JP2022040695A - Optical fiber strain measurement device and brillouin frequency shift offset adjustment method - Google Patents

Abstract

To enable measurement of a correct BFS by automatically changing a BFS offset in a BOTDR device.SOLUTION: An optical fiber strain measurement device comprises: a light source section for generating probe light; a wavelength control section for receiving an input of back-scattered light to be generated by the probe light in an optical fiber to be a measurement object and outputting a back Brillouin scattered light included in the back-scattered light; a self-delay interferometer for generating a measurement signal to which the back Brillouin scattered light is input; and a signal processing section. The signal processing section comprises: means for generating measurement result data including optical fiber strain data from the measurement signal; and means for changing a BFS offset amount when a standard deviation of a BFS exceeds a threshold and also when signal loss of an area where the standard deviation exceeds the threshold is equal to or less than allowable loss.SELECTED DRAWING: Figure 2

Description

The present invention relates to an optical fiber strain measuring device using backward Brillouin scattered light and a method for adjusting a Brillouin frequency shift offset performed by the optical fiber strain measuring device.

With the development of optical fiber communication, distributed optical fiber sensing using the optical fiber itself as a sensing medium is being actively researched. In particular, optical fiber sensing using scattered light can measure the physical quantity of the entire object to be measured because it can be sensed as a long-distance distribution, unlike an electric sensor that measures point by point.

In distributed optical fiber sensing, optical pulse (probe light) is incident from one end of the optical fiber, and the light scattered backward in the optical fiber is measured with respect to the elapsed time from the probe light incident. : Optical Time Domain Reflectometry) is typical.

Backscattering in an optical fiber includes Rayleigh scattering, Brillouin scattering and Raman scattering. Those that use natural Brillouin scattering as the measurement target of OTDR are called Brillouin OTDR (BOTDR: Brillouin OTDR). In the following description, natural Brillouin scattering may be simply referred to as Brillouin scattering.

Brillouin scattering is observed as a frequency shifted by several GHz to the Stokes side and the anti-Stokes side with respect to the center frequency of the probe light incident on the optical fiber. The spectrum of Brillouin scattering is called the Brillouin Gain Spectrum (BGS). The frequency shift amount and spectral line width of BGS are Brillouin frequency shift (BFS), respectively.
It is called Shift) and Brillouen line width. The BFS and Brillouin line widths differ depending on the material of the optical fiber and the wavelength (frequency) of the probe light incident on the optical fiber. For example, when a probe light having a wavelength of 1.55 μm is incident on a quartz-based single-mode optical fiber, the BFS is about 11 GHz and the Brillouin line width is about 30 MHz.

It is known that BFS changes linearly at a rate of about 500 MHz /% with respect to the distortion of the optical fiber. When this is converted into tensile strain and temperature, they correspond to 0.058 MHz / με and 1.18 MHz / ° C, respectively.

As described above, BOTDR can measure strain and temperature distribution in the longitudinal direction of an optical fiber, and is attracting attention as a monitoring technique for large buildings such as bridges and tunnels.

In BOTDR, in order to measure the spectral waveform of Brillouin scattered light generated in an optical fiber, heterodyne detection with a separately prepared reference light is generally performed. The intensity of Brillouin scattered light is about 2 to 3 orders of magnitude smaller than the intensity of Rayleigh scattered light used in general OTDR. Therefore, heterodyne detection is useful for improving the minimum light receiving sensitivity.

Since BOTDR handles information on the frequency spectrum distribution in the longitudinal direction of the optical fiber, it is necessary to acquire three-dimensional information on time, amplitude, and frequency. Here, since the Brillouin scattered light is very weak as described above, the signal-to-noise ratio is sufficient even if heterodyne detection is applied.
S / N) cannot be secured. As a result, an averaging process for improving S / N is required. Due to this averaging process and the acquisition of the above-mentioned three-dimensional information, it is difficult to shorten the measurement time with the conventional BOTDR apparatus.

As a method of shortening the measurement time in the BOTDR apparatus, a method using a self-delay interferometer has been proposed. In this method, two-dimensional information on time and phase is acquired by measuring the frequency change of light as the phase difference of the beat signal obtained by coherently detecting the backscattered light with a self-delay interferometer. Therefore, the measurement time is shortened as compared with the conventional BOTDR apparatus that requires acquisition of three-dimensional information.

In the BOTDR apparatus using a self-delayed interferometer, the BFS is calculated from the difference from the BGS of the reference fiber by comparing the backscattered light with the self-delayed interferometer with the sine wave. When using a self-delayed interferometer, it measures up to a measurable frequency determined by the optical path length difference of this self-delayed interferometer. Here, when the BFS rises and exceeds the measurable frequency, folding occurs and the BFS decreases.

FIG. 1 is a schematic diagram for explaining the behavior of a self-delay interferometer when BFS rises. In FIG. 1 (A), BFS (MHz) is shown on the horizontal axis, and the intensity (a.u.) of the output signal from the self-delay interferometer is shown on the vertical axis. Further, in FIG. 1B, the horizontal axis represents the position (m) and the vertical axis represents the BFS (MHz). Here, an example is shown in which the measurable frequency is 500 MHz and the BFS changes in the range of 0 to 1000 MHz.

As shown in FIGS. 1A and 1B, the BFS is folded back at a measurable frequency (= 500 MHz), and when the BFS is 500 MHz or more, the correct BFS cannot be measured (see, for example, Patent Document 1).

JP-A-2020-051941

Here, when measuring with a preset BFS offset, the BFS at the same temperature and the same strain differs depending on the type of optical fiber, and the BFS is within the measurable range specified by the set measurable frequency. However, there are cases where the correct BFS cannot be measured due to wrapping. Further, since the BFS performs a phase comparison between the output from the self-delay interferometer and the sine wave, the standard deviation of the BFS becomes large at the timing when the folding occurs, and it seems that the measurement accuracy is deteriorated.

The present invention has been made in view of the above-mentioned problems. An object of the present invention is an optical fiber strain measuring device and an optical fiber strain measuring device that enable correct BFS to be measured by automatically changing the BFS offset in a BOTDR device having a self-delay interferometer. To provide a method of adjusting the BFS offset.

In order to achieve the above object, in the optical fiber strain measuring apparatus of the present invention, the light source unit that generates the probe light and the backward scattered light generated by the optical fiber to be measured by the probe light are input, and the backward scattered light is input. It is configured to include a wavelength control unit that outputs the rear Brilluan scattered light included in the above, a self-delay interferometer that generates a measurement signal to which the rear Brilluan scattered light is input, and a signal processing unit. The signal processing unit is a means for generating measurement result data including distortion data of the optical fiber from the measurement signal, and a signal loss in a region where the standard deviation of BFS exceeds the threshold value and the standard deviation exceeds the threshold value. Is provided with a means for changing the BFS offset amount when is equal to or less than the allowable loss.

According to the preferred embodiment of the above-mentioned optical fiber strain measuring apparatus, the self-delay interferometer has a branch portion for bifurcating the rear Brilluan scattered light into the first optical path and the second optical path, and the first optical path and the second optical path. It propagates through the optical frequency shifter portion provided on either one to give a frequency shift of the beat frequency, the delay portion provided on either one of the first optical path and the second optical path, and the first optical path and the second optical path. A combined wave section that combines light to generate combined wave light, a coherent detection section that detects the combined wave light by heterodyne detection and outputs a difference frequency as the first electric signal, and a second electric unit having the same frequency as the first electric signal. It is configured to include a locally-generated electric signal source that generates a signal, and a mixer unit that homodyne-detects a first electric signal and a second electric signal and outputs a difference frequency as a phase difference signal.

Further, in order to achieve the above-mentioned object, the BFS offset adjusting method of the present invention includes a standard deviation measuring process for measuring the standard deviation σ of the BFS at each position over the entire longitudinal direction of the optical fiber, and a length of the optical fiber. It is performed when there is an interval in which the standard deviation σ is larger than the threshold σth as a result of the judgment in the standard deviation judgment process for determining whether or not the standard deviation σ is larger than the threshold σth over the entire direction. As a result of the strength loss reference process that refers to the strength loss of the section, the strength loss determination process that determines whether the strength loss of the section is less than the allowable loss, and the determination in the intensity loss determination process, the section. It comprises a BFS offset changing process of changing the BFS offset, which is performed when the strength loss of is less than the permissible loss. The standard deviation measurement process, the standard deviation determination process, the intensity loss measurement process, the intensity loss determination process, and the BFS offset change process are repeated until the standard deviation σ becomes equal to or less than the threshold value σth.

Further, according to another preferred embodiment of the BFS offset adjusting method, a strength loss reference process that refers to the strength loss at each position over the entire longitudinal direction of the optical fiber and the strength loss in the section is less than the permissible loss. Each position throughout the longitudinal direction of the optical fiber, which is performed when the strength loss in the section is less than the allowable loss as a result of the strength loss determination process for determining the presence or absence and the determination in the strength loss determination process. In the standard deviation measurement process for measuring the standard deviation σ of BFS in, the standard deviation determination process for determining whether the standard deviation σ is larger than the threshold σth over the entire longitudinal direction of the optical fiber, and the standard deviation determination process. As a result of the determination, the BFS offset changing process for changing the BFS offset, which is performed when there is a section in which the standard deviation σ is larger than the threshold σth, is provided. The strength loss measurement process, the strength loss determination process, the standard deviation measurement process, the standard deviation determination process, and the BFS offset change process are repeated until the standard deviation σ becomes equal to or less than the threshold value σth.

According to the optical fiber strain measuring device and the BFS offset adjusting method of the present invention, the correct BFS can be measured by automatically changing the BFS offset.

It is a schematic diagram for demonstrating the behavior of the self-delay interferometer when BFS rises. It is a schematic block diagram of an optical fiber strain measuring apparatus. It is a schematic diagram for demonstrating the principle of offset adjustment. It is a flowchart which shows an example of the processing of an offset adjusting means. It is a figure (1) which shows the example of the signal waveform to be measured. It is a figure (2) which shows the example of the signal waveform to be measured.

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.

The optical fiber strain measuring apparatus of the present invention will be described with reference to FIG. FIG. 2 is a schematic block diagram of an optical fiber strain measuring device. Although described here as an optical fiber strain measuring device, it is possible not only to measure the strain of the optical fiber but also to measure the temperature based on the strain of the optical fiber.

The optical fiber distortion measuring device includes a light source unit 10, a circulator 20, an optical amplifier 30, a wavelength separation filter (optical band path filter) 32 as a wavelength control unit, and a self-delayed heterodyne interferometer (hereinafter referred to as a self-delayed interferometer). It is configured to include 40 and a signal processing unit 90.

The light source unit 10 generates probe light. The light source unit 10 includes a laser light source 12, an optical pulse generator 14, and a timing controller 16.

The laser light source 12 produces continuous light. The continuous light generated by the laser light source 12 is sent to the optical pulse generator 14.

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 16. The repetition period of this optical pulse is set longer than the time required for the optical pulse to reciprocate in the optical fiber (also referred to as the measured 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. Instead of the circulator 20, an optical coupler and an isolator may be used in combination.

The backscattered light from the optical fiber 100 to be measured is sent to the optical amplifier 30 via the circulator 20. The backscattered light amplified by the optical amplifier 30 is sent to the wavelength separation filter 32.

The wavelength separation filter 32 blocks the Rayleigh scattered light component from the backscattered light amplified by the optical amplifier 30, and transmits only the Brillouin scattered light component. The light transmitted through the wavelength separation filter 32 is sent to the self-delay interferometer 40.

The signal E ₀ (t) at time t of the natural Brillouin scattered light emitted from the wavelength separation filter 32 is represented by the following equation (1).

E ₀ ( _t ) = A ₀ exp {j (2πf bt + φ ₀ )} (1)
Here, A ₀ indicates the amplitude, f _b indicates the optical frequency of the natural Brillouin scattered light, and φ ₀ indicates the initial phase.

The self-delay interferometer 40 includes a branch section 42, an optical frequency shifter section 43, a delay section 48, a combine wave section 50, a coherent detection section 60, a mixer section 70, a low-pass filter (LPF) 72, and a station. It is configured to include a power generation signal source 83.

The local oscillator electric signal source 83 generates an electric signal having a 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 wavelength separation filter 32, and branches into two optical paths, the first optical path and the second optical path.

In this configuration example, 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 83 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 48 is provided in the second optical path. The delay unit 48 gives a delay of time τ to the light propagating in the second optical path. The delay unit 48 may give a delay of time τ to the light propagating in the second optical path as compared with the light propagating in the first optical path, and the configuration thereof is arbitrary.

The combined wave unit 50 generates combined wave light by combining the light propagating in the first optical path and the second optical path. The optical signal E ₁ (t) propagating in the first optical path and the optical signal E ₂ (t—τ) propagating in the second optical path incident on the combine 50 are the following (2) (3), respectively. It is expressed by an expression.

E ₁ (t) = A ₁ exp {j (2πf bt + 2πf _{AOM t} + φ ₁ )} (2)
E ₂ (t) = A ₂ exp [j {2πf _b (t−τ) + φ ₂ }] (3)
Here, A ₁ and A ₂ are the amplitudes of E ₁ (t) and E ₂ (t-τ), respectively, and φ ₁ and φ ₂ are E ₁ (t) and E ₂ (t-τ), respectively. Is the initial phase of.

The coherent detection unit 60 performs heterodyne detection of the combined wave light to generate a beat signal. The coherent detection unit 60 includes, for example, a balanced photodiode (PD) 62 and a FET amplifier 64. The beat signal I ₁₂ given by the heterodyne detection is represented by the following equation (4).

I ₁₂ = 2A ₁ A ₂ cos {2π (f _AOM t + f _b τ) + φ ₁ -φ ₂ } (4)
The beat signal generated by the coherent detection unit 60 is sent to the mixer unit 70 as a first electric signal. Further, the electric signal generated by the local oscillator electric signal source 83 is sent to the mixer unit 70 as a second electric signal.

The mixer unit 70 homodyne detects the first electric signal and the second electric signal to generate a homodyne signal. The LPF 72 removes the sum frequency component of the amplified beat signal and the local oscillator signal from the homodyne signal to generate a measurement signal which is a difference frequency component. The measurement signal is sent to the signal processing unit 90.

Here, since both the first electric signal and the second electric signal are beat signals having a frequency f _AOM , the change of 2πf _b τ is output as a phase difference by homodyne detection of these. The Brillouin frequency f _b changes due to the strain of the optical fiber 100 to be measured.

The signal processing unit 90 can be configured by, for example, an FPGA (Field Programmable Gate Array). Further, the signal processing unit 90 may be realized by executing the software. Since the storage means for storing the contents of each process can have an arbitrarily suitable conventionally known configuration, the description thereof will be omitted here.

The signal processing unit 90 includes, for example, an averaging processing means, a measurement result data generation means, and an offset adjusting means.

The averaging processing means performs averaging of M times (M is an integer of 2 or more), that is, averaging processing. The number of times of this averaging corresponds to the number of electrical pulses generated by the timing controller 16. The number of times M of averaging is set to an arbitrary suitable number, but for example, when the length of the optical fiber 100 is 5 km, it can be set to about 4000 times.

The measurement result data generation means generates measurement result data including BFS and distortion data based on the measurement signal received from the self-delay interferometer 40. Distortion data is obtained based on BFS. The measurement result data also includes information on the temperature of the optical fiber. Since the operation of the measurement result data generation means is conventionally known, the description thereof is omitted here.

The offset adjusting means adjusts the offset. The principle of offset adjustment will be described with reference to FIG.

FIG. 3 is a schematic diagram for explaining the principle of BFS offset adjustment. In FIG. 3, BFS (MHz) is shown on the horizontal axis, and the intensity (a.u.) of the output signal from the self-delay interferometer is shown on the vertical axis. Here, an example in which the measurable frequency is 500 MHz is shown.

FIG. 3 shows an example in which the phase comparison is performed in the range of 0 to π and the corresponding BFS is 0 MHz to 500 MHz. In this case, the negative BFS value cannot be calculated. However, the BFS value may take a negative value. Therefore, in order to make it possible to calculate a negative BFS value, a BFS offset that shifts the reference value for phase comparison is performed.

For example, when the magnitude of the BFS offset (BFS offset amount) is 125 MHz, the measurable BFS region is -125 MHz to 375 MHz. Here, as shown in FIG. 3, BFS and the intensity have a relationship expressed by a cosine function. Therefore, in order to perform accurate measurement, it is preferable to use a region of 50 to 450 MHz as a linear region with a margin of about 50 MHz at both ends of the measurable range of 0 to 500 MHz. Therefore, when the BFS offset amount is 125 MHz, the measurement region is −75 MH to 325 MHz. Similarly, when the BFS offset amount is 220 MHz, the measurement area is −170 MHz to 230 MHz.

The processing of the offset adjusting means will be described with reference to FIGS. 4 to 6. FIG. 4 is a flowchart showing an example of processing of the offset adjusting means. 5 (A) to (C) and FIGS. 6 (A) to 6 (C) are diagrams showing examples of signal waveforms to be measured. 5 (A) to 5 (C) show the case where the BFS offset amount is 125 MHz, and FIGS. 6 (A) to 6 (C) show the case where the BFS offset amount is 220 MHz. 5A and 6A show the position (m) from the input end of the optical fiber on the horizontal axis and BFS (MHz) on the vertical axis. Here, the position (m) of the optical fiber indicates the distance from the input end where the probe light of the optical fiber 100 is input to the position where the backscattered light is generated. In addition, in FIG. 5A and FIG. 6A, the maximum value, the minimum value, and the current value of the data averaged by the averaging processing means are shown superimposed. 5 (B) and 6 (B) are shown on the horizontal axis with the position (m) from the input end of the optical fiber 100 as in FIGS. 5 (A) and 6 (A), and on the vertical axis. The signal strength is shown as the strength loss (dB). Further, FIGS. 5 (C) and 6 (C) show the positions (m) from the input end of the optical fiber 100 on the horizontal axis, as in FIGS. 5 (A) and 6 (A), in the vertical direction. The standard deviation σ (MHz) is shown on the axis.

First, the time axis standard deviation σ of the BFS at each position is measured over the entire longitudinal direction of the optical fiber (S10). By the measurement of S10, for example, the relationship between the BFS and the position shown in FIG. 5 (A) and the relationship between the standard deviation and the position shown in FIG. 5 (C) can be obtained.

Next, it is determined whether or not the standard deviation σ is larger than the threshold value (σth) over the entire longitudinal direction of the optical fiber (S20). The threshold value (σth) of this standard deviation is set to about 2 to 3 times the measurement performance σspec of the BOTDR apparatus. For example, when the measurement performance σspec of BOTDR is 1.5 MHz, σth can be set to 3 MHz.

As a result of the determination of S20, when the standard deviation σ is equal to or less than the threshold value σth over the entire longitudinal direction of the optical fiber (No), the offset adjustment is unnecessary and the process ends.

As a result of the determination in S20, when there is a section in which the standard deviation σ is larger than the threshold value σth (Yes), the strength loss in the section is subsequently referred to (S30). For example, in the example shown in FIG. 5C, the standard deviation σ is larger than the threshold value σth in the two sections where the positions are 72 to 120 m and 127 to 177 m. A section in which this standard deviation σ is larger than the threshold value σth is also referred to as an accuracy deterioration section.

Next, it is determined whether or not the strength loss in the section is less than the allowable loss (for example, 4 dB) (S40).

As a result of the determination in S40, when the intensity loss in the section is equal to or greater than the allowable loss (No), it is considered that the reason why the standard deviation σ exceeds the threshold value σth is other than the turnaround at the measurable frequency. In this case, the process is terminated, and the cause of the strength loss is separately investigated (S42).

As a result of the determination of S40, when the intensity loss in the section is less than the allowable loss (Yes), it is considered that the cause of the standard deviation σ exceeding the threshold value is the turnaround at the measurable frequency.

Deterioration of measurement accuracy mainly occurs due to an increase in strength loss. However, in the example shown in FIG. 5, in the accuracy deterioration section, as shown in FIG. 5 (C), although the standard deviation σ is larger than the threshold value σth, it is shown in FIG. 5 (B). As such, the strength loss is less than 1 dB.

Further, the intensity loss increases as the propagation distance in the optical fiber 100 becomes longer, that is, as the backscattered light generated at a position farther from the input end of the probe light. However, in the example shown in FIG. 5A, the measurement accuracy is better at the position farthest from the input end of the probe light.

As described above, when the strength loss is small, it can be estimated that the deterioration of the measurement accuracy is due to the turnaround at the measurable frequency.

Therefore, as a result of the determination in S40, when the strength loss in the section is less than the allowable loss (Yes), the BFS offset is changed (S50). After that, the measurement of S10 is performed again.

Here, if the change amount of the BFS offset is small, it is easy to obtain the optimum value of the BFS offset amount, but on the other hand, it takes time to adjust the BFS offset. On the other hand, if the change amount of the BFS offset is large, the time required for adjusting the BFS offset can be shortened, but on the other hand, the obtained BFS offset amount is not always the optimum value.

The amount of change in the BFS offset can be arbitrarily set according to the design, and is set to, for example, 25 MHz to 50 MHz.

S10 to S50 are repeated until there is no folding back due to the measurable frequency.

Here, an example is shown in which the standard deviation is determined in S10 and S20, and then the strength loss is determined in S30 and S40, but the present invention is not limited to this. S10 and S20 may be interchanged with S30 and S40, and the strength loss may be determined first in S30 and S40, and then the standard deviation may be determined in S10 and S20.

In FIG. 6, the BFS offset amount is 220 MHz. As shown in FIG. 6A, the BFS measured here is about −150 to −170 MHz in the 70 m to 120 m section. As shown in FIG. 5A, this BFS is a value that cannot be observed correctly when the BFS offset amount is 125 MHz. Further, at this time, as shown in FIG. 6C, the standard deviation σ is also less than the threshold value σth.

In this way, since the optical fiber strain measuring device has a function of automatically adjusting the BFS offset, the BFS offset is automatically adjusted even if an unknown optical fiber having a different refractive index or the like is connected, and the correct BFS is measured. Can be done.

The standard deviation threshold value σth, the permissible loss, and the amount of change in the BFS offset are not limited to the above examples, and may be appropriately set according to the characteristics of the BOTDR apparatus. Further, the BFS offset may be adjusted at all times during the measurement by the optical fiber strain measuring device, or may be performed periodically or irregularly.

10 Light source 12 Laser light source 14 Optical pulse generator 16 Timing controller 20 Circulator 30 Optical amplifier 32 Wavelength separation filter (optical bandpass filter)
40 Self-delay interferometer 42 Branch part 43 Optical frequency shifter part 46 Polarization controller 48 Delay part 50 Combined part 60 Coherent detection part 62 Balanced PD
64 FET amplifier 70 Mixer section 72 Low-pass filter (LPF)
83 Local oscillator 90 Signal processing unit 100 Optical fiber to be measured

Claims (4)

A light source that generates probe light and
A wavelength control unit that inputs the backward scattered light generated in the optical fiber to be measured by the probe light and outputs the rear Brillouin scattered light included in the backward scattered light.
A self-delayed interferometer that generates a measurement signal to which the rear Brillouin scattered light is input,
Equipped with a signal processing unit
The signal processing unit
A means for generating measurement result data including distortion data of the optical fiber from the measurement signal, and
It is characterized by providing a means for changing the Brilluan frequency shift offset amount when the standard deviation of the Brilluan frequency shift exceeds the threshold value and the signal loss in the region where the standard deviation exceeds the threshold value is equal to or less than the allowable loss. Optical fiber strain measuring device. The self-delay interferometer
A branch portion in which the rear Brillouin scattered light is bifurcated into a first optical path and a second optical path,
An optical frequency shifter portion provided in either the first optical path or the second optical path to give a frequency shift of the beat frequency, and
A delay portion provided in either the first optical path or the second optical path, and
A wave junction portion that generates combined wave light by combining the light propagating in the first optical path and the second optical path.
A coherent detection unit that heterodyne detects the combined wave light and outputs the difference frequency as the first electric signal.
A local oscillator electric signal source that generates a second electric signal having the same frequency as the first electric signal,
The optical fiber strain measuring apparatus according to claim 1, further comprising a mixer unit that homodyne detects the first electric signal and the second electric signal and outputs a difference frequency as a phase difference signal. A method for adjusting a Brillouin frequency shift offset, which is performed by the optical fiber strain measuring apparatus according to claim 1 or 2.
A standard deviation measurement process that measures the standard deviation σ of the Brilluan frequency shift at each position over the entire longitudinal direction of the optical fiber.
A standard deviation determination process for determining whether or not the standard deviation is larger than the threshold value over the entire longitudinal direction of the optical fiber.
As a result of the determination in the standard deviation determination process, the intensity loss reference process for referring to the intensity loss in the section, which is performed when there is a section in which the standard deviation is larger than the threshold value,
The strength loss determination process for determining whether the strength loss in the section is less than the allowable loss, and
As a result of the determination in the intensity loss determination process, the Brillouen frequency shift offset change process for changing the Brillouan frequency shift offset, which is performed when the intensity loss in the section is less than the allowable loss, is provided.
The standard deviation measurement process, the standard deviation determination process, the intensity loss measurement process, the intensity loss determination process, and the Brilluan frequency shift offset change process are repeated until the standard deviation becomes equal to or less than the threshold value. How to adjust the Brilluan frequency shift deviation. A method for adjusting a Brillouin frequency shift offset, which is performed by the optical fiber strain measuring apparatus according to claim 1 or 2.
A strength loss reference process that refers to the strength loss at each position throughout the longitudinal direction of the optical fiber.
The strength loss determination process for determining whether the strength loss in the section is less than the allowable loss, and
A standard for measuring the standard deviation of the Brillouin frequency shift at each position over the entire longitudinal direction of the optical fiber, which is performed when the intensity loss in the section is less than the allowable loss as a result of the determination in the intensity loss determination process. Deviation measurement process and
A standard deviation determination process for determining whether or not the standard deviation is larger than the threshold value over the entire longitudinal direction of the optical fiber.
As a result of the determination in the standard deviation determination process, the Brillouan frequency shift offset change process for changing the Brillouan frequency shift offset, which is performed when there is a section in which the standard deviation is larger than the threshold value, is provided.
The feature is that the intensity loss measurement process, the intensity loss determination process, the standard deviation measurement process, the standard deviation determination process, and the Brilluan frequency shift offset change process are repeated until the standard deviation becomes equal to or less than the threshold value. How to adjust the Brilluan frequency shift deviation.

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