US-12624972-B2 - Measurement device for measuring temperature or distortion of an optical fiber, method of adjusting measurement device, and measurement method for measuring temperature of distortion of an optical fiber

US12624972B2US 12624972 B2US12624972 B2US 12624972B2US-12624972-B2

Abstract

A measurement device includes a splitter splitting light from a light source into first and second lights; an optical frequency shifter shifting a frequency of the first light; a first optical modulator modulating intensity of the first light and generating probe light having two frequency components; a second optical modulator generating pump light by pulsing the second light; an optical detector detecting, when the probe light is incident from a first end of an optical fiber to be measured and the pump light is incident from a second side of the optical fiber, light emitted from the second side of the optical fiber; and a processor measuring, based on detected light intensity, temperature or distortion of the optical fiber, in which a frequency of a lower one of the two frequency components yields Brillouin gain, and a frequency of a higher frequency component yields Brillouin loss.

Inventors

Yosuke Tanaka
Daiki Saito

Assignees

NATIONAL UNIVERSITY CORPORATION TOKYO UNIVERSITY OF AGRICULTURE AND TECHNOLOGY

Dates

Publication Date: 20260512
Application Date: 20220214
Priority Date: 20210226

Claims (12)

1 . A measurement device comprising: a splitter configured to split light from a laser light source into two split lights; an optical frequency shifter configured with a phase modulator, the phase modulator being configured to shift a frequency of one of the two split lights by an adjusted frequency shift amount by driving the phase modulator with a modulation signal of a sawtooth wave; a first optical modulator configured to modulate an intensity of the one of the two split lights and to generate probe light having two frequency components; a second optical modulator configured to generate pump light by pulsing the other of the two split lights; an optical detector configured to detect, when the probe light is incident from one end side of an optical fiber to be measured and the pump light is incident from the other end side of the optical fiber, a light intensity of light emitted from the other end side of the optical fiber; and a processor configured to be programmed to measure, based on the light intensity, a temperature or a distortion of the optical fiber, wherein a frequency of a lower frequency component out of the two frequency components is a frequency at which Brillouin gain occurs by interaction with the pump light, and a frequency of a higher frequency component out of the two frequency components is a frequency at which Brillouin loss occurs by the interaction with the pump light, an initial frequency shift amount in the optical frequency shifter is set so as to prevent the two frequency components of the probe light from simultaneously receiving a gain by the Brillouin gain and a loss by the Brillouin loss, the light intensity is acquired by the optical detector each time a frequency of a modulation signal of the first optical modulator is changed, a shape of a Brillouin gain spectrum is obtained based on a relationship between the frequency of the modulation signal and the light intensity, a Brillouin frequency shift is obtained based on the initial frequency shift amount and a peak frequency, and the peak frequency is obtained from the relationship between the frequency of the modulation signal and the light intensity, and the initial frequency shift amount is adjusted such that a relationship between the light intensity and one of the temperature and the distortion of the optical fiber becomes a predetermined relationship within a predetermined range based on the shape of the Brillouin gain spectrum and the Brillouin frequency shift to provide the adjusted frequency shift amount.
2 . The measurement device according to claim 1 , wherein the processor is configured to measure, based on a temporal change in the light intensity, a temperature distribution of the temperature or a distortion distribution of the distortion of the optical fiber.
3 . The measurement device according to claim 1 , wherein the first optical modulator is configured to generate the probe light by modulating the intensity of the one of the two split lights in which the frequency thereof has been shifted by the optical frequency shifter.
4 . A measurement method for causing a processor to execute a process, the measurement method comprising executing on the processor the steps of: splitting light from a laser light source into two split lights; shifting, by a phase modulator, a frequency of one of the two split lights by an adjusted frequency shift amount by driving the phase modulator with a modulation signal of a sawtooth wave; modulating an intensity of the one of the two split lights and generating probe light having two frequency components; generating pump light by pulsing the other of the two split lights; detecting, when the probe light is incident from one end side of an optical fiber to be measured and the pump light is incident from the other end side of the optical fiber, a light intensity of light emitted from the other end side of the optical fiber; and measuring, based on the light intensity, a temperature or a distortion of the optical fiber, wherein a frequency of a lower frequency component out of the two frequency components is a frequency at which Brillouin gain occurs by interaction with the pump light, and a frequency of a higher frequency component out of the two frequency components is a frequency at which Brillouin loss occurs by the interaction with the pump light, and the adjusted frequency shift amount, used in the splitting of the light from the laser light source, is obtained by executing on the processor the steps of: setting an initial frequency shift amount of the frequency of the one of the two split lights so as to prevent the two frequency components of the probe light from simultaneously receiving a gain by the Brillouin gain and a loss by the Brillouin loss; acquiring the light intensity each time a frequency of a modulation signal is changed in the modulating the intensity of the one of the two split lights and the generating the probe light; obtaining a shape of a Brillouin gain spectrum based on a relationship between the frequency of the modulation signal and the light intensity; obtaining a Brillouin frequency shift based on the initial frequency shift amount and a peak frequency, and the peak frequency is obtained from the relationship between the frequency of the modulation signal and the light intensity; and adjusting the initial frequency shift amount such that a relationship between the light intensity and one of the temperature and the distortion of the optical fiber becomes a predetermined relationship within a predetermined range based on the shape of the Brillouin gain spectrum and the Brillouin frequency shift to provide the adjusted frequency shift amount.
5 . The measurement method according to claim 4 , wherein the processor is configured to measure, based on a temporal change in the light intensity, a temperature distribution of the temperature or a distortion distribution of the distortion of the optical fiber.
6 . The measurement method according to claim 4 , wherein the processor is configured to generate the probe light by modulating the intensity of the one of the two split lights in which the frequency thereof has been shifted.
7 . A measurement device comprising: a splitter configured to split light from a laser light source into first and second split lights; an optical frequency shifter configured with a phase modulator, the phase modulator being configured to receive the first split light and shift a frequency of the first split light by an adjusted frequency shift amount by driving the phase modulator with a modulation signal of a sawtooth wave so as to provide shifted first split light; a first optical modulator configured to receive the shifted first split light to modulate an intensity thereof and to generate probe light having two frequency components; a second optical modulator configured to receive the second split light to generate pump light by pulsing the second split light; an optical detector configured to detect, when the probe light is incident from one end side of an optical fiber to be measured and the pump light is incident from the other end side of the optical fiber, a light intensity of light emitted from the other end side of the optical fiber; and a processor configured to be programmed to measure, based on the light intensity, a temperature or a distortion of the optical fiber, wherein a frequency of a lower frequency component out of the two frequency components is a frequency at which Brillouin gain occurs by interaction with the pump light, and a frequency of a higher frequency component out of the two frequency components is a frequency at which Brillouin loss occurs by the interaction with the pump light, the adjusted frequency shift amount in the optical frequency shifter is set so as to prevent the two frequency components of the probe light from simultaneously receiving a gain by the Brillouin gain and a loss by the Brillouin loss, the light intensity is acquired by the optical detector each time a frequency of a modulation signal of the first optical modulator is changed, a Brillouin gain spectrum and a Brillouin frequency shift are obtained based on a relationship between the frequency of the modulation signal and the light intensity, the adjusted frequency shift amount is obtained based on the Brillouin gain spectrum and the Brillouin frequency shift, and the adjusted frequency shift amount is set such that a relationship between the light intensity of the light emitted from the other end side of the optical fiber and the temperature or the distortion of the optical fiber satisfies a predetermined function.
8 . The measurement device according to claim 7 , wherein the processor is configured to measure, based on a temporal change in the light intensity, a temperature distribution of the temperature or a distortion distribution of the distortion of the optical fiber.
9 . The measurement device according to claim 7 , wherein the first optical modulator is configured to generate the probe light by modulating the intensity of the shifted first split light in which the frequency thereof has been shifted by the optical frequency shifter.
10 . A measurement method for causing a processor to execute a process, the measurement method comprising executing on the processor the steps of: splitting light from a laser light source into first and second split lights; receiving the first split light and shifting, by a phase modulator, a frequency of the first split light by an adjusted frequency shift amount by driving the phase modulator with a modulation signal of a sawtooth wave so as to provide shifted first split light; receiving, by a first optical modulator, the shifted first split light to modulate an intensity thereof and to generate probe light having two frequency components; receiving, by a second optical modulator, the second split light to generate pump light by pulsing the second split light; detecting, when the probe light is incident from one end side of an optical fiber to be measured and the pump light is incident from the other end side of the optical fiber, a light intensity of light emitted from the other end side of the optical fiber; and measuring, based on the light intensity, a temperature or a distortion of the optical fiber, wherein a frequency of a lower frequency component out of the two frequency components is a frequency at which Brillouin gain occurs by interaction with the pump light, and a frequency of a higher frequency component out of the two frequency components is a frequency at which Brillouin loss occurs by the interaction with the pump light, the adjusted frequency shift amount is set so as to prevent the two frequency components of the probe light from simultaneously receiving a gain by the Brillouin gain and a loss by the Brillouin loss, the light intensity is acquired each time a frequency of a modulation signal of the first optical modulator is changed, a Brillouin gain spectrum and a Brillouin frequency shift are obtained based on a relationship between the frequency of the modulation signal and the light intensity, the adjusted frequency shift amount is obtained based on the Brillouin gain spectrum and the Brillouin frequency shift, and the adjusted frequency shift amount is set such that a relationship between the light intensity of the light emitted from the other end side of the optical fiber and the temperature or the distortion of the optical fiber satisfies a predetermined function.
11 . The measurement method according to claim 10 , wherein the processor is configured to measure, based on a temporal change in the light intensity, a temperature distribution of the temperature or a distortion distribution of the distortion of the optical fiber.
12 . The measurement method according to claim 10 , wherein the processor is configured to generate the probe light by modulating the intensity of the shifted first split light in which the frequency thereof has been shifted.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application is a 371 U.S. National Phase of International Application No. PCT/JP2022/005619, filed on Feb. 14, 2022, which claims priority to Japanese Patent Application No. 2021-030156, filed Feb. 26, 2021. The entire disclosures of the above applications are incorporated herein by reference. TECHNICAL FIELD The present invention relates to a measurement device for measuring temperature or distortion of an optical fiber, a method of adjusting the measurement device, and a measurement method. BACKGROUND ART Conventionally, many researches have been conducted on a technique of a distributed sensor using Brillouin scattering in an optical fiber. The distributed Brillouin sensing utilizes the fact that a Brillouin gain spectrum, which is a frequency area where Brillouin scattering occurs strongly, changes in proportion to distortion and temperature. In many methods, the frequency sweep of probe light is performed to observe a change in the Brillouin gain spectrum. In addition, proposed are a method of collectively acquiring a Brillouin gain spectrum using pump light and probe light made of multi-frequency light, and a method of appropriately shaping a spectrum of probe light while using pump light and probe light similarly made of multi-frequency light to utilize a change in optical power finally received in proportion to temperature or distortion. In addition, a study has been conducted on a method of using an area in which a change in Brillouin gain spectrum can be regarded as linear with respect to a frequency. However, such an area is narrow. CITATION LIST Non Patent Literature NPL 1: Y Tanaka and Y. Ozaki, “Brillouin frequency shift measurement with virtually controlled sensitivity,” Appl. Phys. Exp. 10, 062504 (2017).NPL 2: Y. Endo and Y. Tanaka, “Sensitivity enhancement of distributed Brillouin fiber optic sensing using two-frequency pump and probe,” SPIE Conf 11525. on Future Sensing Technologies, Paper 11525-3 (2020).NPL 3: A. Voskoboinik, J. Wang, B. Shamee, S. R. Nuccio, L. Zhang, MChitgarha, A. E. Willner, and M. Tur, “SBS-based fiber optical sensing using frequency-domain simultaneous tone interrogation,” J. Lightwave Technol. 29, 1729-1735 (2011).NPL 4: C. Jin, L. Wang, Y Chen, N. Guo, W. Chung, H. Au, Z. Li, H-Y. Tam, and C. Lu, “Single-measurement digital optical frequency comb based phase-detection Brillouin optical time domain analyzer,” Opt. Exp. 25, 9213-9224 (2017).NPL 5: Y. Tanaka, Y. Ozaki, and Y. So, “Scanless Brillouin gain spectrum measurement based on multiheterodyne detection,” in Tech. Digest of International Cof. on Optical Fiber Sensors 2018, TuE88 (2018).NPL 6: Y. Tanaka and T. Hasegawa, “Brillouin optical time domain analysis using spectrally reshaped 12-GHz spacing multimode pump and probe,” Conference on Lasers and Electro-Optics (CLEO) 2020, paper SF3P.7 (2020).NPL 7: Y. Peled, A. Motil, L. Yaron, and M. Tur, “Slope-assisted fast distributed sensing in optical fibers with arbitrary Brillouin profile,” Opt. Express 19, 19845-19854 (2011).NPL 8: H. Lee, N. Hayashi, Y. Mizuno, and K. Nakamura, “Slope-assisted Brillouin optical correlation domain reflectometry: proof of concept,” Photon. Jour. 8, 6802807 (2016). Technical Problem In a method using the frequency sweep of probe light, measurement time is inherently required, and complicated control is required for the frequency sweep of a light source. In addition, in a method of shaping a spectrum of light including a large number of frequency components, devices for performing spectrum shaping are required, and as such the number of system components increases, and modulation adjustment for spectrum generation becomes complicated. The present invention has been made in view of the above problems, and an object of the present invention is to provide a measurement device and the like capable of simplifying a device configuration while shortening measurement time. SUMMARY (1) The present invention relates to a measurement device including: a splitter configured to split light from a laser light source into two lights; an optical frequency shifting unit configured to shift a frequency of one of the two split lights; a first optical modulation unit configured to modulate intensity of one of the split lights and to generate probe light having two frequency components; a second optical modulation unit configured to generate pump light by pulsing the other split light; an optical detection unit configured to detect, when the probe light is incident from one end side of an optical fiber to be measured and the pump light is incident from the other end side of the optical fiber, light emitted from the other end side of the optical fiber; and a processing unit configured to measure, based on light intensity detected by the optical detection unit, temperature or distortion of the optical fiber, in which a frequency of a lower frequency component out of the two frequency components is a frequency a