 74, and has a second frequency f2 with respect to the light propagating in the _second optical path. Gives a frequency shift.

The second measuring device is different from the first measuring device in that the self-delayed heterodyne interferometer 41 is provided with an optical frequency shifter unit in both the first optical path and the second optical path, and the configuration of the electric signal generation unit 71 is different from that of the first measuring device. .. Since other configurations are the same as those of the first measuring device, overlapping description will be omitted.

Since the first measuring device has one optical frequency shifter unit and one local oscillator electric signal source, it is advantageous in terms of manufacturing cost as compared with the second measuring device. On the other hand, in the second measuring device, since the frequencies of the two lights combined in the combined wave portion are close to each other, more accurate measurement can be performed from the viewpoint of performing homodyne detection.

(Third Embodiment)
The optical fiber strain and temperature measuring device (hereinafter, also referred to as a third measuring device) of the third embodiment will be described with reference to FIG. FIG. 5 is a schematic block diagram of the third measuring device.

The third measuring device includes a light source unit 10, an optical circulator 20, an optical amplifier 30, an optical bandpass filter 32, a branch unit 34, a self-delayed homodyne interferometer 140, a signal strength acquisition unit 180, a signal processing unit 90, and a timing controller 88. Is configured with.

Since the configuration and operation from the light source unit 10 to the optical bandpass filter 32 are the same as those of the first measuring device, overlapping description will be omitted.

The optical bandpass filter 32 transmits only natural Brillouin scattered light among the backscattered light. This natural Brillouin scattered light is sent to the branch portion 34.

The branching unit 34 branches the natural Brillouin scattered light into two, one of which is sent to the self-delayed homodyne interferometer 140 and the other of which is sent to the signal strength acquisition unit 180.

The self-delayed homodyne interferometer 140 includes an interferometer branching section 142, a delay adjusting section 144, an interferometer interferometer section 146, and a coherent detection section 50. The natural Brillouin scattered light sent to the self-delayed homodyne interferometer 140 is bifurcated into a first optical path and a second optical path at the interferometer branching portion 142. A delay adjusting unit 144 is provided in the first optical path. The light sent to the first optical path is sent to the interferometer combiner unit 146 via the delay adjusting unit 144. The light sent to the second optical path is sent to the interferometer combiner unit 146 as it is.

The interferometer combiner unit 146 combines the light received through the first optical path and the second optical path to generate interference light, and sends the interference light to the coherent detection unit 50.

The coherent detection unit 50 includes a balanced photodiode (PD) 52 and a FET amplifier 54. The interference light is converted into an electric signal by the balanced PD 52 and then amplified by the FET amplifier 54. The amplified electric signal is sent to the signal processing unit 90 as an interference signal. The frequency shift amount is acquired from this interference signal.

The signal intensity acquisition unit 180 includes a delay path 182 for intensity measurement and a scattered light receiving unit 184. The scattered light that has been bifurcated by the branching unit 34 and sent to the signal intensity acquisition unit 180 receives a predetermined delay in the intensity measurement delay path 182 and is then sent to the scattered light receiving unit 184. The scattered light receiving unit 184 is composed of, for example, a PD186 and a FET amplifier 188. The scattered light is converted into an electric signal by the PD186 and then amplified by the FET amplifier 188. The amplified electric signal is sent to the signal processing unit 90 as intensity information.

The signal processing unit 90 is configured in the same manner as the first measuring device.

Here, in order to prevent the influence of the polarization fluctuation, the self-delayed homodyne interferometer 140 is preferably configured by a so-called spatial coupling system. FIG. 6 is a schematic diagram of a self-delayed homodyne interferometer composed of a spatial coupling system.

The self-delaying homodyne interferometer 140 includes, for example, first and second half mirrors 152 and 156, first and second mirrors 154 and 155, and a phase control element 153.

The first half mirror 152, the first mirror 154, the second mirror 155, and the second half mirror 156 are arranged, for example, at the vertices of a rectangle. The light input to the first half mirror 152 is split into two by the first half mirror 152. One split into two is sent to the second half mirror 156, and the other is sent to the second half mirror 156 via the first mirror 154 and the second mirror 155. The light input to the second half mirror 156 is sent to the coherent detection unit 50.

Further, the phase control element 153 is provided on an optical path connecting the first half mirror 152, the first mirror 154, the second mirror 155, and the second half mirror 156.

In this case, the first half mirror 152 functions as an interferometer branching unit 142, and the second half mirror 156 functions as an interferometer combiner unit 146. Further, the first mirror 154, the second mirror 155, and the phase control element 153 function as the delay adjusting unit 144.

When the side connecting the first half mirror 152 and the second half mirror 156 is the first side and the side connecting the first mirror 154 and the second mirror 155 is the second side, the second side is used. By moving the light in a direction orthogonal to the first side, the optical path length of the light propagating in the first optical path, that is, the delay amount is changed. Further, in the phase control element 153, for example, the phase, that is, a minute delay amount changes depending on the electric signal from the signal processing unit 90.

Next, the normalization performed in the optical fiber strain and temperature measuring device will be described with reference to FIG. 7. Here, the signal after homodyne detection is a direct current (DC) signal, and the initial phase that serves as a reference for the amount of phase change is unknown. Therefore, when looking at the intensity of the section in which BFS occurs (the section shown by II in FIG. 7A), the reference section in which BFS does not occur (the section shown by I in FIG. 7A). The magnitude and direction of change are not determined with respect to the strength. Therefore, in order to calculate the frequency shift amount, standardization processing is required. This standardization process is performed as follows, for example.

First, the voltage of the phase control element 153 is changed so that the phase is swept from 0 to 2π, the average intensity of the reference section in which BFS does not occur is measured, and the minimum value (Min) and the maximum value (Max) of the average intensity are measured. ) (See FIG. 7 (B)).

Next, the voltage applied to the phase control element 153 is adjusted so that the average intensity of the reference section becomes an intermediate value (Mid = (Max + Min) / 2) between the acquired minimum value and the maximum value.

The intensity P of the section where BFS is generated is standardized by the following formula.

Pn = (P-Mid) / (Max-Min)
Pn indicates the strength after normalization. From the intensity Pn after this normalization, the frequency shift amount of BFS can be obtained.

Here, the absolute value of Pn indicates the magnitude of the frequency shift due to distortion or temperature change. Further, the sign of Pn indicates whether the strain is in the compression direction or the tension direction, or the temperature change is in the ascending direction or the decreasing direction.

Further, here, the average intensity of the reference section is used as an intermediate value, but the value is not limited to this. The average intensity can be set to any suitable reference value between the minimum and maximum values.

For example, if the temperature change in the range of -250 ° C to + 250 ° C can be measured when the average intensity of the reference section is set as the median value, the average intensity of the reference section should be lower than the median value. Therefore, for example, the temperature change in the range of −100 ° C. to + 400 ° C. can be set to be measurable. Also, Max-Min corresponds to a temperature range of 500 ° C.

The signal processing unit can acquire the BFS from the interference signal, and can separately acquire the strain change amount and the temperature change amount as in the first measuring device. Since the configuration and operation of the reference intensity change rate acquisition unit 192, the intensity change rate acquisition unit 194, the subtraction unit 196, and the strain / temperature separation unit 198 provided in the signal processing unit 90 are the same as those of the first measuring device, overlapping description is omitted. do.

Since this third measuring device performs measurement by a self-delayed homodyne interferometer, it can be expected to have an effect of improving the signal-to-noise ratio (S / N) of 3 dB as compared with the measurement by a heterodyne interferometer. In addition, since the self-delayed homodyne interferometer can reduce the number of devices such as frequency shifters, it is also effective in reducing the size and cost of the device.

10 Light source 20 Circulator 30 Optical amplifier 32 Optical band path filter 34, 42 Branch 40, 41 Self-delayed heterodyne interferometer 43 Optical frequency shifter 44 1st optical frequency shifter 45 2nd optical frequency shifter 46 Delay 48 Wave part 50 Coherent detection part 52 Balanced PD
54 FET amplifier 56,75 Mixer section 58,76,84 Low-pass filter (LPF)
70, 71 Electric signal generator 72 Local oscillator electric signal source 73 1st local oscillator electric signal source 74 2nd local oscillator electric signal source 80 Signal strength acquisition unit 82 Squared circuit 86 1/2 power circuit 88 Timing controller 90 Signal Processing unit

Claims (5)

A light source that generates probe light and
A branch portion that bifurcates the rear Brillouin scattered light generated in the optical fiber to be measured by the probe light, and
A self-delayed homodyne interferometer that generates an interference signal by inputting one of the scattered lights branched into two at the branch portion.
A signal intensity acquisition unit that acquires the intensity of the scattered light by inputting the other scattered light that has been branched into two at the branch unit.
A signal processing unit having a reference intensity change rate acquisition means, an intensity change rate acquisition means, a subtraction means, and a strain / temperature separation means is provided.
The self-delayed homodyne interferometer
An interferometer branch that splits the input scattered light into the first and second optical paths,
A delay adjusting unit provided in the first optical path and capable of changing the phase of scattered light in response to an instruction from the signal processing unit.
An interferometer combiner that generates interference light by combining the light received through the first optical path and the second optical path, and
It is provided with an interference light receiving unit that converts the interference light into an electric signal and generates an interference signal.
The reference intensity change rate acquisition means obtains the intensity information obtained by the signal intensity acquisition unit in the initial state in the longitudinal direction of the optical fiber based on the intensity of scattered light at the input end of the optical fiber. The rate of change in intensity at each point along the line is obtained as the reference intensity change rate.
The intensity change rate acquisition means is along the longitudinal direction of the optical fiber based on the intensity of scattered light at the input end of the optical fiber from the intensity information obtained by the signal intensity acquisition unit in the used state. The rate of change in intensity at each point is obtained as the rate of change in intensity .
The subtracting means subtracts the reference intensity change rate from the intensity change rate acquired by the intensity change rate acquisition means to obtain the offset removal intensity change rate.
The strain / temperature separating means acquires a frequency shift amount from the interference signal, and separates and acquires a strain change amount δε and a temperature change amount δT from the frequency shift amount and the offset removal intensity change rate.
The signal processing unit
An instruction is sent to the delay adjusting unit to change the phase of the scattered light propagating in the first optical path so as to sweep from 0 to 2π.
Measure the average intensity of the reference interval where the Brillouan frequency shift does not occur in each phase,
Obtain the minimum and maximum values of the average intensity,
The phase of the light propagating in the first optical path is set so that the average intensity becomes a reference value between the minimum value and the maximum value, and the intensity of the interference signal is determined by using the minimum value and the maximum value. An optical fiber strain and temperature measuring device characterized by standardization. The optical fiber strain and temperature measuring device according to claim 1 , wherein the self-delayed homodyne interferometer is composed of a space-coupled system. The signal processing unit
The frequency shift amount δν _B and the intensity change rate δP _B , the strain-dependent coefficient C _νε of the frequency shift of the rear Brillouin scattering in the optical fiber, the temperature-dependent coefficient C _νT , and the scattering of the rear Brillouin scattering obtained in advance. By solving the following binary simultaneous equations (1) and (2) from the strain-dependent coefficient _CPε and the temperature-dependent coefficient _CPT of the coefficient, the strain change amount δε and the temperature change amount δT in the optical fiber are obtained. The optical fiber strain and temperature measuring apparatus according to claim 1 or 2 .

The process of generating probe light and
The process of bifurcating the rear Brillouin scattered light generated in the optical fiber to be measured by the probe light, and
The process of bifurcating one of the two branched light paths into the first optical path and the second optical path,
The process of delaying the scattered light propagating in the first optical path and
A process of combining scattered light propagating in the first optical path and the second optical path to generate interference light.
The process of generating an interference signal, which is an electric signal, by photoelectric conversion of the interference light, and
The process of acquiring the frequency shift amount from the strength of the interference signal and
The process of acquiring the intensity of the scattered light from the other scattered light in which the rear Brillouin scattered light is branched into two, and
The process of acquiring information on the rate of change in intensity and
The process of acquiring the offset removal intensity change rate by subtracting the reference intensity change rate acquired in advance from the intensity change rate, and
A process of separating and acquiring the strain change amount δε and the temperature change amount δT from the frequency shift amount and the offset removal intensity change rate is provided.
The reference intensity change rate is the intensity at each point along the longitudinal direction of the optical fiber, based on the intensity of scattered light at the input end of the optical fiber, based on the intensity information acquired in the initial state. Obtained as a rate of change,
The intensity change rate is a change in intensity at each point along the longitudinal direction of the optical fiber, based on the intensity of scattered light at the input end of the optical fiber, based on the intensity information acquired in the used state. Obtained as a percentage of
Moreover,
The process of changing the phase of the scattered light propagating in the first optical path so as to sweep from 0 to 2π and measuring the average intensity of the reference section in which the Brillouin frequency shift does not occur in each phase.
The process of acquiring the minimum and maximum values of the average intensity and
A process of setting the phase of light propagating in the first optical path so that the average intensity becomes a reference value between the minimum value and the maximum value is provided.
A method for measuring optical fiber strain and temperature, characterized in that, in the process of acquiring the frequency shift amount, the intensity of the interference signal is standardized by using the minimum value and the maximum value. The strain change amount δε and the temperature change amount δT are the frequency shift amount δν _B and the intensity change rate δP _B , and the strain dependence coefficient C _νε of the frequency shift of the rear Brillouin scattering in the optical fiber, which has been obtained in advance. It is characterized by being obtained by solving the following binary simultaneous equations (1) and (2) from the temperature-dependent coefficient C _νT , the distortion-dependent coefficient C _Pε of the scattering coefficient of backward Brillouin scattering, and the temperature-dependent coefficient C _PT . The optical fiber strain and temperature measuring method according to claim 4 .

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