Optical fiber sensor and Brillouin frequency shift measurement method

Abstract

To achieve low cost by direct modulation of a semiconductor laser.SOLUTION: A optical fiber sensor includes: a light source that generates optical pulses as probe light using a direct modulation method; an optical bandpass filter that extracts anti-Stokes light, which is a component of Brillouin scattering light on an anti-Stokes side, from backscattered light generated by the probe light in an optical fiber to be measured; a split section that splits the anti-Stokes light extracted by an optical bandpass filter into two branches; an interference signal acquisition unit that receives one of the two branches of the anti-Stokes light split by the split section and generates an interference signal by self-delayed homodyne interference; an intensity acquisition unit to which the other anti-Stokes light split into two branches at the split section is input and an intensity signal indicating the intensity of the anti-Stokes light is generated; and a Brillouin frequency shift acquisition unit that acquires an amount of Brillouin frequency shift from the interference signal and the intensity signal.SELECTED DRAWING: Figure 1

Description

[[Technical Field]]

[0001] The present invention relates to an optical fiber sensor using Brillouin scattered light and a Brillouin frequency shift measuring method. [[Background Art]]

[0002] Along with the development of optical fiber communication, distributed optical fiber sensing that uses the optical fiber itself as a sensing medium has been actively researched. In particular, unlike electrical sensors that perform measurement point by point, optical fiber sensing using scattered light can perform measurement as a long-distance distribution, so it can measure the physical quantity of the entire object to be measured.

[0003] As a distributed optical fiber sensing capable of measuring as a long-distance distribution, Optical Time Domain Reflectometry (OTDR), which injects an optical pulse from one end of an optical fiber and measures the light backscattered in the optical fiber with respect to time, is typical. Backscattering in an optical fiber includes Rayleigh scattering, Brillouin scattering and Raman scattering. Among them, the method that measures spontaneous Brillouin scattering is called BOTDR (Brillouin OTDR) (for example, see Non-Patent Document 1).

[0004] Brillouin scattering is observed at frequencies shifted by approximately GHz on the Stokes and anti-Stokes sides relative to the center frequency of the light pulse incident on an optical fiber, and its spectrum is called the Brillouin Gain Spectrum (BGS). The frequency shift and spectral linewidth of the BGS are called the Brillouin Frequency Shift (BFS) and Brillouin Linewidth, respectively. The BFS and Brillouin Linewidth vary depending on the material of the optical fiber and the wavelength of the incident light. For example, in the case of a silica-based single-mode optical fiber, the magnitude of the BFS and the Brillouin Linewidth at a wavelength of 1.55 μm have been reported to be approximately 11 GHz and approximately 30 MHz, respectively. Furthermore, Non-Patent Literature 1 states that the magnitude of the BFS associated with strain and temperature changes in a single-mode fiber at a wavelength of 1.55 μm is 0.049 MHz / με and 1.0 MHz / ℃, respectively.

[0005] Thus, BFS is dependent on strain and temperature. For this reason, BOTDR is attracting attention as it can be used for purposes such as deterioration diagnosis of large structures like bridges and tunnels, temperature monitoring of plants, and monitoring of areas at risk of landslides.

[0006] In BOTDR (Bio-Optical Timer Detection), heterodyne detection with a separately prepared reference light is commonly used to measure the spectral waveform of spontaneous Brillouin scattered light generated in an optical fiber. The intensity of spontaneous Brillouin scattered light is 2 to 3 orders of magnitude smaller than that of Rayleigh scattered light. Therefore, heterodyne detection is also useful for improving the minimum light detection sensitivity.

[0007] Here, because the natural Brillouin scattered light is very weak, a sufficient signal-to-noise ratio (S / N) cannot be ensured even when heterodyne detection is applied. As a result, averaging processing is necessary to improve the S / N. Conventional optical fiber strain measurement devices that perform BOTDR acquire three-dimensional information of time, amplitude, and frequency, but it is difficult to shorten the measurement time due to the averaging processing and the acquisition of this three-dimensional information.

[0008] In response to this, the inventors of this application have proposed a self-delayed heterodyne BOTDR (SDH-BOTDR). A fiber optic strain measurement device and method utilizing this technology have been proposed (see, for example, Patent Document 1). In SDH-BOTDR, the change in BFS is observed as a phase change in the beat signal by comparing the phase of the received beat signal and the local oscillator signal. In this way, SDH-BOTDR can directly calculate BFS without requiring a frequency sweep, thus enabling fast and inexpensive measurement.

[0009] Furthermore, the inventors of this application have also proposed an optical fiber strain measurement device and method that utilize a self-delayed homodyne type BOTDR as a self-delayed interferometer (see, for example, Patent Document 2). The self-delayed homodyne type BOTDR not only has a simpler configuration but also improves the signal-to-noise ratio and spatial resolution. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2016-191659 [Patent Document 2] Japanese Patent Publication No. 2019-060743 [Non-patent literature]

[0011] [Non-Patent Document 1] T. Kurashima et al., “Brillouin Optical-fiber time domain reflectometry”, IEICE Trans. Commun., vol.E76-B, no.4, pp.382-390 (1993) [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] In the self-delayed homodyne or self-delayed heterodyne (hereinafter sometimes collectively referred to as self-delayed) BOTDRs described above, the configuration of the transmitting section is almost the same as that of a normal BOTDR that is not self-delayed. That is, in a conventional self-delayed BOTDR, the light source section consists of a semiconductor laser as the light source, an optical modulator, a polarization scrambler if necessary, and an optical amplifier. In this configuration, the optical pulse is generated by an external modulation method, in which continuous light generated by the semiconductor laser is converted into an optical pulse by the optical modulator. The external modulation method is suitable for generating ultrashort optical pulses of nanoseconds or less, and high-quality optical pulses that are chirp-free or have low chirp.

[0013] On the other hand, external modulation methods require expensive equipment such as optical modulators used to convert continuous light into optical pulses, making it difficult to reduce the cost of optical fiber sensors.

[0014] This invention has been made in view of the above circumstances. The object of this invention is to provide an optical fiber sensor that achieves cost reduction by employing a direct modulation method that directly modulates a semiconductor laser, and a Brillouin frequency shift measurement method that generates optical pulses using a direct modulation method. [Means for solving the problem]

[0015] To achieve the above-mentioned objectives, the optical fiber sensor of this invention includes: a light source that generates optical pulses as probe light using a direct modulation method; an optical bandpass filter that extracts anti-Stokes light, which is the anti-Stokes component of Brillouin scattered light, from the backscattered light generated in the optical fiber to be measured by the probe light; a branching unit that splits the anti-Stokes light extracted by the optical bandpass filter into two; an interference signal acquisition unit that receives one of the anti-Stokes light split by the branching unit and generates an interference signal by self-delayed homodyne interference; an intensity acquisition unit that receives the other anti-Stokes light split by the branching unit and generates an intensity signal indicating the intensity of the anti-Stokes light; and an intensity acquisition unit that acquires the Brillouin frequency shift amount from the interference signal and the intensity signal. It is configured to include a Brillouin frequency shift acquisition unit.

[0016] Furthermore, another preferred embodiment of the present invention provides an optical fiber sensor comprising: a light source that generates optical pulses in a direct modulation manner as probe light; an optical bandpass filter that extracts anti-Stokes light, which is the anti-Stokes component of Brillouin scattered light, from the backscattered light generated in the optical fiber to be measured by the probe light; an interference signal acquisition unit that receives the anti-Stokes light extracted by the optical bandpass filter and generates an interference signal by self-delayed heterodyne interference; and a Brillouin frequency shift acquisition unit that acquires the amount of Brillouin frequency shift from the interference signal.

[0017] Furthermore, the Brillouin frequency shift measurement method of this invention comprises the steps of: generating an optical pulse as a probe light using a direct modulation method; extracting anti-Stokes light, which is the anti-Stokes component of Brillouin scattered light, from the backscattered light generated in the optical fiber to be measured by the probe light; splitting the extracted anti-Stokes light into two; obtaining an interference signal from one of the split anti-Stokes light streams by self-delayed homodyne interference; obtaining an intensity signal indicating the intensity of the anti-Stokes light from the other split anti-Stokes light stream; and obtaining the Brillouin frequency shift from the interference signal and the intensity signal.

[0018] Furthermore, another preferred embodiment of the Brillouin frequency shift measurement method of this invention comprises the steps of: generating an optical pulse as a probe light using a direct modulation method; extracting anti-Stokes light, which is the anti-Stokes component of Brillouin scattered light, from the backscattered light generated in the optical fiber to be measured by the probe light; acquiring an interference signal from the extracted anti-Stokes light by self-delayed heterodyne interference; and acquiring the Brillouin frequency shift from the interference signal. [Effects of the Invention]

[0019] According to the optical fiber sensor and the Brillouin frequency shift measurement method of the present invention, cost reduction can be achieved by adopting a direct modulation scheme that directly modulates a semiconductor laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] [Figure 1] It is a schematic block diagram showing the optical fiber sensor of the present invention. [Figure 2] It is a diagram showing an example of a time waveform of probe light. [Figure 3] It is a diagram showing an example of a spectral waveform of probe light. [Figure 4] It is a diagram showing an example of a spectral waveform of backscattered light. [Figure 5] It is a diagram showing an example of a BFS waveform. MODE FOR CARRYING OUT THE INVENTION

[0021] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but each drawing is merely schematically illustrated to an extent that allows the present invention to be understood. Further, preferred configuration examples of the present invention will be described below, which are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many modifications and variations can be made without departing from the scope of the configuration of the present invention and while still achieving the effects of the present invention.

[0022] With reference to FIG. 1, an optical fiber sensor using self-delayed homodyne BOTDR according to the present invention will be described. FIG. 1 is a schematic block diagram showing the optical fiber sensor of the present invention.

[0023] The optical fiber sensor includes a semiconductor laser 11, a circulator 20, an optical amplifier 30, an optical bandpass filter 32, a branching unit 34, an interference signal acquisition unit 140, an intensity acquisition unit 141, a BFS acqui sition unit 170 and a timing controller 90.

[0024] The semiconductor laser 11 generates rectangular optical pulses from continuous light using a so-called direct modulation method in response to the input of electrical pulses generated by the timing controller 90. A general-purpose distributed feedback (DFB) laser with a linewidth of several MHz can be used as the semiconductor laser 11.

[0025] The repetition period of the optical pulses generated by the semiconductor laser 11 is set to be longer than the time required for the optical pulses to travel back and forth through the optical fiber under measurement, which is the optical fiber to be measured, 100. The optical pulses generated by the semiconductor laser 11 are output from the semiconductor laser 11 as probe light.

[0026] Referring to Figures 2 and 3, the waveforms when an optical pulse is generated using an external modulation method and when it is generated using a direct modulation method will be explained.

[0027] Figure 2 shows an example of the time waveform of probe light. In Figures 2(A) and 2(B), time [unit: ns] is shown on the horizontal axis and intensity in an arbitrary unit [au] is shown on the vertical axis. Figure 2(A) is an optical pulse generated using an external modulation method, similar to the conventional technology, and Figure 2(B) is an optical pulse generated using the direct modulation method of this invention.

[0028] Figure 3 shows an example of the spectral waveform of probe light. Figures 3(A) and 3(B) show wavelength [unit: nm] on the horizontal axis and intensity [unit: dBm] on the vertical axis. Figure 3(A) is an optical pulse generated using an external modulation method, similar to the conventional technology, and Figure 3(B) is an optical pulse generated using the direct modulation method of this invention.

[0029] As mentioned above, when generated using an external modulation scheme, the optical pulse is chirp-free or has minimal chirp. Therefore, the optical spectral width is inversely proportional to the pulse width. Assuming the optical pulse is a chirp-free Gaussian pulse, the relationship between the pulse width Δt and the spectral width Δν is Δt·Δν≧0.441. Thus, when the pulse width is 10 ns, the 3 dB spectral width is approximately 44.1 MHz. Furthermore, as shown in Figure 3(A), the width including the tail region of the optical spectrum is about 20 GHz, which can be judged as sufficiently narrow.

[0030] On the other hand, when optical pulses are generated using a direct modulation method, a strong frequency chirp occurs during modulation due to carrier fluctuations. As a result, the spectral width extends to over 85 GHz, as shown in Figure 3(B).

[0031] The probe light generated by the laser light source 11 is incident on the optical fiber 100 to be measured via the circulator 20. Alternatively, an optical coupler may be used instead of the circulator 20.

[0032] The probe light incident on the optical fiber 100 under test propagates through the optical fiber 100. While propagating through the optical fiber, the probe light generates backscattered light. The backscattered light propagates through the optical fiber 100 in the opposite direction to the probe light and is sent to the circulator 20.

[0033] The backscattered light generated in the optical fiber 100 under test is sent via the circulator 20 to an optical amplifier 30, which is composed of, for example, an erbium-doped optical fiber amplifier (EDFA). The backscattered light amplified by the optical amplifier 30 is then sent to an optical bandpass filter 32.

[0034] The optical bandpass filter 32 filters out the anti-Stokes component of the natural Brillouin scattered light (Ant Also known as i-Stokes light, it allows light to pass through while blocking other frequency components.

[0035] The spectral waveform of backscattered light will be explained with reference to Figure 4. Figure 4 shows an example of the spectral waveform of backscattered light. Figures 4(A) and 4(B) show wavelength [unit: nm] on the horizontal axis and intensity [unit: dBm] on the vertical axis. Figure 4(A) is the spectral waveform of backscattered light when generated using an external modulation method, similar to the conventional technology, and Figure 4(B) is the spectral waveform of backscattered light when generated using the direct modulation method of this invention.

[0036] As shown in Figure 4(A), the backscattered light obtained by the external modulation method has a sufficiently narrow spectral width, so the Stokes component of the backscattered light (also called Stokes light) and the anti-Stokes light do not overlap. Therefore, it is easy to accurately extract only the Stokes light with the optical bandpass filter 32.

[0037] On the other hand, as shown in Figure 4(B), in the backscattered light obtained by the direct modulation method, the chirp components of Rayleigh scattered light and Stokes light overlap, making it difficult to extract only Stokes light with the optical bandpass filter 32. However, since anti-Stokes light can be observed, in the optical fiber sensor of this invention, the optical bandpass filter 32 extracts anti-Stokes light and removes Stokes light and Rayleigh scattered light. The optical fiber sensor of this invention measures BFS using the anti-Stokes light extracted by the optical bandpass filter 32.

[0038] Normally, to use a strongly chirped optical pulse, such as in a direct modulation scheme, as a sensing light source, pulse shaping is required to remove the chirped component from the probe light using an optical filter. However, adding an optical filter to the optical path from the probe light generated by the light source to the optical fiber under measurement leads to increased equipment costs.

[0039] In contrast, the optical fiber sensor of this invention does not require an optical filter to pulse-shape the probe light; instead, it is possible to remove the chirp component and extract the Brillouin scattered light using only the optical bandpass filter 32 to which the backscattered light is incident. Therefore, an optical filter is not required near the semiconductor laser 11, which is the light source. Furthermore, a general-purpose DFB laser can be used as the light source.

[0040] The chirp generated by the direct modulation method is a red chirp, where the frequency decreases (shifts to longer wavelengths) near the peak of the probe light pulse, and the main component of the pulse is on the shorter wavelength side. Furthermore, the anti-Stokes light is a component on the shorter wavelength side relative to the probe light.

[0041] As a result, the chirp component generated by the direct modulation method overlaps with Rayleigh scattered light. Therefore, extracting Anti-Stokes light means removing the chirp component and Rayleigh scattered light, as well as extracting the Brillouin scattered light component.

[0042] The light that has passed through the optical bandpass filter 32 is sent to the branching unit 34. The branching unit 34 splits the light that has passed through the optical bandpass filter 32 into two, sending one of the scattered light streams to the interference signal acquisition unit 140 and the other scattered light stream to the intensity acquisition unit 141.

[0043] The interference signal acquisition unit 140 comprises a self-delayed homodyne interferometer 150, an interference light receiving unit 160, and a phase control circuit 142. The self-delayed homodyne interferometer 150 comprises a branching unit 152, a delay unit 156, and a multiplexing unit 158. The branching unit 152 and the multiplexing unit 158 ​​are configured with any suitable conventionally known optical couplers, such as a 3dB coupler. Cut.

[0044] The branching section 152 splits the light sent to the interference signal acquisition section 140 into a first optical path and a second optical path.

[0045] In this configuration example, a delay unit 156 is provided in the first optical path. The delay unit 156 imposes a time delay τ on the light propagating through the first optical path. Furthermore, the delay unit 156 can change the phase of the propagating light based on instructions from the phase control circuit 142.

[0046] The wave-combining unit 158 ​​combines the light propagating through the first optical path and the second optical path to generate combined light.

[0047] The combined light, which is interference light generated by the self-delayed homodyne interferometer 150, is sent to the interference light receiving unit 160. The interference light receiving unit 160 receives the combined light and generates an interference signal. The interference light receiving unit 160 is configured, for example, with a balanced photodiode (PD) 162, an FET amplifier 164, and an analog-to-digital converter (A / D) 166. The combined light sent to the interference light receiving unit 160 is input to the balanced PD 162. The balanced PD 162 generates a balanced detection signal, which is an electrical signal, from the combined light. The balanced detection signal is amplified as appropriate by the FET amplifier 164, and then converted into a digital signal by the A / D 166 to obtain the interference signal. This interference signal generated in the interference light receiving unit 160 is sent to the BFS acquisition unit 170.

[0048] The intensity acquisition unit 141 is comprised of a delay unit 157 and a light receiving unit 161. Light sent to the intensity acquisition unit 141 is delayed by a predetermined delay in the delay unit 157 before being sent to the light receiving unit 161. The light receiving unit 161 is comprised of, for example, a PD 163, an FET amplifier 165, and an A / D converter 167. Light sent to the light receiving unit 161 is converted into an electrical signal by the PD 163 and then amplified as appropriate by the FET amplifier 165. The amplified electrical signal is converted into a digital signal by the A / D converter 167 to obtain an intensity signal. This intensity signal generated in the light receiving unit 161 is sent to the BFS acquisition unit 170.

[0049] The BFS acquisition unit 170 can be configured, for example, using a commercially available personal computer equipped with software that implements the functions described later. Alternatively, the BFS acquisition unit 70 can be configured using an FPGA (Field-Programmable Gate Array).

[0050] Furthermore, the interference signal generated in the interference light receiving unit 160 is very weak. Therefore, in order to improve the signal-to-noise ratio (S / N), it is necessary to perform averaging processing in the averaging processing means described later. For speed, it is desirable to perform this averaging processing using an FPGA.

[0051] The BFS acquisition unit 170 acquires the BFS distribution in the optical fiber 100 under test from the interference signal received from the interference light receiving unit 160 and the intensity signal received from the light receiving unit 161.

[0052] Each process performed in the BFS acquisition unit 170 to acquire the BFS distribution, including the averaging process, is conventionally known, as disclosed in, for example, Patent Document 2, and therefore will not be explained here.

[0053] Since BFS is dependent on strain and temperature, after determining the BFS, the strain and temperature of the optical fiber 100 under test can be obtained using any suitable conventional known technique. In other words, the optical fiber sensor of this invention can be applied to distributed strain and temperature sensors, Furthermore, the Brillouin frequency shift measurement method of this invention can be applied to strain and temperature measurement methods.

[0054] The BFS waveform will be explained with reference to Figure 5. Figure 5 shows an example of a BFS waveform. In Figures 5(A) and 5(B), the horizontal axis represents the distance from the input end of the optical fiber under test [unit: m], and the vertical axis represents the BFS [unit: MHz]. Figure 5(A) is the BFS waveform generated using an external modulation method, similar to the conventional technology, and Figure 5(B) is the BFS waveform generated using the direct modulation method of this invention.

[0055] Figure 5 shows the results when the optical fiber under test is 1 km long, and a temperature difference of approximately 45 degrees Celsius is applied between the near end (input end) at 42-72 m and the far end (opposite end to the input end) at 960-990 m. The measurement time is 1 second. The measurement accuracy is approximately ±0.2 MHz for the external modulation method shown in Figure 5(A), and approximately ±2 MHz for the direct modulation method shown in Figure 5(B). Here, when converting BFS to temperature, approximately 1 MHz = 1°C.

[0056] The optical fiber sensor of this invention uses a direct modulation method to generate the optical pulse of the probe light, and does not require a relatively expensive optical modulator. Therefore, it is possible to reduce the cost of the optical fiber sensor. On the other hand, if an external modulation method is used to generate the optical pulse of the probe light, it is difficult to reduce the cost of the optical fiber sensor, but it allows for more accurate measurements.

[0057] Therefore, depending on the application, one can select between an optical fiber sensor using the direct modulation method of the present invention and an optical fiber sensor using the external modulation method.

[0058] In optical fiber sensors using an external modulation method, the noise during BFS measurement is mainly due to the degradation of the signal-to-noise ratio caused by optical loss in the optical fiber. In contrast, in the optical fiber sensor using the direct modulation method of the present invention, there is noise caused by fluctuations in the amount of chirp component due to the shift in the optical frequency of the light source and the shift in the center frequency of the FBG used in the optical bandpass filter 32.

[0059] Therefore, it is best to configure the system to determine the noise level and issue a warning that measurement is difficult if it exceeds a predetermined level. The warning that measurement is difficult can be given by any suitable means, such as displaying it on the display of the operating terminal used by the operator.

[0060] Here, an example using a self-delayed homodyne interferometer has been described, but in the optical fiber sensor of this invention, a self-delayed heterodyne interferometer may also be used in the interference signal acquisition section. When a self-delayed heterodyne interferometer is used, the intensity signal acquisition section is unnecessary. Furthermore, the self-delayed heterodyne interferometer is configured to include a branching section, a delay section, a frequency shifter, and a multiplexing section.

[0061] The branching section splits the light sent to the self-delayed heterodyne interferometer into a first optical path and a second optical path.

[0062] A delay section is provided in one of the first and second optical paths. The delay section imparts a time delay of τ to the light propagating through it. Additionally, a frequency shifter is provided in one of the first and second optical paths. The frequency shifter imparts a frequency shift to the light propagating through it.

[0063] The multiplexing section combines the light propagating through the first and second optical paths to generate combined light. In this case as well, a direct modulation method is used to generate the optical pulse of the probe light, and a relatively expensive optical modulator is not required. Therefore, even when using a self-delayed heterodyne interferometer, by using a direct modulation method, it is possible to avoid the use of conventional optical interferometers equipped with a self-delayed heterodyne interferometer that uses an external modulation method. It can be implemented at a lower cost than an IBA sensor. While optical fiber sensors equipped with a self-delayed heterodyne interferometer have a frequency shifter in the interferometer, from the standpoint of cost reduction, the self-delayed homodyne interferometer is more effective. [Explanation of Symbols]

[0064] 11 Semiconductor lasers 20 Circulators 30 Optical Amplifiers 32 Optical bandpass filters 34 Branching point 90 Timing controller 100 Optical fibers under measurement 140 Interference signal acquisition unit 141 Strength acquisition section 142 Phase control circuit 150 Self-delayed homodyne interferometer 152 Branching point 156, 157 Delay section 158 Wave section 160 Interferometric light receiving section 161 Light receiving part 162 Balanced PD 163 PD 164, 165 FET amplifier 166, 167 A / D 170 BFS Acquisition Section

Claims

1. As probe light, a light source that generates optical pulses using a direct modulation method, An optical bandpass filter that extracts anti-Stokes light, which is the anti-Stokes component of Brillouin scattered light, from the backscattered light generated in the optical fiber to be measured by the probe light, A branching section that splits the anti-Stokes light extracted by the aforementioned optical bandpass filter into two, An interference signal acquisition unit receives one of the anti-Stokes light beams that has been split into two at the aforementioned branching point and generates an interference signal by self-delayed homodyne interference, An intensity acquisition unit receives the other anti-Stokes light that has been split into two at the aforementioned branching point and generates an intensity signal indicating the intensity of the anti-Stokes light, A Brillouin frequency shift acquisition unit that acquires the Brillouin frequency shift amount from the interference signal and the intensity signal. Equipped with A fiber optic sensor characterized by the following features.

2. As probe light, a light source that generates optical pulses using a direct modulation method, An optical bandpass filter that extracts anti-Stokes light, which is the anti-Stokes component of Brillouin scattered light, from the backscattered light generated in the optical fiber to be measured by the probe light, An interference signal acquisition unit receives anti-Stokes light extracted by the aforementioned optical bandpass filter and generates an interference signal by self-delayed heterodyne interference, A Brillouin frequency shift acquisition unit that acquires the Brillouin frequency shift amount from the aforementioned interference signal. Equipped with A fiber optic sensor characterized by the following features.

3. A warning is issued if the noise contained in the probe light, which is caused by fluctuations in the chirp component of the optical pulse, exceeds a predetermined level. The optical fiber sensor according to claim 1 or 2.

4. The process of generating optical pulses as probe light using a direct modulation method, The process involves extracting anti-Stokes light, which is the anti-Stokes component of Brillouin scattered light, from the backscattered light generated in the optical fiber to be measured by the aforementioned probe light, and The process of splitting the extracted anti-Stokes light into two, The process involves obtaining an interference signal from one of the two split anti-Stokes light beams by self-delayed homodyne interference, The process of obtaining an intensity signal indicating the intensity of the anti-Stokes light from the other of the two branched anti-Stokes light, The process of obtaining the Brillouin frequency shift from the aforementioned interference signal and the aforementioned intensity signal, Equipped with A method for measuring Brillouin frequency shift, characterized by the following features.

5. The process of generating optical pulses as probe light using a direct modulation method, The process involves extracting anti-Stokes light, which is the anti-Stokes component of Brillouin scattered light, from the backscattered light generated in the optical fiber to be measured by the aforementioned probe light, and The process involves obtaining an interference signal from the extracted anti-Stokes light using self-delayed heterodyne interference, and The process of obtaining the Brillouin frequency shift from the aforementioned interference signal, Equipped with A method for measuring Brillouin frequency shift, characterized by the following features.

6. A process for issuing a warning when noise contained in the probe light, caused by fluctuations in the chirp component of the optical pulse, exceeds a predetermined level. The Brillouin frequency shift measurement method according to claim 4 or 5, characterized by comprising the following:

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