JP5222513B2 - Optical fiber measurement method, optical fiber measurement system, and optical fiber measurement device - Google Patents

Description

  The present invention relates to a technique for measuring strain distribution and temperature distribution of an optical fiber such as an optical fiber.

  B-OTDR (Brillouin Optical Time Domain Reflectmetry) is known as a measurement method for detecting distortion of a fiber optic line and temperature abnormality (see, for example, Patent Document 1). This method uses a Brillouin frequency shift. That is, the optical pulse is branched into test light that enters the optical fiber to be measured and reference light, and the test light enters the optical fiber line. A beat signal is generated by combining the back Brillouin scattered light of the test light and the reference light. The beat signal has a frequency corresponding to a difference between the frequencies of the reference light and the scattered light of about 10 GHz and is an amount indicating a Brillouin frequency shift. The Brillouin frequency shift at each point in the optical fiber line can be measured by measuring the time from incidence to light reception (that is, recursion from the light incident end). Since this Brillouin frequency shift varies depending on the strain distribution and the temperature distribution, the strain and temperature distribution in the optical fiber can be measured by measuring the Brillouin frequency shift. The above is the basic principle of B-OTDR.

By the way, when measuring the temperature distribution or strain distribution of an optical fiber by B-OTDR, it is not known whether the measured Brillouin frequency shift change is due to temperature change or strain. In other words, the existing technology cannot separate the effects of temperature distribution and distortion. Accordingly, the strain distribution and the temperature distribution cannot be measured unless the temperature distribution of the optical fiber to be measured is kept constant or has no distortion.
JP-A-3-120437

As described above, the existing technology cannot separate the effects of temperature distribution and strain on the Brillouin frequency shift. Therefore, to measure the temperature distribution, the measured optical fiber must be kept in a strain-free state. Conversely, to measure the strain distribution, the temperature distribution must be kept constant. Is on.
The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical fiber measurement method, an optical fiber measurement system, and an optical fiber measurement device capable of measuring the temperature distribution and strain distribution of an optical fiber at a time. .

In order to achieve the above object, according to one aspect of the present invention, the back Brillouin scattered light of light incident on the optical fiber to be measured is observed, and the time from the incidence of the light to the recurrence is measured. In the optical fiber measurement method for measuring the distribution of characteristics of the optical fiber in association with the position in the direction, the main optical fiber is installed along the path to be measured, and the temperature dependence coefficient C _{T of} the Brillouin frequency shift is A sub optical fiber equal to the main optical fiber is installed tension-free under the same temperature environment as the main optical fiber, the output light of the light source is branched into reference light and test light, and the test light is pulse-modulated to modulate the main optical fiber. The back Brillouin scattered light generated in the main optical fiber and the back Brillouin scattered light generated in the sub optical fiber are combined with the reference light. And by heterodyne detection to calculate the Brillouin frequency shift Delta] f ₂ of the Brillouin frequency shift Delta] f ₁ from the obtained frequency spectrum caused by the main optical fiber caused by the secondary optical fiber by the heterodyne detection, ΔT = Δf ₂ / C _T calculating a distortion [Delta] [epsilon] by when to calculate the temperature change ΔT to calculate the temperature distribution of the secondary optical fiber, the strain-dependent coefficient of Delta] f ₁ was _{C ε, Δε = (Δf 1} -Δf 2) / C ε by Then, a strain distribution of the main optical fiber is calculated, and an optical fiber measuring method is provided. “Installing tension-free” means installing in a state without internal stress to such an extent that the distortion can be ignored.

By taking such means, it can be considered that the sub optical fiber has the same temperature distribution as the main optical fiber, but has no strain distribution. By performing B-OTDR measurement based on the same test light for both fibers, a Brillouin frequency shift can be obtained for each optical fiber. Of these, the Brillouin frequency shift Δf ₂ of the sub optical fiber is affected only by the temperature, so the proportional expression Δf ₂ = C _T × ΔT holds. Solving this with respect to ΔT, first, the temperature change of the sub-fiber is obtained.

The main optical fiber has the same temperature distribution as the sub optical fiber while having a strain distribution. Therefore, the Brillouin frequency shift Δf ₁ in the main optical fiber can be expressed as the sum of the component C _ε × Δε due to the strain Δε and the component due to the same temperature distribution Δf ₂ as that of the sub fiber, that is, Δf ₁ = C _ε × Δε + Δf _2. Is established. By solving this with respect to Δε, the strain change of the main optical fiber can also be obtained.

  In the process so far, the procedure from the incidence of the test light to the measurement is only once. That is, both the temperature and strain characteristics can be obtained by simultaneously injecting test light into two optical fibers and observing the return light. Therefore, the temperature distribution and strain distribution of the optical fiber can be measured at a time.

  According to the present invention, it is possible to provide an optical fiber measurement method, an optical fiber measurement system, and an optical fiber measurement device that can measure the temperature distribution and strain distribution of an optical fiber at a time.

  FIG. 1 is a diagram showing an embodiment of an optical fiber measurement system according to the present invention. In FIG. 1, the output light of the laser light source 1 is branched into two by the light branching unit 3, one of which is incident as a test light on the system to be measured and the other as a reference light on the data processing system. Among these, the test light is externally modulated by the pulse modulator 8, and the pulsed test light enters the optical multiplexing / demultiplexing unit 4.

  Two optical fibers 6 and 7 are connected to the output end of the optical multiplexing / demultiplexing unit 4. These are all optical fibers to be measured that are to be measured by the B-OTDR. Of these, the optical fiber 6 is installed by the support portion 9 or the like, and has different strains depending on the location. The feature of this embodiment is that two optical fibers are installed in parallel to observe both Brillouin backscattering. Since these optical fibers are installed in parallel, they are therefore in the same temperature environment. For distinction, the optical fiber 6 is referred to as a main optical fiber 6, and the optical fiber 7 is referred to as a sub optical fiber 7. Both fibers can be used for actual information transmission.

  The lengths of the main optical fiber 6 and the sub optical fiber 7 are the same, the temperature dependency coefficients for the Brillouin frequency shift are also the same, and both are placed in the same environment, but have the following differences. In other words, the main optical fiber 6 is installed under the same conditions as the region in which the strain distribution is desired to be measured (for example, an air route in which the optical fiber is to be installed), and thus has a strain distribution in the longitudinal direction. On the other hand, the sub optical fiber 7 is installed in a tension-free state with no internal stress to such an extent that the distortion can be ignored. In short, it should be installed in a relaxed state.

  As shown in FIG. 2, the test light (solid arrow in the figure) from the pulse modulator 8 is incident on the main optical fiber 6 and the sub optical fiber 7 via the optical multiplexing / demultiplexing unit 4. Then, backward Brillouin scattered light (dotted arrow in the figure) is generated in both fibers. The backward Brillouin scattered light returns to the optical multiplexing / demultiplexing unit 4, is guided to the optical coupler 5, is combined with the reference light, and enters the observation unit 20. FIG. 3 is a block diagram illustrating an example of the optical multiplexing / demultiplexing unit 4. The test light from the pulse modulator 8 is branched into two by an optical splitter 41 and is incident on the main optical fiber 6 and the sub optical fiber 7 via the optical circulators 42 and 43. Conversely, the backward Brillouin scattered light from the main optical fiber 6 and the sub optical fiber 7 is multiplexed by the optical coupler 44 via the optical circulators 42 and 43, and guided to the optical coupler 5.

  That is, in the optical coupler 5 of FIG. 1, interference light according to the frequency difference (νB) between the reference light and the backward Brillouin scattered light is generated. This interference light is optically / electrically converted by a light receiving unit (PD) 2 such as a photodiode of the observation unit 20 to extract a beat signal. The beat signal is amplified by the amplifier 22 and then input to the spectrum analyzer 30 for heterodyne detection.

  That is, the beat signal is mixed by the mixer 27 with the local signal of the local oscillator (LO) 29 that generates a frequency signal close to νB, and converted into a baseband signal. An unnecessary wave component is removed from the baseband signal by a low-pass filter (LPF) 28, and thereby a beat component based on the Brillouin frequency shift νB is extracted. That is, the Brillouin frequency shift is detected as a beat signal having a frequency νB by combining the Brillouin backscattered light ν0 ± νB and the reference light ν0 and performing heterodyne detection. This beat signal is an amount reflecting the frequency spectrum of the backward Brillouin scattered light generated in each of the optical fibers 6 and 7, that is, the Brillouin frequency shift. The obtained beat signal is digitally converted by the A / D converter 31 and used for data processing in the control unit 50.

The control unit 50 includes a shift calculation unit 50a and a characteristic calculation unit 50b as processing functions according to the present embodiment. Among these, the shift calculation unit 50a calculates Brillouin frequency shifts Δf _{1 and} Δf ₂ generated in the optical fibers 6 and 7 from the beat signals corresponding to the optical fibers 6 and 7, respectively. Brillouin scattered light is affected by both the temperature distribution and the strain distribution in the main optical fiber 6, and is only affected by the temperature distribution in the sub optical fiber 7. Therefore, both can be distinguished as Δf ₁ > Δf ₂ . The characteristic calculation unit 50b calculates the temperature distribution of the sub optical fiber 7 and the temperature distribution and strain distribution of the main optical fiber 6 from the Brillouin frequency shift at each position. The procedure will be described in detail below.

  FIG. 4 is a flowchart showing an optical fiber measurement method according to the present invention. First, the main optical fiber 6 is installed in a desired state (step S1), and then the sub optical fiber 7 is installed in a free state in parallel with the main optical fiber (step S2). Of course, this procedure may be reversed.

Next, test light is incident on both optical fibers 6 and 7 to perform B-OTDR measurement (step S3). In this step, the Brillouin scattered light spectrum of the test light is obtained for each of the optical fibers 6 and 7 in association with the position in the longitudinal direction. That is, in the B-OTDR measurement, the reception time is observed together with the spectral intensity of the Brillouin backscattered light. Since the time taken from the incidence of test light to reception reflects the length of the propagation path of the optical signal, the measurement position common to the main optical fiber 6 and the sub optical fiber 7 can be obtained by analyzing this. Based on these, in this step, the distribution of Brillouin frequency shifts Δf _{1 and} Δf ₂ is calculated.

Next, a temperature distribution and a strain distribution are calculated from the obtained Δf _{1 and} Δf ₂ (step S4). In this calculation, the temperature dependence coefficient of the Brillouin frequency shift of the main optical fiber 6 and the sub optical fiber 7 is C _T, and the distortion dependence coefficient of the main optical fiber 6 is C _ε . Strain is applied only to the main optical fiber 6, and both optical fibers 6 and 7 have a common temperature distribution. Accordingly, when the temperature change amount and strain to be obtained are ΔT and Δε, the following equations (1) and (2) are established.

  By solving the equations (1) and (2) and solving for ΔT and Δε, the temperature change ΔT and the strain Δε can be obtained simultaneously. By performing this calculation for each spectrum at each scattering position, it is possible to simultaneously obtain the temperature distribution and strain distribution in one measurement.

As described above, in this embodiment, the two optical fibers 6 and 7 are installed in parallel under the same temperature environment, and the other optical fiber is provided so that only one of the optical fibers 6 has a strain distribution. 7 is set free. Then, B-OTDR measurement is performed on the two optical fibers to determine the Brillouin frequency shifts Δf ₁ and Δf ₂ of the fibers 6 and 7. The temperature distribution is calculated by dividing the smaller value (Δf ₂ ) of the two Brillouin frequency shifts obtained by the temperature dependence coefficient C _T , and the difference (Δf ₁ −Δf ₂ ) between the two Brillouin frequency shifts is calculated. The strain distribution is calculated by dividing by the strain dependence coefficient _Cε .
That is, the temperature change ΔT is calculated from ΔT = Δf ₂ / C _T , the temperature distribution is calculated from the distribution in the longitudinal direction, the strain Δε is calculated from Δε = (Δf ₁ −Δf ₂ ) / C _ε, and the longitudinal direction The strain distribution can be calculated from the distribution.

  As a result, temperature and strain changes due to Brillouin frequency shift changes can be measured with a single B-OTDR measurement. Therefore, temperature and strain measurements using Brillouin scattered light measurement are easier. It can be carried out. In addition, the system of FIG. 1 can be constructed simply because it is only necessary to install two optical fibers used as sensors in parallel, and therefore the cost of the system can be reduced. Therefore, the present invention can be applied to expand the practicality of an optical fiber sensor using Brillouin scattering, and eventually the practicality of a temperature distribution and strain distribution measurement method using an optical fiber. Furthermore, convenience can be improved for other applications of Brillouin scattering measurement such as an optical fiber line maintenance test using Brillouin frequency shift.

  In addition, this invention is not limited to the said embodiment, In an implementation stage, a component can be deform | transformed and embodied in the range which does not deviate from the summary. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment.

The figure which shows embodiment of the optical fiber measurement system in connection with this invention. FIG. 4 is a schematic diagram showing light propagation in the optical multiplexing / demultiplexing unit 4. FIG. 3 is a block diagram illustrating an example of an optical multiplexing / demultiplexing unit 4. The flowchart which shows the optical fiber measuring method in connection with this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Laser light source, 2 ... Light-receiving part, 3 ... Optical branching part, 4 ... Optical multiplexing / demultiplexing part, 5 ... Optical coupler, 6 ... Main optical fiber, 7 ... Sub optical fiber, 8 ... Pulse modulator, 9 ... Support part , 41 ... Optical splitter, 42, 43 ... Optical circulator, 44 ... Optical coupler, 20 ... Observation section, 27 ... Mixer, 28 ... Low pass filter, 29 ... Local oscillator, 30 ... Spectrum analyzer, 31 ... A / D (Analog / Digital) converter

Claims (4)

In parallel with the first optical fiber for communication having a strain distribution, a second optical fiber having the same temperature dependence coefficient with respect to the Brillouin frequency shift is installed in a tension-free and same temperature environment. ,
Before SL enters the same test light to the first and second optical fiber B-OTDR (Brillouin Optical Time Domain Reflectmetry) was measured,
Of the frequency shifts of the back Brillouin scattered light from each of the first and second optical fibers obtained by the B-OTDR measurement, a frequency shift having a small value is divided by the temperature dependency coefficient to obtain the second. Temperature distribution of the optical fiber
The difference in the frequency shift of the back Brillouin scattered light from each of the first and second optical fibers is divided by the distortion dependence coefficient for the Brillouin frequency shift of the first optical fiber, and the first optical fiber. An optical fiber measuring method characterized by obtaining a strain distribution. Observe the backward Brillouin scattered light of the light incident on the optical fiber to be measured, and measure the distribution of the optical fiber characteristics by associating the time from the incident light to the recurrence with the position in the longitudinal direction of the optical fiber. In the optical fiber measurement method to
Install a main optical fiber for communication along the path to be measured,
A sub-optical fiber having a Brillouin frequency shift temperature dependency coefficient C _T equal to that of the main optical fiber is installed tension-free in the same temperature environment along the same path as the main optical fiber;
Branch the output light of the light source into reference light and test light,
Pulse-modulating the test light and entering the main optical fiber and the sub optical fiber,
The back Brillouin scattered light generated in the main optical fiber and the back Brillouin scattered light generated in the sub optical fiber are combined with the reference light to perform heterodyne detection,
Calculating a Brillouin frequency shift Δf ₁ generated in the main optical fiber and a Brillouin frequency shift Δf ₂ generated in the sub optical fiber from the frequency spectrum obtained by the heterodyne detection;
A temperature change ΔT is calculated by ΔT = Δf ₂ / C _T to calculate a temperature distribution of the sub optical fiber,
The strain distribution of the main optical fiber is calculated by calculating the strain Δε by Δε = (Δf ₁ −Δf ₂ ) / C _ε when the strain dependence coefficient of Δf ₁ is C _ε. Measuring method. A main optical fiber for communication installed along the path to be measured;
A sub-optical fiber installed tension-free under the same temperature environment along the same path as the main optical fiber, and having a Brillouin frequency shift temperature dependency coefficient C _T equal to the main optical fiber;
A laser light source;
A light branching unit that branches the output light of the laser light source to generate test light and reference light;
A pulse modulator for pulse modulating the test light;
An optical combining unit that injects the pulse-modulated test light into the main optical fiber and the sub optical fiber, and combines the back Brillouin scattered light generated in the main optical fiber and the back Brillouin scattered light generated in the sub optical fiber. Namibe,
An optical coupler for multiplexing the light combined in the optical multiplexing / demultiplexing unit with the reference light;
A light receiving unit that optically / electrically converts the output light of the optical coupler and outputs a beat signal;
A frequency analysis unit for obtaining a frequency spectrum of backward Brillouin scattered light generated in the main optical fiber and a frequency spectrum of backward Brillouin scattered light generated in the sub optical fiber from the beat signal;
The Brillouin frequency shift Δf ₁ generated in the main optical fiber is calculated from the frequency spectrum of the back Brillouin scattered light generated in the main optical fiber, and the Brillouin generated in the sub optical fiber from the frequency spectrum of the back Brillouin scattered light generated in the sub optical fiber. A shift calculation unit for calculating the frequency shift Δf ₂ ;
The temperature change ΔT is calculated by ΔT = Δf ₂ / C _T to calculate the temperature distribution of the sub optical fiber, and Δε = (Δf ₁ −Δf ₂ ) / where the strain dependence coefficient of Δf ₁ is C _ε. An optical fiber measurement system comprising: a characteristic calculation unit that calculates a strain distribution of the main optical fiber by calculating a strain Δε by C _ε . A main optical fiber for communication installed along the path to be measured, and installed in a tension-free manner in the same temperature environment along the same path as the main optical fiber, and the temperature dependence coefficient C _{T of the} Brillouin frequency shift Is an optical fiber measuring device used in connection with a secondary optical fiber equal to the main optical fiber,
A laser light source;
A light branching unit that branches the output light of the laser light source to generate test light and reference light;
A pulse modulator for pulse modulating the test light;
An optical combining unit that injects the pulse-modulated test light into the main optical fiber and the sub optical fiber, and combines the back Brillouin scattered light generated in the main optical fiber and the back Brillouin scattered light generated in the sub optical fiber. Namibe,
An optical coupler for multiplexing the light combined in the optical multiplexing / demultiplexing unit with the reference light;
A light receiving unit that optically / electrically converts the output light of the optical coupler and outputs a beat signal;
A frequency analysis unit for obtaining a frequency spectrum of backward Brillouin scattered light generated in the main optical fiber and a frequency spectrum of backward Brillouin scattered light generated in the sub optical fiber from the beat signal;
The Brillouin frequency shift Δf ₁ generated in the main optical fiber is calculated from the frequency spectrum of the back Brillouin scattered light generated in the main optical fiber, and the Brillouin generated in the sub optical fiber from the frequency spectrum of the back Brillouin scattered light generated in the sub optical fiber. A shift calculation unit for calculating the frequency shift Δf ₂ ;
ΔT = Δf ₂ / C _T to calculate the temperature change [Delta] T by calculating the temperature distribution of the secondary optical fiber, when the strain dependence coefficient Delta] f ₁ was _{C ε, Δε = (Δf 1} -Δf 2) / An optical fiber measuring apparatus comprising: a characteristic calculating unit that calculates a strain distribution of the main optical fiber by calculating a strain Δε by C _ε .

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