JP2019035724A - Optical fiber strain measurement device and optical fiber strain measurement method - Google Patents

Abstract

To compensate an optical loss characteristic of an optical fiber to be measured without causing extra power loss in measurement of optical fiber strain.SOLUTION: An optical fiber strain measurement device comprises: a light source unit that generates probe light; a first wavelength separation filter that separates and extracts a Stokes component of backward Brillouin scattering light from backward scattering light generated in an optical fiber to be measured by probe light; a self-delay heterodyne interferometer that has a polarization controller controlling polarization of the Stokes component, and that detects change of a frequency shift quantity of the Stokes component as a phase difference; and a polarization control unit that has a second wavelength separation filter which separates and extracts anti-Stokes component from a remaining component of the first wavelength separation filter. The polarization control unit acquires a loss characteristic from the optical fiber to be measured as a reference signal from the anti-Stokes component, subtracts the reference signal from the observed signal obtained by a self-delay heterodyne interferometer, and controls the polarization controller in such a manner that a detection level of the observed signal after subtraction is maximized.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical fiber strain measuring apparatus and an optical fiber strain 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 is actively studied. A typical example of distributed optical fiber sensing is time domain reflectometry (OTDR) in which a light pulse is incident from one end of an optical fiber and the light backscattered in the optical fiber is measured with respect to time. is there. Backscattering in an optical fiber includes Rayleigh scattering, Brillouin scattering, and Raman scattering. Among them, 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 about GHz in 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 a Brillouin gain spectrum (BGS). be called. The frequency shift and spectral line width of BGS are called Brillouin frequency shift (BFS) and Brillouin line width, respectively. The BFS and Brillouin line widths differ depending on the material of the optical fiber and the incident light wavelength. 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, it is known that the magnitude of BFS accompanying changes in strain and temperature in a single mode optical fiber is 0.049 MHz / με and 1.0 MHz / ° C. at a wavelength of 1.55 μm, respectively.

  Here, since BFS has dependency on strain and temperature, BOTDR is attracting attention as being usable for monitoring purposes such as large buildings such as bridges and tunnels, and places where landslides may occur. ing.

  BOTDR generally performs heterodyne detection with reference light prepared separately in order to measure the spectral waveform of natural Brillouin scattered light generated in an optical fiber. The intensity of natural Brillouin scattered light is two to three orders of magnitude smaller than the intensity of Rayleigh scattered light. For this reason, heterodyne detection is useful for improving the minimum light receiving sensitivity.

  Here, since natural Brillouin scattered light is very weak, even if heterodyne detection is applied, a sufficient signal-to-noise ratio (S / N) cannot be ensured. As a result, an averaging process for improving S / N is required. Due to this averaging process and the acquisition of three-dimensional information of time, amplitude, and frequency, it is difficult to shorten the measurement time in the conventional optical fiber strain measurement device.

  In view of the above problems, the inventors of the present invention obtain two-dimensional information of time and phase by measuring the frequency change of light as a phase difference of a beat signal given by coherent detection. An optical fiber strain measurement device and an optical fiber strain measurement method using Brillouin scattered light are being studied (see, for example, Patent Document 1). In Patent Document 1, a self-delayed heterodyne type BOTDR (SDH-BOTDR: Self-Delayed Heterodyne BOTDR) technique is used.

  In this SDH-BOTDR, continuous (CW) light output from a light source is modulated into a rectangular optical pulse by a modulator and input to a measured optical fiber. Back Brillouin scattered light from the optical fiber to be measured is separated into Rayleigh scattered light and Brillouin scattered light by a narrow band optical filter (OBPF), and only the Brillouin scattered light is input to the self-delayed heterodyne interferometer.

  FIG. 2 is a schematic diagram for explaining the operation of the self-delay heterodyne interferometer. In the self-delayed heterodyne interferometer, Brillouin scattered light (indicated by I in FIG. 2) is branched into two in a first optical path and a second optical path. An optical frequency shifter unit is provided in one of the first optical path and the second optical path, and a delay unit is provided in the other, and detects a change in BFS as a phase difference. Further, a delay time difference is given between the light propagating through 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 (indicated by II and III in FIG. 2, respectively) to generate combined light (indicated by IV and V in FIG. 2). Generate.

  In the self-delay heterodyne interferometer, in order to perform reception with higher accuracy, it is important to control the polarization states of the first optical path and the second optical path to be the same.

  Control of the polarization state in the self-delay heterodyne interferometer is performed by using a signal from a photodiode (PD) downstream of the self-delay heterodyne interferometer by arranging a polarization controller in one of the first optical path and the second optical path. Is called. The intensity of the signal from the PD is observed through a bandpass filter (BPF) in the same frequency band as that of the local electrical signal, and the intensity of the observed peak (indicated by VI in FIG. 2) is the maximum. Thus, the polarization is controlled.

  Here, the optical fiber to be measured has optical loss characteristics. For this reason, the observed waveform of the backward Brillouin scattered light from the optical fiber to be measured has an intensity gradient due to the optical loss characteristic of the optical fiber (see FIG. 3A). As a result, the observed peak has a wider bandwidth and lower intensity (indicated by II in FIG. 3 (B)) than when there is no loss (indicated by I in FIG. 3 (B)). .) Therefore, accurate polarization control becomes difficult.

  As a method for compensating for the inclination of the intensity, there is a method in which an optical loss characteristic is measured in advance as a reference signal in OTDR and compared with an observation signal of scattered light from the optical fiber to be measured (see, for example, Patent Document 2). ). However, for example, when intensity fluctuations occur in real time, such as fluctuations in the output intensity of the light source, accurate comparison is difficult with the method of comparing the reference signal and the observation signal.

  As another method, the scattered light from the optical fiber to be measured is branched into two before being input to the self-delay heterodyne interferometer, one is used as a received signal, and the other is used as a reference for compensating for optical loss characteristics. There is a method used as a signal.

Japanese Unexamined Patent Publication No. 2016-191659 JP 2000-346741 A

K. Koizumi, et al. , “High-Speed Distributed Strain Measurement using Brillouin Optical Time-Domain Reflectometry Based-on Self-Delayed Heterodyne Detection 201,“ High-Speed Distributed Strain Measurement using Brillouin Optical Time-Domain 1.07, Sep. 2015

  However, in the method of branching into two before inputting to the above-described self-delay heterodyne interferometer and using one as a received signal and using the other as a reference signal for compensating for optical loss characteristics, the received signal is affected by the branch loss. The strength of the decreases.

  Therefore, when weak power such as BOTDR is received, the reception sensitivity is further deteriorated.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an optical fiber strain measuring device that compensates for optical loss characteristics of an optical fiber to be measured without causing an extra power loss, and An object of the present invention is to provide an optical fiber strain measurement method.

  In order to achieve the above-described object, an optical fiber strain measuring device according to the present invention includes a light source unit that generates probe light and a Stokes component of backward Brillouin scattered light from back scattered light generated in the measured optical fiber by the probe light. A first wavelength separation filter for separating and extracting the Stokes component, a polarization controller for controlling the polarization of the Stokes component, a self-delay heterodyne interferometer for detecting a change in the frequency shift amount of the Stokes component as a phase difference, and a first wavelength And a polarization controller having a second wavelength separation filter that separates and extracts the anti-Stokes component from the residual component of the separation filter. The polarization controller obtains the loss characteristics from the measured optical fiber as the reference signal from the anti-Stokes component, subtracts the reference signal from the observation signal obtained by the self-delay heterodyne interferometer, and detects the observation signal after subtraction. The polarization controller is controlled so that the level becomes maximum.

  The optical fiber strain measurement method of the present invention includes a process of generating probe light, a process of separating and extracting the Stokes component of the back Brillouin scattered light from the back scattered light generated in the measured optical fiber by the probe light, Using a delay heterodyne interferometer, the process of detecting the change in the frequency shift amount of the Stokes component as a phase difference, the process of separating and extracting the anti-Stokes component from the residual component from which the Stokes component has been separated and extracted, and the anti-Stokes component In order to maximize the detection level of the observation signal after subtraction, the process of obtaining the loss characteristics from the measured optical fiber as the reference signal, the process of subtracting the reference signal from the observation signal obtained by the self-delay heterodyne interferometer And a step of controlling a polarization controller included in the self-delay heterodyne interferometer.

  According to the optical fiber strain measurement device and the optical fiber strain measurement method of the present invention, it is possible to suppress waveform deterioration by compensating for the tilt due to the optical loss characteristic of the optical fiber, so that the measurement accuracy is improved. In addition, since an anti-Stokes component that is not used for coherent detection is used for the reference signal, no extra power loss occurs. In addition, since backscattered light is used, the tilt due to the light loss characteristic can be compensated even when intensity fluctuations occur in real time.

It is a typical block diagram of an optical fiber distortion measuring device. It is a schematic diagram for demonstrating operation | movement of a self-delay heterodyne interferometer. It is a schematic diagram for demonstrating an optical loss characteristic.

(First embodiment)
With reference to FIG. 1, an embodiment of an optical fiber strain measuring apparatus of the present invention will be described. FIG. 1 is a schematic block diagram of an optical fiber strain measuring apparatus.

  An optical fiber strain measuring device (hereinafter also simply referred to as a measuring device) includes a light source unit 10, an optical circulator 20, a first wavelength separation filter 30, a self-delay heterodyne interferometer 40, and a polarization control unit 200. Is done.

  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 optical pulses from the continuous light.

  Here, the measuring apparatus measures the phase difference according to the frequency change. For this reason, the frequency fluctuation of the light source 12 must be sufficiently smaller than the Brillouin shift. Therefore, a frequency stabilized laser is used as the light source 12. For example, when the distortion of an optical fiber 100 (hereinafter also referred to as a measured optical fiber) 100 to be measured is 0.008%, the Brillouin shift corresponds to 4 MHz. For this reason, in order to measure distortion of about 0.008%, it is desirable that the frequency fluctuation of the light source 12 is sufficiently smaller than 4 MHz.

  The optical pulse generator 14 is configured using an intensity modulator such as an acousto-optic (AO) modulator or an electro-optic (EO) modulator, which is well known in the art. The optical pulse generator 14 generates an optical pulse from continuous light according to the input electric pulse. The repetition period of this light pulse is set longer than the time required for the light pulse to reciprocate through the measured optical fiber 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 is incident on the measured optical fiber 100 through the optical circulator 20. Instead of the optical circulator 20, an optical coupler and an isolator may be used in combination.

  Backscattered light from the measured optical fiber 100 is sent to the first wavelength separation filter 30 through the optical circulator 20. The first wavelength separation filter 30 has two output ports.

  The first wavelength separation filter 30 separates and extracts the Stokes component of the back Brillouin scattered light from the back scattered light. The Stokes component is output from one output port of the first wavelength separation filter 30 and sent to the self-delay heterodyne interferometer 40. In addition, the first wavelength separation filter 30 sends the residual component of the backscattered light to the polarization controller 200.

The signal E ₀ (t) at time t of the natural Brillouin scattered light emitted from the first wavelength separation filter 30 is expressed by the following equation (1).

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

  The self-delay heterodyne interferometer 40 includes a branching unit 42, an optical frequency shifter unit 43, a delay unit 48, a polarization controller 46, a multiplexing unit 50, a light receiving unit 60, a local electric signal source 83, A phase comparison unit 70 is provided.

The local electric signal source 83 generates an electric signal having a frequency f _AOM .

  The branching unit 42 receives the Stokes component of the backward Brillouin scattered light generated in the optical fiber 100 to be measured by the probe light through the first wavelength separation filter 30 and splits it into the first optical path and the second optical path.

The optical frequency shifter unit 43 is provided in the first optical path. The optical frequency shifter 43 gives a frequency shift of the frequency f _AOM to the light propagating through the first optical path using the electric signal of the frequency f _AOM generated by the local electric signal source 83.

  In this configuration example, the polarization controller 46 is provided in the second optical path. The polarization controller 46 controls the polarization of light propagating through the second optical path according to an instruction from the polarization controller 200.

  In this configuration example, the delay unit 48 is provided in the second optical path. The delay unit 48 gives a delay of time τ to the light propagating through the second optical path. In addition, the delay part 48 should just give delay of time (tau) to the light which propagates a 2nd optical path compared with the light which propagates a 1st optical path, The structure is arbitrary. For example, when the polarization controller 46 functions as a delay device, the delay device need not be provided separately. Further, it can be constituted by a so-called delay line.

The multiplexing unit 50 combines the light propagating through the first optical path and the second optical path to generate combined 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 incident on the multiplexing unit 50 are respectively (2) and (3) below. It is expressed by a formula.

E ₁ (t) = A ₁ exp {j (2πf _b t + 2πf _AOM t + φ ₁ )} (2)
E ₂ (t−τ) = A ₂ exp [j {2πf _b (t−τ) + φ ₂ }] (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 light receiving unit 60 generates a beat signal by heterodyne detection of the combined light. The light receiving unit 60 is configured using, for example, a balanced photodiode and an FET amplifier. The beat signal I given by heterodyne detection is expressed 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 light receiving unit 60 is branched into two, and one is sent to the phase comparison unit 70 as the first electric signal. The electrical signal generated by the local electrical signal source 83 is sent to the phase comparison unit 70 as a second electrical signal.

The phase comparison unit 70 performs homodyne detection on the first electric signal and the second electric signal to generate a homodyne signal. Here, since both the first and second electric signals are beat signals having the frequency f _AOM , a change of 2πf _b τ is output as a phase difference by homodyne detection. Brillouin frequency f _b is changed by two factors distortion and fluctuation of the oscillation frequency measured optical fiber 100 of the light source 12. However, by using a frequency stabilized laser as the light source 12, the influence of the distortion of the optical fiber 100 to be measured becomes dominant. Here, the phase comparison unit 70 is configured by a personal computer (PC), but may be configured by an analog device.

  Note that the other of the two beat signals generated by the light receiving unit 60 is sent to the polarization control unit 200.

  The polarization controller 200 includes a reference signal delay unit 202, a second wavelength separation filter 204, a photoelectric converter 206, a low-pass filter (LPF) 208, a first analog-digital converter (ADC) 210, a second ADC 212, and a subtractor. 214, FFT 216, band pass filter (BPF) 218, intensity measuring device 220 and controller 222.

  The signal sent from the first wavelength separation filter 30 to the polarization control unit 200 passes through the reference signal delay unit 202, the second wavelength separation filter 204, the photoelectric converter 206, the LPF 208, and the first ADC 210 to the subtractor 214. Sent. The signal sent from the self-delay heterodyne interferometer 40 to the polarization controller 200 is sent to the subtracter 214 via the second ADC 212.

  The reference signal delay unit 202 is used to synchronize the timing of the signals that have passed through the first ADC 210 and the second ADC 212, respectively. The reference signal delay unit 202 can be configured in the same manner as the delay unit 48 provided in the second optical path.

  The second wavelength separation filter 204 extracts the anti-Stokes component from the residual component from which the Stokes component has been separated and extracted by the first wavelength separation filter 30 and sends it to the photoelectric converter 206. The first wavelength separation filter 30 and the second wavelength separation filter 204 can be configured similarly except that the Stokes component and the anti-Stokes component are extracted, that is, the wavelength band is different.

  The anti-Stokes component extracted by the second wavelength separation filter 204 is converted into an electric signal by the photoelectric converter 206 and sent to the LPF 208.

  The LPF 208 detects the slope (intensity slope) of the envelope of the light intensity signal. The detected intensity gradient is sent to the first ADC 210 as a reference signal indicating optical loss characteristics.

  Further, the signal sent from the self-delay heterodyne interferometer 40 to the polarization controller 200 is sent to the second ADC 212 as an observation signal.

  The subtracter 214 subtracts the signals that have passed through the first ADC 210 and the second ADC 212, respectively. That is, the subtractor 214 compensates for the optical loss characteristic by subtracting the reference signal from the observation signal. The observation signal subtracted by the subtracter 214 is sent to the BPF 218 via the FFT 216. The transmission frequency band of the BPF 218 is the same as the electric signal generated by the local electric signal source 83.

  The intensity measuring device 220 measures the detection level (peak intensity) of the signal received through the BPF 218. The intensity measuring device 220 only needs to have a function of measuring the intensity of the electric signal, and can have any suitable conventionally known configuration.

  The controller 222 controls the polarization controller 46 so that the peak intensity becomes maximum based on the measurement result of the intensity measuring device 220. The controller 222 may be integrated with the intensity measuring device 220 or the polarization controller 46.

  Here, an example has been described in which the other of the two beat signals generated by the light receiving unit 60 is sent to the polarization control unit 200, but the branching point is not limited to this. A configuration may be adopted in which the light is split into two before the light receiving unit 60, one is sent to the light receiving unit 60 and the other is sent to the polarization control unit 200. In this case, a photoelectric converter that converts an optical signal sent from the self-delay heterodyne interferometer 40 to the polarization controller 200 into an electrical signal is separately provided.

  According to this measuring apparatus, since the waveform deterioration can be suppressed by compensating for the inclination due to the optical loss characteristic of the optical fiber, the measurement accuracy is improved. In addition, since an anti-Stokes component that is not used for coherent detection is used for the reference signal, no extra power loss occurs. In addition, since backscattered light is used as a reference signal, even when an intensity fluctuation occurs in real time, it is possible to compensate for the tilt due to the light loss characteristic.

(Other configuration examples)
In the above-described embodiment, the example in which the optical frequency shifter unit is provided in one of the two optical paths of the self-delay heterodyne interferometer 40 has been described, but the present invention is not limited to this. For example, a first optical frequency shifter unit that provides a frequency shift of the first frequency f ₁ is provided in the first optical path, and a second optical frequency that provides a frequency shift of the second frequency f ₂ (≠ f ₁ ) in the second optical path. A shifter portion may be provided. In this case, two local electric signal sources may be prepared, one that generates an electric signal having the first frequency f _{1 and one} that generates an electric signal having the second frequency f ₂ . The second electric signal can use a difference frequency component between these two electric signals.

DESCRIPTION OF SYMBOLS 10 Light source part 12 Light source 14 Optical pulse generator 20 Optical circulator 30 1st wavelength separation filter 40 Self-delay heterodyne interferometer 42 Branch part 43 Optical frequency shifter part 46 Polarization controller 48 Delay part 50 Multiplexing part 60 Light receiving part 70 Phase comparison Part 72 Low-pass filter (LPF)
83 Local Electric Signal Source 100 Optical Fiber to be Measured 200 Polarization Control Unit 202 Reference Signal Delay Device 204 Second Wavelength Separation Filter 206 Photoelectric Converter 208 LPF
210 First ADC
212 Second ADC
214 Subtractor 216 FFT
218 BPF
220 Strength measuring instrument 222 Controller

Claims (5)

A light source unit for generating probe light;
A first wavelength separation filter that separates and extracts a Stokes component of the back Brillouin scattered light from the back scattered light generated in the measurement optical fiber by the probe light;
A self-delay heterodyne interferometer that has a polarization controller that controls the polarization of the Stokes component, and detects a change in the frequency shift amount of the Stokes component as a phase difference;
A polarization controller having a second wavelength separation filter that separates and extracts an anti-Stokes component from a residual component of the first wavelength separation filter;
The polarization controller is
A reference signal indicating loss characteristics of the optical fiber to be measured is acquired from the anti-Stokes component,
Subtracting the reference signal from the observation signal obtained with the self-delayed heterodyne interferometer; and
The optical fiber distortion measuring apparatus, wherein the polarization controller is controlled so that a detection level of the observation signal after the subtraction is maximized. The polarization controller is
A reference signal delay device that gives a predetermined delay to a residual component of the first wavelength separation filter and sends the residual component to the second wavelength separation filter;
A photoelectric converter that photoelectrically converts the light of the anti-Stokes component;
A low-pass filter that obtains an envelope of intensity from the photoelectrically converted signal as the reference signal;
The optical fiber strain measuring apparatus according to claim 1, comprising: The self-delay heterodyne interferometer is
A branching portion for bifurcating the rear Brillouin scattered light into a first optical path and a second optical path;
An optical frequency shifter for providing a frequency shift of a beat frequency, provided in one of the first optical path and the second optical path;
A delay unit provided in one of the first optical path and the second optical path;
A multiplexing unit that combines the light propagating through the first optical path and the second optical path to generate combined light;
A light receiving unit that heterodyne detects the combined light and outputs a difference frequency as a first electrical signal;
A local electrical signal source for generating a second electrical signal having the same frequency as the first electrical signal;
A phase comparison unit that performs homodyne detection of the first electric signal and the second electric signal and outputs a difference frequency as a phase difference signal;
The optical fiber distortion measuring apparatus according to claim 1 or 2, wherein the combined light or the first electric signal is sent to the polarization control unit as the observation signal. The self-delay heterodyne interferometer is
A branching portion for bifurcating the rear Brillouin scattered light into a first optical path and a second optical path;
A first optical frequency shifter provided in the first optical path for giving a frequency shift of a first frequency;
A second optical frequency shifter provided in the second optical path for giving a frequency shift of a second frequency;
A delay unit provided in one of the first optical path and the second optical path;
A multiplexing unit that combines the light propagating through the first optical path and the second optical path to generate combined light;
A light receiving unit that heterodyne detects the combined light and outputs a difference frequency as a first electrical signal;
A local electrical signal source for generating a second electrical signal having the same frequency as the first electrical signal;
A phase comparison unit that performs homodyne detection of the first electric signal and the second electric signal and outputs a difference frequency as a phase difference signal;
The optical fiber distortion measuring apparatus according to claim 1 or 2, wherein the combined light or the first electric signal is sent to the polarization control unit as the observation signal. A process of generating probe light;
Separating and extracting the Stokes component of the back Brillouin scattered light from the back scattered light generated in the optical fiber to be measured by the probe light;
Detecting a change in the amount of frequency shift of the Stokes component as a phase difference using a self-delay heterodyne interferometer;
Separating and extracting the anti-Stokes component from the residual component from which the Stokes component has been separated and extracted;
From the anti-Stokes component, obtaining a loss characteristic from the measured optical fiber as a reference signal;
Subtracting the reference signal from the observation signal obtained with the self-delayed heterodyne interferometer;
And a step of controlling a polarization controller included in the self-delay heterodyne interferometer so that the detection level of the observation signal after the subtraction is maximized.

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