Optical fiber characteristics analysis device

Abstract

To provide an optical fiber type sensor capable of measuring temperature and strain in the vicinity of a reflection point with a simple configuration in an optical fiber sensing device in which optical fiber itself is made into a sensor.SOLUTION: With an optical fiber characteristics analysis device that analyzes Brillouin scattering of an optical fiber to be measured using a Brillouin optical correlation area reflectometer, signal light subjected to frequency modulation with a periodic signal is incident to the optical fiber to be measured, and reflected light from the other end of the optical fiber to be measured is used as reference light.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to optical fiber sensing technology that uses optical fibers as sensors. [Background technology]

[0002] Optical fiber sensors that use optical fiber as temperature sensors or strain sensors have been researched because of their advantages of long distance, light weight, resistance to electromagnetic noise, and excellent explosion-proofing. Fiber Bragg Grating (FBG) type sensors and long-period fiber grating (LPG) type sensors have been realized as optical fiber temperature sensors (see, for example, Non-Patent Documents 1 and 2).

[0003] However, FBG / LPG type sensors require a sensor part with wavelength characteristics to be inserted into the optical fiber to be measured. To insert the sensor part, it is necessary to draw an FBG (periodic refractive index change). There is also a method of fusion splicing different fibers, but both FBG / LPG type sensors require some kind of special processing on the optical fiber.

[0004] Brillouin Optical Correlation-Domain Reflectometry has been proposed as a temperature sensor that does not require a sensor section to be inserted into the optical fiber (see, for example, Non-Patent Document 3). In Non-Patent Document 3, the correlation between the signal light and the reference light makes it possible to obtain temperature information at any position in the measured optical fiber that has an equal optical path to the reference optical path. However, in Non-Patent Document 3, the position at which the temperature is measured is determined by the length of the delay line inserted in the reference optical path. Therefore, in order to change the position at which the temperature is measured, it is necessary to adjust the length of the delay line inserted in the reference optical path.

[0005] In a Brillouin optical correlation domain reflectometer, a configuration has been proposed in which the reference light path is eliminated by using the reflected light at the far end of the optical fiber under test as the reference light (see, for example, Non-Patent Document 4). However, even when the optical fiber under test has a far-end reflection, the signal-to-noise ratio in Non-Patent Document 4 is low, and the spatial resolution is limited to about 20 cm. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] AD Kersey. et. al., “Fiber Grating Sensors”, J. Lightw. Technol. 15, 1442 (1997). [Non-Patent Document 2] N. Zhao, et. al., “High temperature probe sensor with high sensitivity based on Michelson interferometer”, Opt. Commun, 343, 131(2015). [Non-Patent Document 3] Otsubo et al., "Proposal of a low-coherence Brillouin optical correlation domain reflectometer," 69th Spring Meeting of the Japan Society of Applied Physics, 22p-D215-4, 2022. [Non-Patent Document 4] Toda et al., "Proposal of an optical fiber-type temperature measurement probe using a low-coherence light source," 83rd Autumn Meeting of the Japan Society of Applied Physics, 23p-C302-6, 2022. Summary of the Invention [Problem to be solved by the invention]

[0007] Therefore, an object of the present disclosure is to provide an optical fiber sensor capable of measuring temperature and strain near a reflection point with a simple configuration in an optical fiber sensing device that uses an optical fiber itself as a sensor. [Means for solving the problem]

[0008] The present disclosure relates to an optical fiber characteristic analysis device that uses a Brillouin optical correlation domain reflectometer to analyze Brillouin scattering in a test optical fiber, in which signal light that has been frequency modulated with a periodic signal is incident on one end of the test optical fiber, and reflected light from the other end of the test optical fiber is used as reference light.

[0009] The optical fiber characteristic analysis device executes an optical fiber characteristic analysis method, specifically, the optical fiber characteristic analysis device detects a correlation peak between the Brillouin scattered light and the reflected light in the test optical fiber, the Brillouin scattered light being emitted from the one end of the test optical fiber.

[0010] The present disclosure does not use a reference optical path, but uses periodic frequency modulation that always causes interference at the reflection point of the fiber under test, thereby improving the measurement accuracy of Brillouin scattering near a point where reflection is strong. m For multiple correlation peaks of modulation frequency f m By designing the reflector, it is possible to measure the Brillouin scattering at only one point near the reflection point. Therefore, the present disclosure can provide an optical fiber sensor that can measure the temperature and strain near the reflection point with a simple configuration.

[0011] The optical fiber characteristic analysis device comprises: A light source that generates continuous light; an optical frequency modulation means for frequency-modulating the continuous light with a periodic signal; an optical branching means for inputting the signal light generated by frequency-modulating the continuous light into the one end of the test optical fiber and outputting backscattered light output from the one end of the test optical fiber to an optical detection means; a light detection means for detecting the backscattered light output from the light branching means; a signal analysis means for analyzing the backscattered light detected by the optical detection means; Equipped with.

[0012] The optical frequency modulation means may perform the frequency modulation at a modulation frequency such that an interval between correlation peaks of the Brillouin scattered light and the reflected light is longer than the fiber length of the optical fiber to be measured. The frequency modulation may be, for example, a sinusoidal modulation.

[0013] The signal analyzing means may detect a correlation peak near the other end of the test optical fiber.

[0014] The above disclosures may be combined to the greatest extent possible. Effect of the Invention

[0015] According to the present disclosure, it is possible to provide an optical fiber sensor capable of measuring temperature and strain in the vicinity of a reflection point with a simple configuration. [Brief description of the drawings]

[0016] [Figure 1] 1 shows an example configuration of a Brillouin optical correlation domain reflectometer according to the present disclosure. [Diagram 2] FIG. 1 is an explanatory diagram of Brillouin scattering. [Diagram 3] 1 is an example of a correlation peak that occurs in the present disclosure. [Figure 4] 1 is an example of a periodic signal applied to a signal light. [Diagram 5] An example of the Brillouin measurement result is shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings are assumed to indicate the same components.

[0018] An example of the configuration of an optical fiber characteristic analysis device according to the present disclosure is shown in Fig. 1. The optical fiber characteristic analysis device 91 is a device that analyzes Brillouin scattering of a measured optical fiber 92 using a Brillouin optical correlation domain reflectometer, and includes, for example, a light source 11, an optical branching means 12, an optical detection means 14, a signal analysis means 15, and an optical frequency modulation means 16.

[0019] The light source 11 generates continuous light, and the optical frequency modulation means 16 frequency-modulates the continuous light. As a result, frequency-modulated light is output from the light source 11. The light output from the light source 11 is input as signal light from the optical branching means 12 to the measured optical fiber 92. As a result, the signal light is scattered and reflected in the measured optical fiber 92. The scattered and reflected backscattered light includes Brillouin scattered light and reflected light at the far end of the measured optical fiber 92.

[0020] The backscattered light from the measured optical fiber 92 is incident on the optical detection means 14 from the optical branching means 12. The optical detection means 14 detects the backscattered light from the measured optical fiber 92. The signal analysis means 15 uses the backscattered light detected by the optical detection means 14 to analyze the Brillouin scattering in the measured optical fiber 92.

[0021] Brillouin scattering is a phenomenon in which incident light undergoes a frequency shift and is scattered by acoustic phonons. When Brillouin scattering occurs in a signal light with a frequency of f0, the frequency of the Brillouin scattered light becomes f0-f B The Brillouin frequency shift f B is the refractive index n of the optical fiber 92 to be measured, and the sound velocity v A Using the wavelength λ0 of the signal light propagating through the optical fiber 92 to be measured, it is expressed by the following equation. (Number 1) f B =2nν A / λ0(1)

[0022] The signal analysis means 15 analyzes the frequency f0-f B and the Brillouin frequency shift f based on the reference light frequency f0 BFor example, the signal analysis means 15 calculates the signal intensity at a measurement point P E (Iso-optical path point) is detected and measurement point P E Brillouin frequency shift f B In this case, since the equal optical path point is always the reflection point, it is possible to measure the temperature change ΔT and the strain ε at the reflection point of the optical fiber 92 to be measured.

[0023] In Non-Patent Document 3, a measurement point P E In the present disclosure, the signal light is incident on one end of the measured optical fiber 92, and the reflected light from the other end of the measured optical fiber 92 is used as the reference light. In the present disclosure, the one end of the measured optical fiber 92 may be referred to as the incident end, and the other end of the measured optical fiber 92 may be referred to as the distal end.

[0024] The principle of the present disclosure will be described with reference to Figures 3 and 4. Signal light incident on one end of the measured optical fiber 92 is reflected at the far end in the longitudinal direction of the measured optical fiber 92. The signal light (reference light) reflected at the far end of the measured optical fiber 92 interferes with the Brillouin scattered light. In the present disclosure, this interference light is observed.

[0025] Measurement point P E can be detected using the optical correlation domain method. The position of the correlation peak obtained using the optical correlation domain method represents the position where the reference light and the scattered light interfere most, and the amplitude of the correlation peak represents the interference intensity. When attempting to detect interference light using the optical correlation domain method that uses reflected light as the reference light, the inventors discovered that by performing frequency modulation with a periodic signal as shown in Figure 4, it is possible to measure Brillouin scattering near the reflection point with a higher signal-to-noise ratio than in the case of random frequency modulation. In addition, the interval d at which the correlation peaks of the signal light and the reference light are equal can be measured. m Therefore, the distance d is longer than the fiber length of the fiber 92 to be measured. m By setting the length of the fiber 92 to be measured, it is possible to always measure the Brillouin scattering at the reflection point in the fiber 92 under test without making any special adjustments.

[0026] Therefore, the optical fiber characteristic analysis device 91 executes the optical fiber characteristic analysis method of the present disclosure. Specifically, the optical fiber characteristic analysis device 91 uses the optical frequency modulation means 16 to frequency-modulate the continuous light generated by the light source 11 with a periodic signal. In this embodiment, the optical frequency modulation means 16 modulates the continuous light with a modulation frequency f m In this example, the frequency-modulated signal light is input to one end of the optical fiber 92 under test, and the reflected light from the far end of the optical fiber 92 under test is used as the reference light.

[0027] The optical fiber characteristic analysis device 91 includes: A light source 11 for generating continuous light; an optical frequency modulation means 16 for frequency-modulating the continuous light with a periodic signal; an optical branching means for inputting the signal light generated by frequency-modulating the continuous light into the one end of the test optical fiber and outputting the backscattered light output from the one end of the test optical fiber to an optical detection means; a light detection means 14 for detecting the backscattered light output from the light branching means 12; A signal analysis means 15 for analyzing the backscattered light detected by the optical detection means 14; Equipped with.

[0028] Here, the signal analysis means 15 detects a correlation peak between the Brillouin scattered light generated in the measured optical fiber 92 by the signal light and the reflected light. At this time, the modulation frequency f m The correlation peak spacing d depends on m The correlation peak interval is expressed by the following formula: (Number 2) d m =c / 2nf m (2) Here, c is the speed of light, and n is the refractive index in the optical fiber. E The modulation frequency f of the wave number modulation means 16 is set so that m Adjust.

[0029] For example, the signal analysis means 15 may calculate the correlation peak interval d m The modulation frequency f of the optical frequency modulation means 16 is set so that is longer than the fiber length of the test optical fiber 92. m Then, the optical frequency modulation means 16 performs frequency modulation at a modulation frequency such that the interval between correlation peaks becomes longer than the fiber length of the test optical fiber 92. Specifically, the modulation frequency f m and the length L of the optical fiber to be measured is as follows: (Number 3) L <d m =c / 2nf m (3) From equation (3), the modulation frequency is designed to satisfy the following equation (4). (Number 4) f m <c / 2nL (4) At this time, a correlation peak occurs only at one point near the reflection point in the longitudinal direction of the measured optical fiber 92, so that the signal analysis means 15 can obtain information on the temperature or strain at that position.

[0030] That is, the modulation frequency f m By setting to a low frequency, the correlation peak spacing d m By widening the distance, the position of the correlation peak can be measured near the far end of the test optical fiber 92. Therefore, the optical fiber characteristic analyzer 91 can realize an optical fiber temperature probe device that can measure the temperature near the far end of the test optical fiber 92 with high spatial resolution.

[0031] An example of the temperature measurement result is shown in Fig. 5. Fig. 5(a) shows an example of the measurement of the spectrum of the correlation peak when the frequency modulation is sinusoidal modulation, Fig. 5(b) shows an example of the spectrum after normalizing and moving averaging the correlation peak, Fig. 5(c) shows an example of an enlarged view of the vicinity of the peak, and Fig. 5(d) shows an example of the measurement of the temperature dependence of the Brillouin frequency shift.

[0032] In Fig. 5(a) to Fig. 5(c), the horizontal axis indicates frequency and the vertical axis indicates amplitude. The spectrum in Fig. 5(a) is the beat spectrum of the interference light between the Brillouin scattered light and the reference light (far-end reflected light). The frequency at which the amplitude is maximum in the beat spectrum is the Brillouin frequency shift f B In Fig. 5(d), the Brillouin frequency shift f B As shown in Fig. 5(d), the Brillouin frequency shift f B As described above, according to the present disclosure, it is possible to measure the temperature near the reflection point with high accuracy.

[0033] As described above, the present disclosure makes it possible to measure Brillouin scattering near the reflection point of the measured optical fiber 92 in a Brillouin optical correlation domain reflectometer without using a reference optical path. Therefore, the present disclosure makes it possible to measure the temperature and strain near the reflection point of the measured optical fiber 92 with high accuracy in a Brillouin optical correlation domain reflectometer without performing special processing on the measured optical fiber 92. Furthermore, the present disclosure can always measure the temperature and strain near the reflection point with high accuracy without special adjustments. Therefore, the present disclosure can provide an optical fiber sensor with a high signal-to-noise ratio.

[0034] The signal analysis means 15 included in the optical fiber characteristic analysis device 91 of the present disclosure can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network. [Explanation of symbols]

[0035] 11:Light source 12: Optical branching means 14: Light detection means 15: Signal analysis means 16: Optical frequency modulation means 91: Optical fiber characteristic analyzer 92: Optical fiber under test

Claims

1. An optical fiber characteristic analyzer for analyzing Brillouin scattering of an optical fiber under test using a Brillouin optical correlation domain reflectometer, comprising: A signal light frequency-modulated with a periodic signal is input to one end of the optical fiber to be measured; A reflected light from the other end of the optical fiber to be measured is used as a reference light. Optical fiber characteristic analysis device.

2. detecting a correlation peak between the reflected light and the Brillouin scattered light generated in the test optical fiber by the signal light; 2. The optical fiber characteristic analyzer according to claim 1.

3. performing the frequency modulation at a modulation frequency such that an interval between the correlation peaks is longer than a fiber length of the optical fiber to be measured; 3. The optical fiber characteristic analyzer according to claim 2.

4. detecting a correlation peak near the other end of the test optical fiber; 3. The optical fiber characteristic analyzer according to claim 2.

5. The frequency modulation is a sinusoidal modulation.

2. The optical fiber characteristic analyzer according to claim 1.

6. A light source that generates continuous light; an optical frequency modulation means for frequency-modulating the continuous light with a periodic signal; an optical branching means for inputting the signal light generated by frequency-modulating the continuous light into the one end of the test optical fiber and outputting backscattered light output from the one end of the test optical fiber to an optical detection means; a light detection means for detecting the backscattered light output from the light branching means; a signal analysis means for analyzing the backscattered light detected by the optical detection means; The optical fiber characteristic analyzer according to claim 1 ,

7. An optical fiber characteristic analysis method executed by an optical fiber characteristic analysis device that analyzes Brillouin scattering of a test optical fiber using a Brillouin optical correlation domain reflectometer, comprising: A signal light frequency-modulated with a periodic signal is input to one end of the optical fiber to be measured; A reflected light from the other end of the optical fiber to be measured is used as a reference light. Optical fiber characterization methods.

8. detecting a correlation peak between the Brillouin scattered light in the test optical fiber and the reflected light emitted from the one end of the test optical fiber; The optical fiber characteristic analysis method according to claim 7.

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