Measurement device and measurement method

Abstract

Provided are a measurement device, etc., with which it is possible to improve convenience while simplifying device configuration.　This measurement device branches light from a laser light source into two, intensity-modulates one branched light to generate pump light that is pulsed light composed of two frequency components, shifts the frequency of the other branched light to generate probe light, multiplexes the probe light and light emitted from one end of an optical fiber to be measured when the pump light is incident from the one end of the optical fiber, detects a beat signal of the probe light and Brillouin scattered generated in a frequency band between the two frequency components, and measures strain or the temperature of the optical fiber on the basis of the detected beat signal. The interval between the two frequency components is set so that the Brillouin gain spectrum generated on the low-frequency side of the higher frequency component and the Brillouin loss spectrum generated on the high-frequency side of the lower frequency component of the two frequency components overlap on the frequency axis.

Description

Measuring device and measuring method

[0001] The present invention relates to a measuring device and a measuring method for measuring the temperature or strain of an optical fiber.

[0002] Numerous distributed sensor technologies utilizing Brillouin scattering in optical fibers have been studied. Distributed Brillouin sensing utilizes the fact that the Brillouin gain spectrum, which is the frequency region where Brillouin scattering is strongest, changes in proportion to strain and temperature. One typical method, Brillouin analysis, involves injecting a probe beam into the opposite end of the optical fiber (the opposite end from the pump beam) and observing changes in the Brillouin gain spectrum with a relatively high signal-to-noise ratio. However, this requires frequency sweeping of the probe beam, which can be time-consuming. Another typical method, Brillouin reflectometry, such as Brillouin Optical Time Domain Reflectometry (BOTDR), involves heterodyne interference of the probe beam with the Brillouin scattered light from the optical fiber, rather than injecting it directly into the optical fiber. The beat signal obtained by photoelectric conversion in a photodetector is then input to an electrical spectrum analyzer to observe changes in the Brillouin gain spectrum. However, electrical spectrum analyzers, which consist of high-frequency circuits, are expensive.

[0003] Other proposed methods include simultaneously acquiring the Brillouin gain spectrum using multi-frequency pump and probe light, and using multi-frequency pump and probe light with appropriate shaping of the probe light spectrum to exploit the fact that the final received optical power changes in proportion to temperature and strain. While these methods realize a configuration without frequency sweeping, shaping the spectrum of light consisting of multiple frequency components requires equipment for spectrum shaping, resulting in an increase in the number of system components. Additionally, methods have been considered that utilize the region in which the change in the Brillouin gain spectrum can be considered linear with respect to frequency. However, this region is narrow.

[0004] In addition, a method has been proposed that enables simple measurements with fewer system components by using the Brillouin gain spectrum and Brillouin loss spectrum obtained with single-frequency pump light and dual-frequency probe light.

[0005] Y. Tanaka and Y. Ozaki, “Brillouin frequency shift measurement with virtually controlled sensitivity,” Appl. Phys. Exp. 10, 062504 (2017).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).A. Voskoboinik, J.Wang, B. Shamee, S. R. Nuccio, L. Zhang, M.Chitgarha, A. E. Willner, and M. Tur, “SBS-based fiber optical sensing using frequency-domain simultaneous tone interrogation,” J. Lightwave Technol. 29, 1729-1735 (2011).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).Y. Tanaka, Y. Ozaki, and Y. So, “Scanless Brillouin gain spectrum measurement based on multiheterodyne detection,” in Tech. Digest of International Conf. on Optical Fiber Sensors 2018,TuE88 (2018).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).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).H. Lee, N. Hayashi, Y. Mizuno, and K. Nakamura, “Slope-assisted Brillouin optical correlation-domain reflectometry: proof of concept,” Photon. Jour. 8, 6802807 (2016).K. Hoshino, D. Saito, Y. Endo, T. Hasegawa, and Y. Tanaka, “Brillouin gain spectrum manipulation using multifrequency pump and probe for slope-assisted BOTDA with wider dynamic range,” Applied Physics Express, vol.15,022009, 2022.

[0006] The technique using two-frequency probe light has a configuration in which the probe light is incident from the opposite side of the optical fiber (the side opposite to the side where the pump light is incident), so there is room for improvement in terms of convenience.

[0007] The present invention has been made in consideration of the above-mentioned problems, and its object is to provide a measuring device etc. that can simplify the device configuration while improving convenience.

[0008] (1) The present invention relates to a measurement device including: a branching unit that branches light from a laser light source into two; an optical modulation unit that intensity-modulates one of the branched light beams to generate pump light, which is pulsed light consisting of two frequency components; an optical frequency shift unit that generates probe light by shifting the frequency of the other branched light beam; an optical detection unit that receives light obtained by combining the probe light and light emitted from one end of an optical fiber to be measured when the pump light is incident on the one end of the optical fiber, and detects a beat signal between the probe light and Brillouin scattered light generated in a frequency band between the two frequency components; and a processing unit that measures a temperature or strain of the optical fiber based on the beat signal detected by the optical detection unit, wherein the interval between the two frequency components is set so that a Brillouin gain spectrum occurring on the low-frequency side of the higher of the two frequency components and a Brillouin loss spectrum occurring on the high-frequency side of the lower of the two frequency components overlap on the frequency axis.

[0009] The present invention also relates to a measurement method including a branching step of branching light from a laser light source into two, an optical modulation step of intensity-modulating one of the branched lights to generate pump light which is pulsed light consisting of two frequency components, an optical frequency shifting step of shifting the frequency of the other of the branched lights to generate probe light, an optical detection step of receiving light obtained by combining the probe light and light emitted from one end of an optical fiber to be measured when the pump light is incident on the one end of the optical fiber, and detecting a beat signal between the probe light and Brillouin scattered light generated in a frequency band between the two frequency components, and a processing step of measuring a temperature or strain of the optical fiber based on the beat signal detected in the optical detection step, wherein the interval between the two frequency components is set so that a Brillouin gain spectrum occurring on the low-frequency side of the higher of the two frequency components and a Brillouin loss spectrum occurring on the high-frequency side of the lower of the two frequency components overlap on the frequency axis.

[0010] According to the present invention, the only light incident on the optical fiber is pump light incident from one end of the optical fiber, so that the device configuration can be simplified and the convenience can be improved.

[0011] (2) In the measuring device and measuring method according to the present invention, the processing unit (in the processing step) may measure a temperature distribution or a strain distribution of the optical fiber based on a time change of the beat signal detected by the light detection unit (in the light detection step).

[0012] According to the present invention, it is possible to measure the temperature distribution and strain distribution from the time change of the beat signal without the need for frequency sweeping.

[0013] Fig. 1 is a diagram showing the configuration of a measurement device according to this embodiment. Fig. 2 is a diagram showing the relationship between pump light, Brillouin gain spectrum, Brillouin loss spectrum, Brillouin scattered light, and probe light. Fig. 3 is a diagram showing an experimental system for an experiment confirming the principle of the method according to this embodiment. Fig. 4 is a diagram showing the experimental results of an experiment confirming the principle of the method according to this embodiment. Fig. 5 is a diagram showing an example of the configuration of a measurement device when no probe light is used.

[0014] Hereinafter, an embodiment of the present invention will be described. Note that the embodiment described below does not unduly limit the content of the present invention described in the claims. Furthermore, not all of the configurations described in the embodiment are necessarily essential constituent elements of the present invention.

[0015] 1 is a diagram showing the configuration of a measurement device according to this embodiment. The measurement device 1 is a device for measuring the temperature or strain of a sensing area SA (measurement target) of an optical fiber 2, and includes a single-wavelength (single-mode) laser light source 10, an optical coupler 11 (branching section), an optical modulator 20 (optical modulation section), a pulse signal generator 30, an oscillator 40, an optical circulator 50, an optical frequency shifter 60 (optical frequency shift section), an optical coupler 12 (combining section), a photodetector 70 (optical detection section), and a data processing device 80 (processing section).

[0016] The optical coupler 11 splits the light from the laser light source 10 into two beams at a predetermined split ratio (for example, 1:1).

[0017] The optical modulator 20 modulates the intensity of one of the lights branched by the optical coupler 11 to generate two frequency components (with a center frequency ν ｐｕｍｐ１ , ν ｐｕｍｐ２ ), and generates pump light Pm, which is pulsed light composed of the pulsed light and the center frequency ν . The pulse signal generator 30 generates an intensity-modulated signal (pulse signal) for pulsing, and the oscillator 40 generates an intensity-modulated signal for frequency-doubling. The pulse signal from the pulse signal generator 30 and the intensity-modulated signal from the oscillator 40 are combined and supplied to the optical modulator 20. When a Mach-Zehnder optical modulator is used as the optical modulator 20, it is possible to suppress the frequency components of the incident light by controlling the bias voltage and generate only the sideband components. In this case, the interval (frequency interval) between the two frequency components of the pump light Pm is twice the frequency of the oscillator 40. Furthermore, the interval between the two frequency components of the pump light Pm is determined by the higher frequency component (center frequency ν ) of the two frequency components. ｐｕｍｐ２ ) and the lower frequency component (center frequency ν ｐｕｍｐ１ ) overlap on the frequency axis. The optical modulation unit may be configured with an optical modulator for pulsing (an optical modulator that performs intensity modulation based on the pulse signal from the pulse signal generator 30) and an optical modulator for frequency duplication (an optical modulator that performs intensity modulation based on the intensity-modulated signal from the oscillator 40). In this case, the optical modulator for pulsing may be a Mach-Zehnder optical modulator, or an optical modulator based on an acousto-optic modulator (AOM) or a semiconductor optical amplifier (SOA).

[0018] The optical circulator 50 outputs the pump light Pm input to the first port from the second port and guides it to one end of the sensing area SA, and also outputs the light (light including Brillouin scattered light Br generated in a frequency band between the two frequency components of the pump light Pm) emitted from the one end of the sensing area SA and input to the second port from the third port and guides it to the optical coupler 12. The optical circulator 50 may be configured with an optical isolator and an optical coupler.

[0019] The optical frequency shifter 60 shifts the frequency of the other light branched by the optical coupler 11 to generate probe light Pr. The amount of frequency shift by the optical frequency shifter 60 is determined by the frequency (ν ｐｒｏｂｅ ) is set so as not to overlap with the spectrum of the Brillouin scattered light Br. The optical frequency shifter 60 may be an acousto-optical modulator, or may be configured with an optical modulator and an optical wavelength filter.

[0020] The optical coupler 12 multiplexes the light emitted from one end of the sensing area SA and output from the third port of the optical circulator 50 with the probe light Pr output from the optical frequency shifter 60. The optical receiver 70 receives the light multiplexed by the optical coupler 12 and detects a beat signal between the Brillouin scattered light Br and the probe light Pr. The intensity of the beat signal detected by the optical receiver 70 (optical heterodyne detection) is output as an electrical signal and input to the data processing device 80.

[0021] The data processing device 80 is a computer equipped with a processor (e.g., CPU) and a storage unit (e.g., RAM, hard disk), and calculates the temperature or strain of the optical fiber 2 (sensing area SA) based on the signal intensity detected by the photodetector 70. For example, a function approximating the relationship (e.g., a proportional relationship) between the signal intensity and the temperature or strain is calculated by pre-calibration, and the signal intensity detected by the photodetector 70 is substituted into the function to calculate the temperature or strain of the sensing area SA. The data processing device 80 also calculates the temperature or strain distribution of the sensing area SA based on the temporal change in the signal intensity detected by the photodetector 70. The length of light propagation during one pulse period of the pump light Pm corresponds to the length of the sensing area SA. In other words, the temperature and strain distribution in the sensing area SA can be measured from the change in signal intensity measured within one pulse period of the pump light Pm. The timing of generation of each pulse of the pump light Pm is supplied as a trigger signal from the pulse signal generator 30.

[0022] As shown in FIG. 2, in the optical fiber 2 (sensing area SA), the higher frequency component (center frequency ν ｐｕｍｐ２), a Brillouin gain spectrum (BGS) is generated on the low frequency side of the ｐｕｍｐ１ ) on the high frequency side, a Brillouin loss spectrum (BLS) is generated. ｐｕｍｐ１ , ν ｐｕｍｐ２ The spacing between the BGS and BLS is set so that the BGS and BLS overlap on the frequency axis, which reduces the BGS gain (Brillouin gain) and the intensity (power) of the Brillouin scattered light Br generated in the BGS frequency band. The BSS shown in Figure 2 is the spectrum of the Brillouin scattered light Br with reduced power. ｐｕｍｐ２ and the difference between the center frequency of BGS and ν ＢＦＳ (and ν ｐｕｍｐ１ and the difference between the center frequency of the BLS ＢＦＳ ) is called the Brillouin frequency shift. The Brillouin frequency shift ν ＢＦＳ changes in proportion to the change in temperature or strain of the sensing area SA. ＢＦＳ As the BGS and BLS approach or separate from each other due to changes in the gain of the BGS, the intensity of the Brillouin scattered light Br changes. By utilizing this phenomenon, the temperature and strain of the sensing area SA can be easily measured by measuring the optical power.

[0023] Probe light Pr (center frequency ν ｐｒｏｂｅ ) is generated by shifting the frequency of the light from the laser light source 10. This frequency shift is expressed as v ｐｒｏｂｅThe BSS is set so that it does not overlap with the BSS. The photodetector 70 observes a beat signal between the Brillouin scattered light Br and the probe light Pr. FIG. 2 illustrates the beat frequency BF between the Brillouin scattered light Br and the probe light Pr. The data processing device 80 observes the time change of the beat signal component by observing the signal intensity of a specific frequency component of the beat signal. It is also possible to observe changes in the intensity of the entire beat signal. The intensity of the beat signal changes depending on fluctuations in the BGS gain (changes in the temperature or strain of the sensing area SA). If the relationship between the intensity of the beat signal and the temperature or strain of the sensing area SA is calibrated in advance, the temperature or strain of the sensing area SA can be measured from the intensity of the observed beat signal using the calibration results.

[0024] Fig. 3 is a diagram showing an experimental system for an experiment to confirm the principle of the method of this embodiment. In Fig. 3, LD is a single-mode laser light source, IM1 to IM3 are optical intensity modulators, SG1 and SG2 are oscillators, EDFA1 to EDFA3 are optical amplifiers, BPF is an optical bandpass filter, OSA is an optical spectrum analyzer, PS is a polarization scrambler, FG is an arbitrary waveform generator, ATT is an attenuator, PD is a photodetector, ESA is an electrical spectrum analyzer, and FUT is the fiber under test (sensing area SA).

[0025] Light from a single-mode laser light source LD with a wavelength of 1.55 μm was split into two by an optical fiber coupler (3 dB coupler). The upper optical path of the experimental system was the optical path for generating probe light Pr, and the lower optical path was the optical path for generating pump light Pm. The fiber under test FUT was a 5 km optical fiber, and its temperature was kept constant at 50°C in an incubator.

[0026] The light from the laser light source LD was intensity-modulated by the optical intensity modulator IM1 to produce light consisting of two frequency components. After branching, the higher-frequency component was extracted by the optical bandpass filter BPF in the upper optical path, and intensity-modulated by the optical intensity modulator IM2 to produce light consisting of two frequency components. The lower-frequency component was used as the probe light Pr. The higher-frequency component becomes a high-frequency signal when optical heterodyne detection is performed, and is therefore not electrically detected. The intensity modulation frequency in the optical intensity modulator IM1 was set to 10.88 GHz, and the intensity modulation frequency in the optical intensity modulator IM2 was set to 10.6 GHz. This shifted the frequency of the light from the laser light source LD by 280 MHz (10.88 - 10.6 = 0.28 GHz) to generate the probe light Pr. In the lower optical path, the branched light (light consisting of two frequency components) was intensity-modulated by an optical intensity modulator IM3 driven by a pulsed modulation signal to generate pump light Pm, which is pulsed light with a pulse width of 25 μs and a period of 50.1 μs.

[0027] The spectrum diagram shown in the lower right of Figure 3 illustrates optical heterodyne detection (enclosed by the dotted line). This diagram illustrates the positional relationship between the Brillouin scattered light Br and the probe light Pr on the frequency axis, and is not to scale. The beat frequency between the Brillouin scattered light Br and the probe light Pr obtained by optical heterodyne detection is approximately 200 MHz, and the frequency interval between the probe light Pr (the lower-frequency component of the two frequency components) and the undetected higher-frequency component is 21.2 GHz in the example of Figure 3.

[0028] In this experiment, the interval between the two frequency components of the pump light Pm was changed so that the relative positions on the frequency axis of the Brillouin gain spectrum BGS, which occurs on the low-frequency side of the higher frequency component of the two frequency components of the pump light Pm, and the Brillouin loss spectrum BLS, which occurs on the high-frequency side of the lower frequency component, changed (the gain of the BGS changed), and the signal intensity detected by the photodetector PD at that time (the integral value of the entire spectrum of the beat signal) was measured. Specifically, the light source frequency and the central frequency ν ｐｕｍｐ２ (The center frequency of the low-frequency component is ν ｐｕｍｐ１ ) and the frequency difference f ｐｕｍｐThe change in signal strength was measured when the frequency was changed from 10.88 GHz to 10.89 GHz in 1 MHz steps.

[0029] The experimental results are shown in Figure 4. As shown in Figure 4, ｐｕｍｐ Experiments have shown that the signal strength (i.e., the power of the Brillouin scattered light Br) increases as the overlap between the BGS and BLS decreases (the BGS gain increases). Strictly speaking, the interaction between the BGS and BLS should be analyzed using the coupled-mode equation, but it is predicted that the change in the frequency difference between the two frequency components of the pump light Pm will be monotonous if the half-width of the BGS or BLS is within a range narrower than 100 MHz, and indeed the experimental results were as predicted.

[0030] According to the technique of this embodiment, by using pump light consisting of two frequency components and setting the frequency interval so that the BGS occurring on the low-frequency side of the higher frequency component and the BLS occurring on the high-frequency side of the lower frequency component overlap on the frequency axis, it becomes possible to measure temperature and strain from the signal strength, and to measure temperature distribution and strain distribution from the time change of the signal strength, without requiring frequency sweeping. Furthermore, since the equipment required for spectral shaping of the pump light and the probe light can be omitted, the device configuration can be simplified. Furthermore, since only the pump light is incident on one end of the optical fiber, convenience is improved compared to when the probe light is incident on the other end of the optical fiber.

[0031] FIG. 5 shows an example of the configuration of a measurement device that does not use probe light Pr. In FIG. 5, components identical to those shown in FIG. 1 are designated by the same reference numerals, and their description will be omitted where appropriate. In the example shown in FIG. 5, light from the laser light source 10 is incident on the optical modulator 20 without being branched. Furthermore, light emitted from one end of the sensing area SA, input to the second port of the optical circulator 50, and output from the third port is incident on an optical filter 90 (optical wavelength filter). The optical filter 90 passes only the Brillouin scattered light Br generated in the frequency band between the two frequency components of the pump light Pm (extracting only the spectrum BSS of the Brillouin scattered light Br). The optical receiver 70 detects the light that has passed through the optical filter 90, and the data processing device 80 calculates the temperature or strain of the optical fiber 2 (sensing area SA) based on the light intensity detected by the optical receiver 70. Extracting the Brillouin scattered light Br using the optical filter 90 in this way further simplifies the device configuration. When the wavelength of the pump light Pm is about 1.5 μm, the Brillouin gain spectrum BGS occurs near a frequency about 11 GHz lower than the respective frequency components of the pump light Pm, and the Brillouin loss spectrum BLS occurs near a frequency about 11 GHz higher than the respective frequency components. The spectral widths of the Brillouin gain spectrum BGS and the Brillouin loss spectrum BLS are generally 100 MHz or less, and the frequencies of these spectra shift by about 1 MHz with a temperature change of 1 degree, and －６ The optical filter 90 preferably has a transmission bandwidth (in the case of a transmission type) or a reflection bandwidth (in the case of a reflection type) of at least 0.1 nm or less. In the case of a transmission type, the transmitted light is detected by the photodetector 70, and in the case of a reflection type, the reflected light is detected by the photodetector 70. For example, an optical fiber grating (FBG) with a bandwidth of 0.1 nm or less can be used as such an optical filter.

[0032] The present invention is not limited to the above-described embodiments, and various modifications are possible. The present invention includes configurations that are substantially the same as the configurations described in the embodiments (for example, configurations with the same functions, methods, and results, or configurations with the same purpose and effects). The present invention also includes configurations in which non-essential parts of the configurations described in the embodiments are replaced. The present invention also includes configurations that achieve the same effects or purposes as the configurations described in the embodiments. The present invention also includes configurations in which publicly known technology is added to the configurations described in the embodiments.

[0033] REFERENCE SIGNS LIST 1... measuring device, 2... optical fiber, 10... laser light source, 11, 12... optical coupler, 20... optical modulator, 30... pulse signal generator, 40... oscillator, 50... optical circulator, 60... optical frequency shifter, 70... photoreceiver, 80... data processing device, 90... optical filter

Claims

1. A measuring device comprising: a branching unit that branches light from a laser light source into two; an optical modulation unit that intensity-modulates one of the branched light beams to generate pump light, which is a pulsed light composed of two frequency components; an optical frequency shift unit that generates a probe light by shifting the frequency of the other branched light beam; an optical detection unit that receives light obtained by combining the probe light and light emitted from one end of an optical fiber to be measured when the pump light is incident on the one end of the optical fiber, and detects a beat signal between the probe light and Brillouin scattered light generated in a frequency band between the two frequency components; and a processing unit that measures a temperature or strain of the optical fiber based on the beat signal detected by the optical detection unit, wherein the interval between the two frequency components is set so that a Brillouin gain spectrum occurring on the low-frequency side of the higher of the two frequency components and a Brillouin loss spectrum occurring on the high-frequency side of the lower of the two frequency components overlap on the frequency axis.

2. A measuring device comprising: an optical modulation unit that generates pump light, which is pulsed light consisting of two frequency components, by intensity-modulating light from a laser light source; an optical filter unit that extracts Brillouin scattered light generated in a frequency band between the two frequency components from the light emitted from one end of an optical fiber to be measured when the pump light is incident on said one end; an optical detection unit that detects the light extracted by the optical filter unit; and a processing unit that measures the temperature or strain of the optical fiber based on the light intensity detected by the optical detection unit, wherein the interval between the two frequency components is set so that the Brillouin gain spectrum occurring on the low-frequency side of the higher of the two frequency components and the Brillouin loss spectrum occurring on the high-frequency side of the lower of the two frequency components overlap on the frequency axis.

3. A measuring device according to claim 1, wherein the processing section measures a temperature distribution or a strain distribution of the optical fiber based on a time change of the beat signal detected by the light detection section.

4. A measuring device according to claim 1 or 2, wherein the optical modulation section generates the pump light by performing intensity modulation based on a signal obtained by combining a pulse signal from a pulse signal generator and an intensity-modulated signal from an oscillator.

5. A measuring device according to claim 1 or 2, wherein the optical modulation section is a Mach-Zehnder type optical modulator.

6. A measurement method comprising: a branching step of branching light from a laser light source into two; an optical modulation step of intensity-modulating one of the branched light beams to generate pump light, which is a pulsed light consisting of two frequency components; an optical frequency shifting step of shifting the frequency of the other branched light beam to generate probe light; an optical detection step of receiving light obtained by combining the probe light and light emitted from one end of an optical fiber to be measured when the pump light is incident on the one end of the optical fiber, and detecting a beat signal between the probe light and Brillouin scattered light generated in a frequency band between the two frequency components; and a processing step of measuring a temperature or strain of the optical fiber based on the beat signal detected in the optical detection step, wherein the interval between the two frequency components is set so that a Brillouin gain spectrum occurring on the low-frequency side of the higher of the two frequency components and a Brillouin loss spectrum occurring on the high-frequency side of the lower of the two frequency components overlap on the frequency axis.

7. A measurement method comprising: an optical modulation step of intensity-modulating light from a laser light source to generate pump light, which is pulsed light consisting of two frequency components; an optical filter step of extracting Brillouin scattered light generated in a frequency band between the two frequency components from the light emitted from one end of an optical fiber to be measured when the pump light is incident from said one end; an optical detection step of detecting the light extracted in the optical filter step; and a processing step of measuring the temperature or strain of the optical fiber based on the light intensity detected in the optical detection step, wherein the interval between the two frequency components is set so that the Brillouin gain spectrum occurring on the low-frequency side of the higher of the two frequency components and the Brillouin loss spectrum occurring on the high-frequency side of the lower of the two frequency components overlap on the frequency axis.

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