JPH04248426A - Apparatus for measuring strain distribution of optical fiber - Google Patents

Abstract

PURPOSE:To enhance measuring accuracy to a large extent by measuring the stress strain of the optical fiber in an optical fiber type strain distribution measuring apparatus separately from thermal strain. CONSTITUTION:This measuring apparatus is equipped with a measuring light source 2 allowing pulse light to be incident on the optical fiber 20 arranged to a structure to be measured from the incident end thereof, an optical system respectively separating and detecting the Brillouin scattering light and Raman scattering light in the back scattering light of the pulse light emitted from the incident end of the optical fiber, a light frequency modulator 4 sweeping specific frequency with respect to the Brillouin scattering light to extract the Brillouin scattering light, a strain distribution operating circuit 9 calculating the distribution of the strain quantity of the optical fiber from the Brillouin frequency shift quantity of the extracted Brillouin scattering light, a temp. distribution operating circuit 10 calculating the temp. distribution of the optical fiber from the intensity of the Brillouin scattering light or that of the Raman scattering light, an operation means 11 calculating the distribution of the thermal strain quantity of the optical fiber from the calculated temp. data and a strain and temp. display and output device 11 subtracting the distribution of thermal strain quantity obtained by the operation means from the strain distribution obtained by the strain distribution operating circuit.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber strain distribution measuring device capable of continuously and highly accurately measuring the strain distribution in the longitudinal direction of an optical fiber.

0002

[Prior Art] Conventionally, as a method of measuring continuous strain distribution in the length direction using an optical fiber, a method has been proposed that utilizes the Brillouin scattered light of the optical fiber caused by pulsed light incident on the optical fiber. There is.

[0003] This Brillouin scattered light is a phenomenon in which the incident light interacts with a sound wave generated in the optical fiber and is backscattered with a frequency that is unique to the medium. This frequency shift is called a Brillouin frequency shift. The amount of shift νB
can be expressed by the following equation.

0004

[Equation 1] νB = 2nVA / λ Here, n: refractive index of optical fiber VA: Speed of sound λ: Wavelength of incident light In addition, the sound velocity VA in the optical fiber is

[0005]

[Math 2]

[0006] Here, E: Young's modulus ρ of the optical fiber:
Density κ of optical fiber: can be expressed as Poisson's ratio of optical fiber.

From Equations 1 and 2 above, the sound velocity VA changes due to the strain generated in the optical fiber, and as a result, the Brillouin frequency shift amount νB also changes.

Therefore, by detecting the Brillouin scattered light included in the backscattered light with an optical frequency tuner while performing a frequency sweep, the amount of Brillouin frequency shift νB at each point of the optical fiber can be determined using the normal OTDR method. The distribution can be measured, and the strain distribution applied to the optical fiber can be continuously determined from this Brillouin frequency shift amount.

[0009]

[Problems to be Solved by the Invention] However, in addition to the stress strain of the optical fiber due to external force, there is also thermal strain caused mainly by the difference in coefficient of thermal expansion between the coating material and the strand of the optical fiber. For example, by installing an optical fiber in another structure and measuring the strain distribution of the optical fiber,
When trying to determine the strain distribution of this structure, the thermal strain of the optical fiber is added and measured, so there is a problem that accurate strain distribution measurement of the object to be measured such as the structure cannot be performed. Ta.

The present invention was devised to solve the problems of the prior art described above, and provides an optical fiber type strain distribution measuring device that can separately measure stress strain and thermal strain of an optical fiber. The purpose is to provide

[0011]

[Means for Solving the Problems] In order to achieve the above object, the optical fiber strain distribution measuring device of the present invention includes an optical fiber disposed in a structure to be measured, and a pulsed light input to the optical fiber from its input end. A measurement light source for input, an optical system for separately detecting Brillouin scattered light and Raman scattered light among the backscattered light of the pulsed light emitted from the input end of the optical fiber, and a light source for measuring the Brillouin scattered light. An optical frequency tuner that sweeps a specific frequency to extract Brillouin scattered light, a strain distribution calculation circuit that calculates the distribution of strain in an optical fiber from the Brillouin frequency shift amount of the extracted Brillouin scattered light, and an optical frequency tuner that extracts Brillouin scattered light intensity. or a temperature distribution calculation circuit that calculates the temperature distribution of the optical fiber from the Raman scattered light intensity, a calculation means that calculates the distribution of the amount of thermal strain of the optical fiber from this temperature information, and a distribution of the amount of thermal strain obtained by the calculation means. and means for subtracting from the strain distribution obtained by the strain distribution calculation circuit.

0012

[Effect] The optical fiber strain is the amount of Brillouin frequency shift ν
This shift amount corresponds to the optical fiber strain distribution (
Figure 3(b)) can be measured. However, this measurement result includes thermal strain in addition to the stress strain of the optical fiber.

On the other hand, the amount of thermal strain of the optical fiber is almost uniquely determined by the configuration of the optical fiber 20, that is, the materials and dimensions of the coating material and the strands, and the amount of thermal strain is determined in advance by the amount related to temperature. It can be found as Therefore,
First, the temperature distribution of the optical fiber (Fig. 3(c)) is determined from the Brillouin scattered light intensity or the Raman scattered light intensity using a temperature distribution calculation circuit, and then the thermal strain distribution of the optical fiber (Fig. 3(d)) is calculated from the temperature information. ) ) is obtained by calculation.

Finally, the distribution of the amount of thermal strain at each point of the optical fiber (FIG. 3(d)) is determined by the amount of strain of the optical fiber (
By subtracting from FIG. 3(b)), the pure stress strain of the optical fiber that does not include the amount of thermal strain, that is, the strain distribution of the structure in which the optical fiber is installed (FIG. 3(e)) is measured.

In this way, if the strain distribution and temperature distribution at each point of the optical fiber are determined simultaneously and the stress strain and thermal strain of the optical fiber are measured separately, the influence of thermal strain can be removed and the measurement accuracy can be improved. can be significantly improved.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

In FIG. 1, an optical fiber 20 is installed in a structure whose strain is to be measured. An optical splitter 3 is provided on the input end side of the optical fiber 20, and one port of the optical splitter 3 is provided with a measurement light source 2, and the other port is provided with a detection system for backscattered light. There is. Measurement light source 2
A light source drive circuit 1 is connected to the light source drive circuit 1, and the measurement light source 2 is driven by a trigger signal from a signal processing circuit 8 at regular intervals. A single wavelength pulsed light having a frequency ω0 is emitted from the measurement light source 2, passes through the optical splitter 3, and is connected to the optical fiber 2.
0. In the optical fiber 20, in addition to the Rayleigh scattered light with the frequency ω0 at the x point, the frequency ω0 ±ωB
Brillouin scattered light is generated.

A portion of the Rayleigh scattered light and Brillouin scattered light generated in the optical fiber 20 enters the optical frequency tuner 4 from the optical splitter 3 as backscattered light. This optical frequency tuner 4 is composed of, for example, a Fabry-Perot interferometer. A frequency sweep circuit 5 and its control circuit 6 are connected to the optical frequency tuner 4, and the tuning frequency of the optical frequency tuner 4 is changed by a constant frequency step change according to a control signal from the signal processing circuit 8. swept, and the tuning frequency is ω0 - ωB , ω0 - ωB', ... or ω0 + ωB
, ω0 + ωB', ... The light emitted from the optical frequency tuner 4 enters the photodetector 7, is converted into an electrical signal, and is input to the signal processing circuit 8. In the signal processing circuit 8, the backscattered light converted into an electrical signal is transmitted to the optical fiber 20 based on the delay time between the trigger signal sending time to the light source driving circuit 1 and the electrical signal input time from the photodetector 7. Identify the location of the occurrence (see Figure 3(a)). That is, the measurement position of the strain or temperature of the optical fiber is determined from the time from when the pulsed light is emitted from the measurement light source 4 until the photodetector 7 detects the backscattered light. If the amount of strain in the optical fiber at each position is known, the strain distribution of the optical fiber can be measured.

First, it is known that the Brillouin frequency shift amount νB of an optical fiber in an undistorted state is approximately 13 GHz. Next, in the Brillouin backscattered light detection system, when a strain is applied to a certain point x in the optical fiber 20, Brillouin scattered light with a frequency ω0 ±ωB shifted by a frequency corresponding to the amount of distortion, that is, Stokes light, is generated. (frequency: ω0 −ωB) and anti-Stokes light (frequency: ω0 +ωB) are generated and enter the optical frequency tuner 4 as backscattered light. Now, if the tuning frequency of this optical frequency tuner 4 is ω0 −ωB, then the above x
Of the Brillouin scattered light generated at the point, a tuning signal light consisting only of Stokes light (frequency: ω0 - ωB) is selectively emitted from the optical frequency tuner 4 and enters the photodetector 7. At this time, the light intensity received by the photodetector 7 corresponds to the Brillouin frequency shift amount (ω0 - ωB) at the point x, so the output of the photodetector 7 is evaluated by the distortion distribution calculation circuit 9. , optical fiber 2 at this point x
The amount of strain (the sum of stress strain and thermal strain) of 0 can be found. The amount of Brillouin frequency shift νB at other points is (ω0
-ωB), the amount of strain (sum of stress strain and thermal strain) at that point can be found in the same way.

Here, the tuning frequency of the optical frequency tuner 4 is changed every fixed frequency step, that is, (ω0 −ωB
), (ω0 −ωB'), ... By repeating the above operation, the amount of Brillouin frequency shift at each point over the entire length of the optical fiber 20 can be measured by the strain distribution calculation circuit 9, Thereby, the strain distribution of the optical fiber at each point (FIG. 3(b)) can be determined.

[0021] The above description was about the Stokes light component, but the anti-Stokes light component (frequency: ω0 + ω
Similarly, the strain distribution of the optical fiber (Fig. 3 (
b) ) can be found.

Next, it is known that the intensity ratio of Stokes light and anti-Stokes light, which are two components of Brillouin scattered light, can be expressed as a function of temperature as shown by the following equation (3).

[0023]

[Math 3]

Therefore, the pair of frequency operations (ω0 ±
If the Stokes light and anti-Stokes light intensity outputs of the photodetector 7 corresponding to The temperature of the fiber can be determined, and the temperature distribution over the entire length of the optical fiber (FIG. 3(C)) can be determined by frequency sweeping operation of the optical frequency tuner 4.

On the other hand, the amount of thermal strain of an optical fiber is almost uniquely determined by the configuration of the optical fiber, that is, the materials and dimensions of the coating material and the strands, and this amount of thermal strain is determined in advance as an amount related to temperature. You can keep it. Therefore, the thermal strain distribution (FIG. 3(d)) can be calculated and determined from the measured temperature distribution of the optical fiber (FIG. 3(C)). Therefore, the strain/temperature display/output device 11 receives the output of the temperature distribution calculation circuit 10, and adds it to the predetermined relational expression between temperature and thermal strain amount to obtain the thermal strain distribution (Fig. 3(d). ) is calculated.

Next, the strain/temperature display/output device 11 subtracts the amount of thermal strain at each point on the optical fiber (FIG. 3(d)) from the amount of strain on the optical fiber (FIG. 3(b)). . In this way, the pure stress strain of the optical fiber, which does not include the amount of thermal strain, ie, the strain distribution of the structure in which the optical fiber is installed (FIG. 3(e)), is measured.

In the above embodiment, a method using Brillouin backscattered light to measure the temperature distribution of an optical fiber was described, but similarly, Raman backscattered light generated by light incident on the optical fiber can also be used. Similarly, the temperature distribution of an optical fiber can be measured.

FIG. 2 shows an example of a method using this Raman backscattered light.

In this embodiment, considering that the Raman backscattered light is weaker than the Brillouin backscattered light, pulsed light is emitted from a separately provided light source 12 and is input into the optical fiber 20. . The Raman backscattered light generated in the optical fiber 20 by this pulsed light returns to the input end again, and then returns to the optical demultiplexer 1 installed in front of the optical frequency tuner 4.
3, the light is separated from the Rayleigh backscattered light and the Brillouin backscattered light. The separated Raman backscattered light is further separated into two components of the Raman scattered light, Stokes light and anti-Stokes light, by the next optical demultiplexer 14, and each is sent to the photodetector 1.
5 and 16, it is converted into an electrical signal. Both signals from each point of the optical fiber are averaged by the signal processing circuit 17 and then input to the temperature distribution calculation circuit 10. Based on these two signals, the temperature distribution calculation circuit 10 calculates the light intensity ratio of the Stokes light and the anti-Stokes light from each point of the optical fiber, and finally calculates the total length of the optical fiber according to the same relationship as the equation 3 described above. Obtain temperature distribution. The thermal strain of the optical fiber is calculated separately from the stress strain using this temperature distribution information of the optical fiber, and the influence of thermal strain is removed from the strain distribution separately determined by the strain distribution calculation circuit 9 to obtain the true strain. The procedure for measuring is the same as in the case of FIG. 1 described above.

Note that since Raman backscattered light is normally weaker than Brillouin backscattered light, the case where a separate light source 12 is used to sufficiently detect this Raman backscattered light has been described above. If the single frequency light source output used for Brillouin backscattered light detection is large enough to be used for Raman backscattered light detection, then this single frequency light source is sufficient as the light source, and the above-mentioned separate light source is of course unnecessary. be.

[0031]

As described above, the present invention recognizes that the strain distribution of an optical fiber measured from the Brillouin frequency shift amount νB includes the thermal strain of the optical fiber as an error, The temperature distribution of the optical fiber is measured from the intensity or the Raman scattered light intensity, and the distribution of the amount of thermal strain of the optical fiber is determined from the temperature information. Since the strain component is subtracted, it is not only possible to measure strain distribution with high precision by separating stress strain and thermal strain, but also to measure both strain and temperature distribution of a single optical fiber, which is economical. Excellent in sex.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an embodiment of an optical fiber strain distribution measuring device of the present invention.

FIG. 2 is a schematic diagram showing a modified embodiment of the optical fiber strain distribution measuring device of the present invention.

FIG. 3 is an explanatory diagram for the example in FIG. 1, where (a) is a Brillouin backscattered light intensity distribution diagram when swept at each Brillouin frequency shift, and (b) is a diagram obtained from the Stokes light component in (a). (c) is the temperature distribution diagram obtained from Stokes light and anti-Stokes light in (a), (d) is the thermal strain distribution diagram obtained from (c), and (e) is the temperature distribution diagram obtained from (c).
It is a stress strain distribution diagram obtained from b) and (d).

[Explanation of symbols]

1 Light source drive circuit 2. Light source for measurement 3 Optical splitter 4 Optical frequency tuner 5 Frequency sweep circuit 6 Control circuit 7 Photodetector 8 Signal processing circuit 9 Distortion distribution calculation circuit 10 Temperature distribution calculation circuit 11　Distortion/temperature display/output device 12 Measurement light source 13,14 Optical demultiplexer 15, 16 Photodetector 17 Signal processing circuit

Claims (1)

[Claims] 1. An optical fiber disposed in a structure to be measured, a measurement light source for inputting pulsed light into the optical fiber from its input end, and a measuring light source for inputting pulsed light to the optical fiber from its input end. An optical system that separates and detects Brillouin scattered light and Raman scattered light among the backscattered light, an optical frequency tuner that sweeps a specific frequency with respect to the Brillouin scattered light and extracts the Brillouin scattered light, and this extracted A strain distribution calculation circuit that calculates the distribution of strain in an optical fiber from the Brillouin frequency shift amount of the Brillouin scattered light, a temperature distribution calculation circuit that calculates the temperature distribution of the optical fiber from the Brillouin scattered light intensity or the Raman scattered light intensity, and this temperature The present invention further comprises a calculation means for calculating the distribution of the amount of thermal strain of the optical fiber from the information, and a means for subtracting the distribution of the amount of thermal strain obtained by the calculation means from the strain distribution obtained by the strain distribution calculation circuit. Characteristic optical fiber strain distribution measuring device.