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Abstract
In this paper, we propose and demonstrate an ultrahigh-speed Brillouin optical correlation domain analysis (BOCDA) with a single-position sampling rate of 200 kS/s and a spatial resolution of 8 cm. The Brillouin gain spectrum (BGS) is obtained by using a data subtraction scheme rather than the conventional lock-in amplifier (LIA) detection configuration, thus removing the limitation of measurement speed imposed by the LIA. Meanwhile, a voltage controlled oscillator (VCO) is used to sweep the frequency interval between the pump and the probe rapidly. As a proof of concept, we implement measurements of various dynamic strains with frequencies up to 20 kHz at arbitrary position. Moreover, to implement high-speed distributed measurements of Brillouin frequency shift (BFS) along the whole fiber under test (FUT), we propose a novel measuring method which moves the correlation peak and sweeps the pump-probe frequency interval simultaneously. A repetition rate of 1 kHz is verified by measuring dynamic strains with frequencies up to 200 Hz, for distributed measurements performed with 200 points.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Brillouin-scattering-based fiber sensors have been intensively investigated and widely used in many applications including border security protection, temperature analysis and structural health monitoring because of its capability to provide continuous information along the whole fiber used for sensing. To implement distributed measurement for locating “events”, there are mainly three methods using different locating techniques in different domains: time, frequency, and correlation [1–3]. Among them, time-domain sensors including Brillouin optical time domain analysis/reflectometry (BOTDA/R) provide the longest measurement range up to 100 km with the help of coding technique or distributed amplification configuration [4–7]. However, the spatial resolution of conventional Brillouin time-domain sensors is limited to meter-level due to the nature of pulse-based operation. Various techniques including differential pulse-width pair (DPP) scheme [8–10], dark-pulse BOTDA [11], and Brillouin echoes scheme have been proposed to improve the spatial resolution to centimeter-level at the expense of extra complexity and measurement time [12].
Recently, dynamic Brillouin measurement has become a hot topic, and various modified BOTDA systems with capability of measuring dynamic strains have been proposed [13–17]. For instance, Voskoboinik et al. proposed a sweep-free BOTDA (SF-BOTDA) system based on frequency-domain multiple simultaneous tone interrogation, having the potential for fast acquisition characteristics but with coarse frequency granularity [13]. Alternately, Tur et al. proposed a slope-assisted BOTDA (SA-BOTDA) technique by exploiting the linear slope of the Brillouin gain spectrum (BGS) [14], which avoids the time-consuming frequency switching process, but suffers from a limited dynamic range (normally ~30 MHz). To achieve a large strain dynamic range, they also presented a fast BOTDA (F-BOTDA) technique based on the use of an arbitrary waveform generator (AWG) to switch the probe frequency rapidly [15]. Moreover, DPP technique was also introduced into the F-BOTDA system [16], achieving a repetition rate of ~500 Hz and a spatial resolution of 10 cm. Although DPP-enhanced F-BOTDA system provides an impressive performance, high-performance AWG (bandwidth >11 GHz) and wideband receiving module (>1 GHz for 10-cm spatial resolution) are required, thus limiting its applications in practical environment.
Alternately, Brillouin-scattering-based correlation-domain sensors (Brillouin optical correlation domain analysis/reflectometry, BOCDA/R) provide a simpler way to achieve ultrahigh spatial resolution, high sampling rate, and random-access capability [3, 18, 19]. For instance, Song et al. achieved a spatial resolution of 1.6 mm by using beat lock-in detection scheme to improve the signal-to-noise ratio (SNR) [20]. In terms of measurement speed, they achieved a 1-kS/s sampling rate for single-position measurement based on a time-division pump-probe generation scheme [21], but the speed of position shift is only 0.2 point/s, making it difficult for high-speed distributed measurement. By using differential frequency modulation, they also demonstrated a distributed measurement over a 100-m-long fiber at a repetition rate of 20-Hz with a spatial resolution of 80 cm [22]. Recently, random accessibility with a 5-kS/s single-position sampling rate has been demonstrated in BOCDA by employing a high-speed lock-in amplifier (LIA) and using a voltage controlled oscillator (VCO) to sweep the pump-probe frequency interval rapidly [23]. However, the sampling rate is limited by the working bandwidth of the LIA. To date, the highest measurement speed (100-kS/s single-position sampling rate) for Brillouin signals is achieved by Mizuno et al. [24]. Nevertheless, this configuration suffers from poor measurement accuracy (10 MHz, or 200 με), deteriorated spatial resolution (40 cm) and limited strain dynamic range (<100 MHz, or <2000 με), which restrict its industrial applications.
In this paper, we propose a novel BOCDA with ultrahigh sampling rate. The Brillouin gain spectrum (BGS) is obtained by subtracting the probe power with and without stimulated Brillouin scattering (SBS) interaction. Thus, the limitation of sampling rate induced by the LIA can be removed [23]. The frequency difference between the pump and the probe is swept around 11 GHz rapidly by using a VCO. In order to compensate the SNR deterioration at the case of ultrafast measurement, an injection-locking technique is used to generate a “pure” probe with high power stability, and high sideband suppression ratio (SBSR) [25–28], leading to a high SNR and a high measurement accuracy. A single-position sampling rate of up to 200 kS/s as well as a spatial resolution of 8 cm is firstly achieved with the proposed method. As a proof of concept, we implement measurements of dynamic strains with frequencies up to 20,000 Hz at an arbitrary position. The measurement accuracy is 1 MHz (or 20 με), 2.7 MHz (or 54 με), and 4.2 MHz (or 84 με) when the sampling rate is set to be 10 kS/s, 100 kS/s, and 200 kS/s, respectively. In the conventional BOCDA, distributed measurement is achieved by point-by-point shifting the correlation peak along the fiber under test (FUT), which is extremely time-consuming, for example 0.2 point/s as mentioned in [21]. Here, we propose a novel measuring method which simultaneously moves the correlation peak and sweeps the pump-probe frequency interval, removing the extra time used for position shifting and instrument communication. When distributed measurements are performed with 200 points, a repetition rate of 1 kHz is achieved, and dynamic strains with frequencies up to 200 Hz are measured in a fully distributed manner.
2. Principle
In a BOCDA system, continuous pump and probe lightwave are launched into the fiber in opposite directions to generate Brillouin interaction. Both lightwave are sinusoidally frequency-modulated to synthesize periodical correlation peaks, and the measurement range is defined as the interval between two adjacent correlation peaks, which is given by [20]
where Vg is the group velocity of light in fiber, and fm is the modulation frequency of the light source. By sweeping fm, the correlation peak can be shifted along the FUT. The spatial resolution of BOCDA is given by [20]where ∆vB is the bandwidth of the BGS, and Δf is the modulation amplitude of the light source. Therefore, a narrow spatial resolution can be achieved by setting a high modulation frequency fm or a large modulation amplitude Δf.As depicted in Fig. 1(a), in the conventional BOCDA system, a LIA accompanying with a pump chopping configuration is used to extract the Brillouin signals, which provides high SNR and measurement accuracy. However, the measurement speed of the BOCDA system is limited by the LIA due to its finite integral time (100 μs in our experiments). As shown in Fig. 1(b), when the sampling rate is set to be 100 S/s, the BGS can be measured accurately. However, when the sampling rate is increased to 500 S/s, the BGS shows a significant distortion, leading to a poor measurement accuracy. In fact, even though a high-speed LIA is used, the sampling rate is limited to 5 kS/s [23].
Fig. 1 (a) Schematic of the conventional detection scheme using lock-in amplifier (LIA). IM: intensity modulator; AWG: arbitrary wave generator; LIA: lock-in amplifier. (b) Brillouin gain spectrum (BGS) measured by using LIA when the sampling is set to be 100 S/s (red line) and 500 S/s (gray line).
Figure 2 shows the schematic of the proposed LIA-free BOCDA scheme. In this system, the LIA detection configuration is not used, and a data subtraction scheme is introduced. Twodata sets of the probe power for the with-pump case and the without-pump case are recorded when the pump-probe frequency interval is swept around the BFS of the FUT. Two low-pass filters (LPFs) are used to filter out the high-frequency noise. Then the BGS is obtained by subtracting these two data sets. In order to compensate the SNR deterioration at the case of ultrafast measurement, an injection-locking scheme is introduced to generate “pure” probe with high power stability and high SBSR, which we explain in the following paragraph.
Fig. 2 Schematic of the proposed LIA-free BOCDA scheme. FUT: fiber under test; PD: photodetector; LPF: low-pass filter.
For the Brillouin optical analysis schemes (BOTDA and BOCDA), the generation of the pump lightwave and the probe lightwave can be realized by simply using two separated lasers or externally modulating the laser with a single-sideband modulator (SSBM) [29, 30]. For the former method, the frequency difference between two lasers is extremely difficult to control with high frequency accuracy. For the latter method, the pump/probe lightwave generated by the SSBM suffers from low optical power, low power stability, and low SBSR. In order to solve these problems, the injection-locking technique is introduced into the system. In this configuration, the probe lightwave generated by the SSBM is used as the master laser, whose emission fields can be written as
where Ak(t) is the intensity of the k-th order sideband, v0(t) is the frequency of the unmodulated optical carrier (the pump lightwave), and fr is the frequency of the radio-frequency (RF) signal. In this formula, the first-order lower sideband is used as the probe which is ~11-GHz down-shifted from the pump. When the output of the SSBM is injected into a slave laser which is injected into the first-order lower sideband, the unwanted sidebands can be suppressed, and the output of the slave laser can be expressed aswhere Ps is the output power of the slave laser. In the BOCDA system, a sinusoidal frequency modulation is applied to the laser source to synthesize periodical correlation peak. Inevitably, a parasitic intensity modulation (sinusoidal shape) comes accordingly [31], which will seriously deteriorate the SNR of the system. With the help of the injection-locking configuration, the intensity modulation can be eliminated, leading to a pure frequency modulation [32]. Moreover, the probe power is improved more than 20 dB, thus removing an additional optical amplifier used to amplify the probe power. Therefore, by using the injection-locking technique, the probe lightwave is purified, stabilized, and amplified.In our experiments, a wideband optical wave, rather than a single-frequency wave is used as the master laser. Thus, the locking area is a key parameter, and it can be given as [32]
where L is the cavity length of the slave laser, and β is the linewidth enhancement factor. ρ is the injection ratio, which can be given by PI/PS, where PI and PS are the injection power and the output power of the slave laser. In order to achieve a large locking area, a distributed feedback laser (DFB) laser diode without internal isolator is used as the slave laser in the experiments.In the conventional BOCDA system, distributed measurement is achieved by point-by-point shifting the correlation peak along the FUT. This process is extremely time-consuming [15], making distributed dynamic measurement very difficult to be realized. Here, we propose a novel measuring method to improve the repetition rate of distributed measurement, as illustrated in Fig. 3. The frequency interval between the pump and the probe is swept rapidly, and the sweeping time is Ts. Meanwhile, the correlation peak is moved along the FUT simultaneously by scanning the modulation frequency fm. For N-points distributed measurement, the scanning time of fm is set to be NTs, which corresponds to the measurement time for the whole FUT. Thus, no extra time is needed to realize distributed measurement.
Fig. 3 The schematic illustration of the proposed measuring method without point-by-point shifting. The blue line shows the sweeping of the pump-probe frequency difference, while the red line shows the scanning of the measurement position (correlation peak). Ts is the sweep time of the voltage controlled oscillator (VCO), and NTs is the total measurement time for N-points when the correlation peak is moved from 0 to LFUT.
3. Experimental setup and results
The detailed experimental setup of the proposed LIA-free BOCDA system is shown in Fig. 4. A 1551-nm DFB laser diode (DFB1, linewidth: ~1 MHz) is used as the light source, whose modulation frequency and amplitude are set to be around 3.4 MHz and 4 GHz, corresponding to a measurement range of ~30 m and a spatial resolution of ~8 cm according to Eq. (1) and (2). The output of the DFB-LD is divided into two branches using a 50/50 optical coupler. The lower branch is used as the pump, whose power is amplified to 21 dBm using an erbium-doped fiber amplifier (EDFA). A delay fiber (single-mode fiber, SMF) with length of ~100 m is used to ensure that a high-order correlation peak is located in the FUT. In the upper branch, the probe lightwave is generated by using a SSBM which is driven by a VCO. A wideband amplifier (Mini-circuits, ZVA-183 + , 2~18 GHz) is utilized to improve the RF power from ~8 dBm to ~20 dBm. An injection-locking configuration connected after the SSBM is used to improve the power stability and the SBSR of the probe. Another DFB laser (DFB2, linewidth: ~3 MHz) without internal isolator is served as the slave laser. The relative polarization state between the master laser and the slave laser is adjusted properly to optimize the injection-locking process. Then the pump lightwave and the probe lightwave are launched into the FUTin opposite directions. A polarization-maintaining fiber (PMF) made by YOFC (Wuhan, China) is used as the FUT to avoid the polarization-dependent gain fluctuation, and its parameters are given in Table 1.To maximize the interacting efficiency, two polarization controllers (PC2 and PC3) are incorporated to align the polarization state of the pump and the probe to the slow axis of the PMF. The optical power of the probe without and with Brillouin interaction is converted to electrical signals using two photodetectors (PD1 and PD2). In order to achieve a high SNR, a high-performance analog-to-digital converter (ADC) with 14-bit resolution and 100-MHz sampling rate is used to record the electrical signals.
Fig. 4 Experimental setup of the proposed LIA-free BOCDA. DFB: distributed feedback laser diode; SSBM: single-sideband modulator; VCO: voltage-controlled oscillator; PC: polarization controller; CIR: optical circulator; EDFA: erbium-doped fiber amplifier; ISO: optical isolator; PMF: polarization-maintaining fiber; VOA: variable attenuator; PD: photodetector; ADC: analog-to-digital converter.
Table 1. The parameters of the PMF.
The instantaneous output frequency of the VCO (sweeps from 10.6 GHz to 11.2 GHz at a repetition rate of 200 kHz) is characterized by using an electrical mixing measurement scheme, as shown in Fig. 5(a). The output of the VCO is injected into the ‘RF’ port of the mixer, while another electrical signal (~10.5 GHz) generated by a microwave synthesizer is injected into the ‘LO’ port. Then, the low-frequency IF signal is recorded by an oscilloscope (10-bit resolution, 20-GHz sampling rate). The frequency changed over time is obtained after short-time Fourier transform (STFT). Here, the measurement accuracy is decided by the calculating granularity of the STFT algorithm, which is set to be 0.5 MHz in our experiments. The measurement results are shown in Figs. 5(b) and 5(c). When a linearly-increased voltage is applied to the VCO (blue line in Fig. 5(b)), the output frequency is distorted (red line in Fig. 5(b)) because of the nonlinear frequency dependence on the applied voltage. The residual errors are near 40 MHz as shown in the inset figure in Fig. 5(b). Therefore, we apply a pre-distorted voltage (blue line in Fig. 5(c)) to the VCO, and the linearly-swept output is shown in Fig. 5(c) (red). The residual errors are decreased to less than 1 MHz with pre-distortion.
Fig. 5 (a) Experimental configuration used to measurement the output frequency of the VCO. The VCO operates with a repetition rate of 200 kHz. (b) Linearly applied voltage (blue line) and nonlinear output frequency of the VCO (red line). The residual errors shown in the inset figure is near 40 MHz. (c) Pre-distorted applied voltage (blue line) and the linear output frequency of the VCO (red line). The residual errors shown in the inset figure is decreased to less than 1 MHz with pre-distortion.
A SSBM driven by the VCO is used to generate the probe lightwave, and its output spectrum is shown in Fig. 6(a) (blue line). There are several unwanted sidebands besides the desired first-order lower sideband which is used as the probe in the SBS interaction process. After injection-locking, the probe is amplified, while other sidebands are suppresseddrastically (red line). Figure 6(b) shows the power fluctuation without (blue line) and with (red line) injection-locking. A photodetector is used to convert the optical power to electrical signal. A sinusoidal power variation can be observed at the SSBM output due to the parasitic intensity modulation of the light source, which leads to a poor measurement accuracy in the proposed LIA-free detection scheme. The red line shows the power variation of the probe after injection-locking. It can be observed that the parasitic intensity modulation is mitigated effectively, resulting in a pure sinusoidal frequency modulation. Figures 6(c) and 6(d) shows the measured BGS without and with injection-locking when the single-position sampling rateis set to be 10 kS/s. Without injection locking, the measured Brillouin gain spectrum (BGS) shows a poor SNR, leading to a low strain measurement accuracy. By introducing the injection locking scheme into the system, the probe is purified, stabilized, and amplified. Thus, the SNR of the measured Brillouin signal is improved, as shown in Fig. 6(d).
Fig. 6 (a) Optical spectra and (b) power fluctuation of the probe lightwave without (blue line) and with (red line) injection locking. The measured Brillouin signals (c) without and (d) with injection locking. The sampling rate is set to be 10 kS/s.
In the proposed LIA-free BOCDA system, the probe power is recorded directly without the usage of LIA, and can be given by
where gB(v) is the Brillouin gain, Δz is the Brillouin interacting length (corresponds to the spatial resolution), Pprobe(t) and Ppump(t) are the probe power and pump power, respectively. In our experiments, although the intensity modulation on the probe wave is removed using an injection-locking configuration, the intensity modulation on the pump is still remained, which may affect the measurement accuracy. In fact, a high-frequency (>3 MHz, corresponding to fm) variation can be observed in the measured BGS. Thus, a low-pass filter (LPF) is introduced into the system to remove the high-frequency noise induced by the intensity modulation of the pump. In this case, the maximum sampling rate is limited to less than 1 MHz.Here, the maximum single-position sampling rate is set to be 200 kS/s, which is the highest measurement speed to date achieved in Brillouin sensors. The strain dependence of the measured BGS is shown in Fig. 7. When the strain applied on a 19-cm-long section increases (the correlation peak is located at the stretched section), the measured BGS shifts to a higher frequency accordingly. To analyze the measurement accuracy of the proposed ultrahigh-speed measurement system, we perform BGS measurement at a fixed position for a duration time of 5 ms, which corresponds to 100 BGSs, 500 BGSs, and 1000 BGSs for 10 kS/s, 100-kS/s, and 200-kS/s sampling rate, and the results are shown in Figs. 8(a)-8(c). Figures 8(d)-8(f) show the BFS variation obtained from Figs. 8(a)-8(c). The standard deviations are calculated to be 1 MHz (or 20 με), 2.7 MHz (or 54 με), and 4.2 MHz (or 84 με), respectively.
Fig. 7 Strain dependence of the measured BGS when the sampling rate is set to be 200 kS/s. The spatial resolution is 8 cm, and the stretched length is 19 cm.
Fig. 8 Measured BGSs when the single-position sampling rate is set to be (a) 10 kS/s, (b) 100 kS/s, and (c) 200 kS/s. The correlation peak is fixed at a certain position, and the measurement time is 5 ms. (d), (e), and (f) are the Brillouin frequency shift (BFS) variation obtained from (a), (b), and (c). The standard deviations of BFS variation are calculated to 1 MHz, 2.7 MHz, and 4.2 MHz for sampling rates of 10 kS/s, 100 kS/s, and 200 kS/s, respectively.
As a proof of concept, the LIA-free BOCDA system is used to measure high-frequency dynamic strains. An electrical motor is used to apply dynamic strains with different frequencies (150 Hz, 600 Hz, 5,000 Hz, and 20,000 Hz) to the fiber, and the stretched length is 19 cm. A static pre-strain is firstly applied to the fiber since it is difficult to stably apply dynamic strains to an unstrained fiber. The experimental results are shown in Fig. 9. The blue line is the measured BFS variation, and the red line is the sinusoidally-fitted data. We can clearly see that the measured data matches well with the theoretical value, confirming the capability of measuring high-frequency dynamic strains.
Fig. 9 The measured BFS variation when dynamic strains with frequency of (a) 150 Hz, (b) 600 Hz, (c) 5,000 Hz, and (d) 20,000 Hz are applied to the fiber. The blue line is the measured data and the red line is the sinusoidally-fitted data.
Subsequently, we demonstrate the high repetition rate of the LIA-free BOCDA system for distributed measurement. For a 200-points measurement, the modulation frequency fm is scanned from 2.8 MHz to 3.8 MHz with a repetition rate of 1 kHz to move the correlation peak along the whole FUT. Most importantly, the scanning of fm and sweeping of the VCO (sweeps from 10.6 GHz to 11.2 GHz with a repetition rate of 200 kHz) should be synchronized accurately. The BFS distribution along the whole FUT is shown in Fig. 10(a). The 19-cm-long stretched section can be measured correctly, and a BFS change of 166 MHz induced by a 3300-με static strain can be observed. The inset figure shows the BGS without (gray line) and with (red line) strain. The distributed BGSs around the stretched section are given in Fig. 10(b).
Fig. 10 (a) Measured BFS distribution along the whole FUT (200 effective sensing points) with a repetition rate of 1 kHz. The inset figure shows the BGS without (gray line) and with (red) strain. (b) Measured BGSs around the stretched section whose length is 19 cm.
Benefiting from the ultrahigh repetition rate of the LIA-free BOCDA system, high-frequency dynamic strains can be measured in a fully distributed manner. To verify this, we apply dynamic strains with frequencies of 50 Hz, 100 Hz, and 200 Hz to a 19-cm-long section(located at 7.9 m). Figures 11(a)-11(c) shows the three-dimensional plot of the distributed BFS change within a duration of 60 ms. A large-value pre-strain is applied to the fiber firstly, and the strained section can be observed clearly. Figure 11(d) shows the BFS variation at 11 m where no strains are applied, which is chaotic due to the limited measurement accuracy (4.2 MHz or 84 με at 200-kS/s sampling rate). Figures 11(e)-11(g) shows the BFS variation at 7.9 m when dynamic strains with frequencies of 50 Hz, 100 Hz, and 200 Hz are applied to the fiber. The BFS changes periodically, and the period match well with the theoretical values,confirming the capability of the LIA-free BOCDA system to measure high-frequency dynamic strains in a fully distributed manner.
Fig. 11 Three-dimensional plot of the distributed BFS change measured by the LIA-free BOCDA at a repetition rate of 1 kHz during 60 ms when dynamic strains with frequencies of (a) 50 Hz, (b) 100 Hz, and (c) 200 Hz are applied to the FUT. (d) The BFS variation at 11 m where no strains are applied. The BFS variation at 7.9 m when dynamic strains with frequencies of (e) 50 Hz, (f) 100 Hz, and (g) 200 Hz are applied. The blue line is the measured data and the red line is the sinusoidally-fitted data.
4. Discussions
In the conventional BOCDA system, the correlation peak (sensing position) is scanned along the FUT by changing the modulation frequency of the laser source step by step (blue line in Fig. 12(a)). In this case, the BGS is measured at a fixed position (Pi). While in the proposed system, a continuous frequency sweeping is utilized to scan the correlation peak along the FUT (red line in Fig. 12(a)). Thus, the BGS is measured within a short segment Δx, where Δx is the length corresponding to the frequency change within TS (TS is the sweeping period of the VCO, as mentioned in Fig. 3). When the stretched length is ΔL, in order to avoid measurement ambiguity, at least one Δx should be located within ΔL. Therefore, Δx and ΔL are supposed to satisfy a relationship given as “2Δx ≤ ΔL”. Thus, the continuously sweeping scheme suffers from a deterioration factor of 2 either on repetition rate or on spatial resolution. For example, when the stretched length is equal to the spatial resolution (8 cm in our experiments), Δx is supposed to be half of the spatial resolution, thus leading to a half repetition rate. When Δx is set to be 8 cm, the actual spatial resolution is deteriorated to be 16 cm. We believe this problem can be solved by applying a step-like frequency-swept signal to the laser source.
Fig. 12 (a) Comparison between two measurement methods based on step-by-step changing fm (blue line) or continuously sweeping fm (red line). (b) Schematic analysis of the distributed measurement process based on continuously sweeping scheme. TS is the sweeping period of the VCO, and Δx is the distance corresponding to the frequency change within Ts.
The proposed ultrahigh-speed measurement system provides a single-position sampling rate of 200 kS/s, which is 40 times better than the results obtained in Ref [23]. Here we also take the spatial resolution (3 cm in Ref [23], 8 cm in this work) and the measurement accuracy (4.13 MHz in Ref [23], 4.2 MHz in this work) into consideration, the improvement factor is 15.
Recently, a novel method using time-gated pump and continuous probe to synthesize multiple correlation peaks along the FUT has been proposed [33, 34]. In this case, multiply correlation peaks are swept concurrently, thus providing a promising method to decrease the measurement time. However, it brings about a deteriorated SNR and a poor strain measurement accuracy. As mentioned in [33], a large number of average is required to achieve an acceptable measurement accuracy. To introduce the time-gated scheme, it is necessary to furtherly improve the SNR of the proposed system. For example, by combining the Brillouin gain and the loss [35], a 3-dB enhancement in SNR can be obtained. We believe that a narrower spatial resolution, as well as a higher measurement accuracy and sampling rate can be achieved by adopting these schemes.
5. Conclusion
In this paper, we propose and experimentally demonstrate a LIA-free BOCDA system with an ultrahigh single-position sampling rate of 200 kS/s and a high spatial resolution of 8 cm. With the help of an injection-locking configuration, an appropriate measurement accuracy of 1 MHz (or 20 με), 2.7 MHz (or 54 με), and 4.2 MHz (or 84 με) is obtained when the single-position sampling rate is set to be 10 kS/s, 100 kS/s, and 200 kS/s, respectively. As a proof of concept, dynamic strains with frequencies up to 20 kHz are measured at arbitrary position. Moreover, we propose a novel measuring method by scanning the modulation frequency fm and sweeping the output frequency of the VCO simultaneously, so as to avoid the time-consuming point-by-point shifting process. When distributed measurements are performed at 200 points, a repetition rate of 1 kHz is firstly achieved. To verify the ultrahigh repetition for distributed measurement, dynamic strains with frequencies up to 200 Hz is measured in a fully distributed manner. The proposed LIA-free BOCDA system provides a record sampling rate both for single-position measurement and for distributed measurement, opening a possibility for Brillouin-scattering-based sensors with high spatial resolution, ultrahigh sampling rate, and high cost-effectiveness.
Funding
This work was supported by the National Natural Science Foundation of China (NSFC) (61775132, 61735015, 61620106015, 61327812).
Acknowledgments
We would like to thank Dr. Mengshi Wu of Shanghai Jiao Tong University, for his helps in experiments.
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