Brillouin scattering measurement system

Abstract

The present disclosure is a Brillouin scattering measurement system in which a pump light pulse and a probe light pulse are introduced into one end of an optical fiber under measurement at different times, the probe light pulse or the pump light pulse is reflected at an arbitrary point of the optical fiber under measurement, the probe light pulse and the pump light pulse are caused to counter-propagate in a predetermined section of the optical fiber under measurement, thereby generating stimulated Brillouin scattered light, and the stimulated Brillouin scattered light is measured.

Description

Brillouin scattering measurement system

[0001] The present disclosure relates to reflectometry, which performs sensing by measuring scattered light of light incident on an optical fiber.

[0002] Optical fiber sensing has been proposed in which vibrations at any point in the longitudinal direction of an optical fiber are observed by measuring the Brillouin Gain Spectrum (BGS) at that point.

[0003] A highly sensitive BGS measurement technique is called Brillouin Optical Time Domain Analysis (BOTDA). In BOTDA, a continuous wave probe light and a pulsed pump light are injected into both ends of the optical fiber under test to generate stimulated Brillouin scattering light, which is then measured. A technique has also been proposed that omits the wavelength sweep process of the conventional BOTDA by combining a single-frequency pump light with a broadband probe light (see, for example, Non-Patent Document 1).

[0004] In addition, a method has been proposed in which the probe light in the BOTDA configuration is pulsed to generate stimulated Brillouin scattering only in a portion of the optical fiber under test, thereby enabling long-distance measurements (see, for example, Non-Patent Document 3).

[0005] However, BOTDA requires devices to be placed at both ends of the optical fiber under test, and different light beams must be input from each end, which increases the operating cost.

[0006] As a method for measuring Brillouin scattered light at one end, there is a technique called Brillouin Optical Time Domain Reflectometry (BOTDR) (see, for example, Non-Patent Document 2). In BOTDR, pulsed light is incident on one end to generate spontaneous Brillouin scattered light in the optical fiber under test, which is then measured.

[0007] However, the power of the natural Brillouin scattered light weakens in long-distance optical fibers under test, which makes it difficult to observe vibrations in long-distance optical fibers under test with the BOTDR.

[0008] T. Ishimaru, Y. Wakisaka, H. Takahashi, D. Iida and Y. Koshikiya, Fast BOTDA acquisition method based on broadband light as a probe signal. 2023 Optical Fiber Communications Conference and Exhibition (OFC), 2023, pp. 1-3. Tu, G. ; Zhang, X. ; Zhang, Y. ; Ying, Z. ; Lv, L. Strain variation measurement with short-time fourier transform-based brillouin optical time-domain reflectometry sensing system. Electron. Lett. 2014, 50, 624-1625. Yongkang Dong, Liang Chen, and Xiaoyi Bao, Time-division multiplexing-based BOTDA over 100km sensing length, Opt. Lett. 2011, 36, 277-279.

[0009] An object of the present disclosure is to enable measurement of Brillouin scattered light from one end of a long-distance optical fiber under test.

[0010] The Brillouin scattering measurement system and Brillouin scattering measurement method disclosed herein are a Brillouin scattering measurement system and a Brillouin scattering measurement method for measuring Brillouin scattered light generated when a pump light pulse and a probe light pulse counter-propagate in an optical fiber under test, comprising: making the pump light pulse and the probe light pulse incident on one end of the optical fiber under test at different timings; reflecting the probe light pulse or the pump light pulse at an arbitrary point in the optical fiber under test; causing the probe light pulse and the pump light pulse to counter-propagate in a predetermined section of the optical fiber under test, thereby generating stimulated Brillouin scattered light; and measuring the stimulated Brillouin scattered light.

[0011] In the present disclosure, the probe light is pulsed and stimulated Brillouin scattering is generated only in the predetermined section, thereby preserving the pump light pulse. Therefore, the present disclosure can measure the BGS in the predetermined section even if the optical fiber under test is long. Therefore, the present disclosure can measure Brillouin scattering over long distances from one end.

[0012] The probe light pulse may be incident on the one end of the optical fiber under test before the pump light pulse is incident on the one end of the optical fiber under test. By employing this configuration, spontaneous Brillouin scattered light generated in a section of the optical fiber under test that is closer to the one end than the predetermined section may be measured.

[0013] A reflector that reflects the probe light pulse or the pump light pulse may be provided at the far end of the optical fiber under test. By adopting this configuration, the predetermined section can be located near the far end of the optical fiber under test.

[0014] The probe light pulse may include a plurality of frequencies. By using the broadband probe light pulse, it is possible to measure vibrations with large amplitudes and high frequencies in a long-distance optical fiber under test.

[0015] Furthermore, narrowing the pulse width of the pump light pulse in the time axis direction can increase the resolution in the distance direction in the measured optical fiber. Furthermore, narrowing the pulse width of the probe light pulse in the time axis direction can set the predetermined section to be a section close to the turning-back point of the probe light pulse or the pump light pulse in the measured optical fiber. As the pulse width of the probe light pulse in the time axis direction is widened, the predetermined section becomes farther away from the turning-back point of the probe light pulse or the pump light pulse in the measured optical fiber. Therefore, the pulse width of the pump light pulse in the time axis direction may be shorter than that of the probe light pulse.

[0016] In the present disclosure, the probe light is pulsed in a BOTDA configuration using a broadband probe light, thereby performing measurements in a short distance section of the optical fiber under test using a BOTDR configuration, and measurements in a long distance section of the optical fiber under test using a BOTDA configuration, and performing measurements over longer distances using the same signal processing.

[0017] The above disclosures can be combined as much as possible.

[0018] According to the present disclosure, it is possible to measure the Brillouin scattered light of a long-distance optical fiber under test from one end.

[0019] 1 shows an example of the configuration of a BOTDA using a broadband probe; FIG. 2 shows an example of the configuration of a BOTDR; FIG. 3 is an explanatory diagram of the optical power and propagation direction of incident light in a BOTDR; FIG. 4 shows the attenuation of scattered light power in the distance direction; FIG. 5 shows the attenuation of pump light power in the distance direction; FIG. 6 shows an example of a system configuration of the present disclosure; FIG. 7 shows an example of incident light of the present disclosure; FIG. 8 shows an example of counter-propagation of a probe light pulse and a pump light pulse at a reflection point of the present disclosure; FIG. 9 shows an example of scattered light power acquired by a scattered light acquisition unit of the present disclosure; and FIG. 10 is a flowchart showing an example of a measurement procedure of the present disclosure.

[0020] Hereinafter, 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 indicate the same components.

[0021] (Configuration example of BOTDA) Figure 1 shows a configuration example of BOTDA using broadband probe light. The explanation will be divided into a scattered light generation unit for generating Brillouin scattering and a scattered light acquisition unit for observing the Brillouin scattered light. The scattered light generation unit generates probe light obtained by amplifying light emitted from a light source having a broad frequency band using an optical amplifier, and pump light obtained by pulsing light emitted from a laser using an intensity modulator and amplified by an optical amplifier. Brillouin scattered light is generated by counter-propagating these lights in the measured optical fiber 190.

[0022] The generated Brillouin scattered light is sent as signal light to an optical receiver through an optical filter. This signal light and the local light that has been split from the laser and modulated are used in the optical receiver to perform coherent detection, converting the analog signal to a digital signal. This allows the Brillouin scattered signal to be acquired and frequency analyzed to calculate the BGS.

[0023] (Configuration example of BOTDR) Figure 2 shows a configuration example of a BOTDR disclosed in Non-Patent Document 2. Pulsed light is generated using electro-optical modulators EOM1 and EOM2, and an erbium-doped fiber amplifier (EDFA). Here, LD is a semiconductor laser, OC is an optical coupler, PS is a polarization scrambler, CIR is a circulator, PD is a photoreceiver, LO is a local frequency oscillator, BPF is a bandpass filter, AMP is an amplifier, and DAQ is an abbreviation for data acquisition.

[0024] This light is input into one end of the optical fiber 190 under test to generate Brillouin scattered light, which is then combined with the continuous light branched by OC1 by OC2 and received by the PD. The received signal by the PD and the signal from the LO are interfered with by an RF mixer to generate a beat component, which is then down-converted to obtain a Brillouin scattered signal. The obtained Brillouin scattered signal is converted into a digital signal by the DAQ, and frequency analysis is then performed in the data processing section to calculate the BGS.

[0025] 3 is an explanatory diagram of light that is incident on the optical fiber 190 under test when Brillouin scattering occurs in Non-Patent Document 1 and Non-Patent Document 2. In the method of Non-Patent Document 1, the pump light and broadband continuous light shown in FIG. 3 are incident on both sides of the optical fiber 190 under test, causing them to counter-propagate, and frequency analysis is performed at the receiving stage to acquire a stimulated Brillouin scattering signal using a BOTDA configuration. In Non-Patent Document 2, of the pump light and broadband continuous light shown in FIG. 3, only pulsed light corresponding to the pump light is incident on one end.

[0026] 4 and 5 are diagrams showing the attenuation of scattered light power and pump light power over distance. As shown in Fig. 4, spontaneous Brillouin scattering has weak scattered light power, which decreases as the distance increases. In contrast, stimulated Brillouin scattering increases scattered light power by counter-propagating a different light to the pump light, and the scattered light power decreases over distance, just like spontaneous Brillouin scattering.

[0027] The scattering phenomenon caused by stimulated Brillouin scattering is a phenomenon in which part of the pump light returns to the input end, so if the scattered light power is weak, the pump light power remains at a greater distance, as shown in Figure 5. In other words, there is a trade-off between the pump light power and the scattered light power between BOTDA and BOTDR, making it difficult to measure large-amplitude vibrations over long distances at high frequencies.

[0028] (Configuration of the Present Disclosure) The present disclosure provides a Brillouin scattering measurement system that measures Brillouin scattered light generated when a pump light pulse and a probe light pulse propagate counter-directionally through an optical fiber under test. In the present disclosure, a BOTDA configuration measurement is performed from one end using a broadband probe light, and the probe light is further pulsed to measure the BOTDA configuration only in a predetermined section of the optical fiber under test. This enables the Brillouin scattered light of a long-distance optical fiber under test to be measured from one end.

[0029] 6 shows an example embodiment of the Brillouin scattering measurement system of the present disclosure. The Brillouin scattering measurement system of this embodiment includes a scattered light generator 91 that generates a probe light pulse and a pump light pulse, and a scattered light acquirer 92 that observes the scattered light generated in the optical fiber 90 under test by the probe light pulse and the pump light pulse, and performs the Brillouin scattering measurement method of the present disclosure.

[0030] The scattered light generating unit 91 inputs a pump light pulse and a probe light pulse at different times into one end of the measured optical fiber 90. In the present disclosure, the probe light pulse is reflected at an arbitrary point in the measured optical fiber 90, and the probe light pulse returned by this reflection is caused to counter-propagate with the pump light pulse. The scattered light acquiring unit 92 then measures stimulated Brillouin scattering light generated by this counter-propagation. The scattered light acquiring unit 92 also measures spontaneous Brillouin scattering light generated in a section of the measured optical fiber 90 closer to the one end than the predetermined section.

[0031] In this embodiment, as an example of reflection from the measured optical fiber 90, a configuration is shown in which a reflecting unit 93 such as a reflection filter is installed at the far end of the measured optical fiber 90, and reflection occurs at the far end of the measured optical fiber 90. The installation position of the reflecting unit 93 is not limited to the far end of the measured optical fiber 90, and the reflecting unit 93 can be installed at any point where the probe light pulse and the pump light pulse are counter-propagated.

[0032] The scattered light generating unit 91 has a function of generating a probe light pulse, a function of generating a pump light pulse, and a function of injecting the generated probe light pulse and pump light pulse into the measured optical fiber 90. The function of generating a probe light pulse includes, for example, a broadband light source 11, an optical filter 12, an intensity modulator 13, and a polarization scrambler 14. The function of generating a pump light pulse includes, for example, a laser 15, an intensity modulator 16, an optical amplifier 17, and an optical filter 18. The function of injecting the generated probe light pulse and pump light pulse into the measured optical fiber 90 includes, for example, an optical coupler 19 and an optical circulator 20.

[0033] The probe light pulse is formed by band-limiting light containing multiple frequencies emitted from a broadband light source 11 using an optical filter 12 and pulsing it using an intensity modulator 13. The pump light pulse is formed by pulsing continuous light of a single frequency emitted from a laser 15 using an intensity modulator 16. The pump light pulse is then amplified by an amplifier 17 and passed through an ASE-cutting optical filter 18. The probe light pulse and pump light pulse are then combined by an optical coupler 19. The combined light pulse is incident on the optical fiber 90 under test via an optical circulator 20, and the scattered light caused thereby is propagated to a scattered light acquisition unit 92 as signal light.

[0034] The light receiving unit 24 of the scattered light acquiring unit 92 performs coherent detection of the scattered light from the optical fiber 90 under test as local light, and then uses an AD converter 25 to acquire a digital signal that can be processed by an analyzing device 26 such as a computer. As a result, the analyzing device 26 can acquire the Brillouin scattering signal and perform frequency analysis to calculate the BGS.

[0035] Here, the Brillouin scattering shifts the frequency of the pump light pulse by approximately 11 GHz. Therefore, the frequency of the local light is modulated using an optical modulator 21 so that the beat components of the local light and the Brillouin scattered light fall within the bandwidth of a balanced photo detector (BPD) and an analog-to-digital converter (ADC) 25. The signal light is then amplified by an optical amplifier 22. The signal light is filtered at the frequency of the Brillouin scattered light using an optical filter 23. Using this local light and the signal light, heterodyne detection is performed for each polarization in a polarization diversity configuration in a light receiving unit 24 to obtain a Brillouin scattered signal.

[0036] In the present embodiment, an example has been shown in which light is split into polarizations using a polarization beam splitter (PBS) and received by a balanced photo detector (BPD) to perform heterodyne detection for each polarization, but the configuration of the light receiving unit 24 is not limited to this.

[0037] A specific light injection method and the occurrence of BOTDA near reflection are described below. Figure 3 shows the light injection method for measurement using a BOTDA configuration with a broadband probe light in Non-Patent Document 1. To cause stimulated Brillouin scattering in the BOTDA measurement configuration, the pump light and the broadband probe light are configured to propagate counter-propagatingly throughout the entire optical fiber 190 under test.

[0038] 7 shows an example of incident light propagating through the measured optical fiber 90. In this embodiment, the probe light pulse is pulsed by the intensity modulator 13 so as to form a BOTDA configuration near the far end of the measured optical fiber 90. The rising edge of the pump light pulse may be adjusted to be immediately after the falling edge of the probe light pulse.

[0039] 8 shows an example of the probe light pulse and the pump light pulse immediately after the broadband probe light pulse is reflected by the reflector 93. In this embodiment, the reflected probe light pulse counter-propagates with the pump light pulse. This causes stimulated Brillouin scattering to occur near the reflection in the optical fiber under test 90, and the Brillouin scattered light can be measured using the BOTDA configuration.

[0040] For example, the length of the optical fiber 90 to be measured is L ｆｕｔ , the pulse width (time width) of the pump light pulse is L ｐ , the pulse width (time width) of the probe light pulse is L ｂｒｏａｄ (i) L ｂｒｏａｄ ≧2×L ｆｕｔ -L ｐ When (ii) L, the BOTDA configuration is achieved over the entire range of distances of the optical fiber 90 under test. Therefore, the BOTDA measurement is performed using a probe light pulse that utilizes reflection. ｂｒｏａｄ <2 x L ｆｕｔ -L ｐ When the distance from the optical fiber 90 to be measured is 0, the distance (L ｆｕｔ -L ｂｒｏａｄ / 2), the BOTDR configuration is ｆｕｔ -L ｂｒｏａｄ / 2) to the distance L ｆｕｔ The section up to this point is configured as BOTDA.

[0041] Therefore, in order to perform a longer distance measurement using a BOTDA configuration only at the far end of the optical fiber 90 under test, ｂｒｏａｄ <2 x L ｆｕｔ -L ｐ The pulse width L of the pump light pulse must be ｐ is difficult to change because it corresponds to the spatial resolution, and the length L of the optical fiber 90 to be measured ｆｕｔ It is also difficult to change. ｂｒｏａｄ is the pulse width of the probe light pulse and can be changed. Therefore, in this embodiment, the pulse width L of the probe light pulse is ｂｒｏａｄ By adjusting the above, it is possible to measure stimulated Brillouin scattering light in any predetermined section near the far end of the optical fiber under test 90. Therefore, the pulse width (time width) of the probe light pulse is shorter than that of the pump light pulse.

[0042] 9 shows an example of the scattered light power acquired by the scattered light acquisition unit 92 when measurements are performed in the first half of the optical fiber 90 under test using the BOTDR configuration and in the second half using the BOTDA configuration. ｂｒｏａｄRegarding (2), while taking the noise floor into consideration, stimulated Brillouin scattering by the BOTDA is set to occur before the spontaneous Brillouin scattering light obtained by the BOTDR disappears. This makes it possible to prevent the occurrence of invisible sections even in the long-distance optical fiber 90 under test.

[0043] FIG. 10 is a flowchart of the measurement procedure disclosed herein. Step S1: Probe and pump optical pulses, as shown in FIG. 7, are incident on the optical fiber 90 under test. At this time, a narrowband local light (LOI) is also generated, taking into account the frequency shift of the BGS. For example, the local light (LOI) is modulated to a frequency close to Brillouin scattering (up to several hundred MHz) to beat with Brillouin scattering using coherent detection. Step S2: The pump and probe optical pulses, each consisting of a single frequency, are counter-propagated only near the reflection, generating natural Brillouin scattering in front of the pulse and stimulated Brillouin scattering near the reflection. Step S3: The scattered light in the optical fiber 90 under test is sent to the scattered light acquisition unit 92, and a Brillouin scattering signal is acquired. At this time, the analyzer 26 receives the signal necessary to form a distributed BGS with a single pump optical pulse. Step S4: The analyzer 26 uses the acquired Brillouin scattering signal to perform frequency analysis for each distance along the optical fiber 90 under test and calculate the BGS. Vibration can be obtained by observing the time series changes in the BGS peak.

[0044] Steps S1 to S3 show the process of measuring the entire optical fiber 90 under test in a distributed manner. The time required for steps S1 to S3 depends on the repetition period at which the pulsed pump light pulse is injected. Since it is necessary to wait for the injection of the pump light pulse while it travels back and forth through the optical fiber 90 under test, there is a trade-off between the repetition period and the length of the optical fiber 90 under test, and when taking an arithmetic average, it is necessary to take the measurement an additional number of times taking that number into account.

[0045] Next, the frequency processing performed by the analyzer 26 after receiving the scattered light will be described in detail. Here, the same processing as in Non-Patent Document 1 is performed. The waveform in the time direction of the scattered light returned from the optical fiber 90 under test corresponds to the distance. To obtain the BGS for each distance for one pump light, the time waveform is divided into time segments and a frequency spectrum is calculated by Fourier transform for each time segment. By calculating the frequency spectrum of the scattered light for one pump light pulse, the BGS for the entire optical fiber 90 under test can be obtained, and by observing the BGS for multiple pump light pulses, it is possible to measure the change in the state of the optical fiber 90 under test over time.

[0046] In this embodiment, a wideband probe light pulse is used, and the probe light pulse and the pump light pulse are incident in this order, so that the probe light pulse and the pump light are reflected in this order at the far end of the measured optical fiber 90. In this configuration, by shortening the pulse width of the probe light pulse relative to the measured optical fiber 90, a BOTDA configuration can be achieved only near the far end of the measured optical fiber 90. Therefore, stimulated Brillouin scattering light can be measured in the measured optical fiber 90 near the reflection.

[0047] By making the probe light pulse incident on one end of the optical fiber under test 90 before the pump light pulse, it becomes possible to measure the spontaneous Brillouin scattering light in the optical fiber under test 90 at the front end, where the broadband probe light pulse is not counter-propagating. Therefore, by using the BOTDA to measure a distance farther than the distance at which the power of spontaneous Brillouin scattering decreases, the distance over which Brillouin scattering can be measured can be extended.

[0048] According to the present disclosure, when measuring large-amplitude, high-frequency vibrations from one end of the optical fiber under test 90, a BOTDR configuration can be used in the vicinity and a broadband BOTDA configuration at one end can be used in the distant portion by using a broadband probe light pulse. By adopting this configuration, the scattered light power at the far end of the optical fiber under test 90 is increased, enabling long-distance measurements. Furthermore, by performing measurements using heterodyne detection, the BGS can be calculated using the same frequency analysis signal processing in both the BOTDR and BOTDA configurations.

[0049] In the present disclosure, instead of the probe light pulse, a pump light pulse may be reflected at any point. The analysis device 26 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.

[0050] 11: Broadband light source 12: Optical filter 13, 16: Intensity modulator 14: Polarization scrambler 15: Laser 17: Optical amplifier 18: Optical filter 19: Optical coupler 20: Optical circulator 21: Optical modulator 22: Optical amplifier 23: Optical filter 24: Light receiving unit 25: ADC 26: Analysis device 90, 190: Optical fiber to be measured 91: Scattered light generating unit 92: Scattered light acquiring unit 93: Reflecting unit

Claims

1. A Brillouin scattering measurement system that measures Brillouin scattered light generated when a pump light pulse and a probe light pulse propagate in a measured optical fiber in a counter-propagating manner, comprising: inputting the pump light pulse and the probe light pulse into one end of the measured optical fiber at different timings; reflecting the probe light pulse or the pump light pulse at an arbitrary point of the measured optical fiber; propagating the probe light pulse and the pump light pulse in a counter-propagating manner in a predetermined section of the measured optical fiber to generate stimulated Brillouin scattered light; and measuring the stimulated Brillouin scattered light.

2. The Brillouin scattering measurement system according to claim 1, wherein the probe light pulse is incident on the one end of the measured optical fiber prior to the pump light pulse, and spontaneous Brillouin scattering light generated in a section of the measured optical fiber closer to the one end than the predetermined section is measured.

3. The Brillouin scattering measurement system according to claim 1, wherein the probe light pulse includes a plurality of frequencies, and the pulse width of the pump light pulse is shorter than that of the probe light pulse.

4. The Brillouin scattering measurement system according to claim 1, further comprising a reflecting section at a far end of the test optical fiber for reflecting the probe light pulse or the pump light pulse, and the predetermined section is in the vicinity of the far end of the test optical fiber.

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