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A novel approach for long-distance sensing through Brillouin optical time-domain analysis (BOTDA) assisted by second-order distributed Brillouin amplification (DBA) was proposed and experimentally demonstrated. To the best of our knowledge, this is the first BOTDA study that used second-order DBA. Compared with BOTDA assisted by first-order DBA, the proposed approach enhanced the signal-to-noise ratio of the Brillouin trace by ~3 dB for a range featuring minimum sensing intensity. Long-distance sensing with ~5 m spatial resolution and ± 1.6°C measurement uncertainty over ~99 km fiber was successfully realized by employing high-efficiency pumping using ~6 dBm second-order and ~1.5 dBm first-order pumps.
© 2016 Optical Society of America
1. Introduction
Brillouin optical time-domain analysis (BOTDA) is widely applied in the fields of fire alarm and structural health monitoring for buildings and oil pipelines. For standard BOTDA, by launching a pump pulse and probe light at the two ends of a fiber, the temperature/strain distribution along the entire sensing fiber can be extracted via stimulated Brillouin scattering (SBS). For long-distance BOTDA, many efforts have been made to overcome the restrictions of fiber attenuation and nonlinear effects, including the broadening of Brillouin gain spectrum (BGS) due to self-phase modulation (SPM) [1], and pulse depletion due to modulation instability (MI) [2, 3]. These problems can lead to severe signal-to-noise ratio (SNR) degradation for BOTDA, especially for long-distance sensing.
To solve the aforementioned problems, several effective methods have been proposed, including distributed Raman amplification (DRA) [4–12], optical pulse coding (OPC) [13], coherent detection [14], wavelet denoising [15], and image restoration [16]. Among these methods, DRA can significantly enhance the Brillouin response by compensating the loss of fiber. To achieve the flattened gain distribution along the fiber, second-order DRA structures have been introduced [9–12]. However, to obtain the expected sensing performance, the pump power of DRA should be as high as a few hundreds or thousands of milliwatts because of the relatively lower gain coefficient of stimulated Raman scattering (SRS). This would cause some potential drawbacks, such as easy burnout, nonlinear penalty, and high-energy consumption. Moreover, a careful structure design must be considered for DRA-based BOTDA to suppress the relative intensity noise (RIN) transfer from high-power and noisy pumps [7, 8, 10–12].
Distributed Brillouin amplification (DBA) is another effective method to intensify the performance of BOTDA and phase-sensitive optical time-domain reflectometer (Φ-OTDR) [17–20]. Owing to the very large gain coefficient of SBS compared with that of SRS, the pump power of the DBA-based BOTDA can be decreased to a milliwatt level [18–20]. In addition, the use of commercial semiconductor laser pumps with relatively lower RIN (<-140 dB/Hz typically) and the group velocity walk-off among Brillouin pump, pulse, and probe light, render the RIN transfer negligible [18–20].
However, for long-distance sensing, SNR degradation would generate a wide segment with worsened sensing accuracy over tens of kilometers. In this study, we generalized our previous approach of first-order DBA-based BOTDA [19] to a second-order pumping case, in which the first-order pump was pushed more deeply inside the fiber.
2. Principle and experimental setup
Figure 1 shows the schematic diagram of BOTDA using second-order DBA. The frequencies of pulse, probe light, and first- and second-order pumps are represented by ʋ0, ʋ1, ʋ2, and ʋ3, respectively. Their frequency separation equals to the Brillouin frequency shift (BFS) denoted by ʋb. To avoid reduction in spatial resolution caused by pulse smoothing, and the additional measurement error due to the common effects of asymmetrical pulse profile and SPM [18, 20], optical-combs [19] for both first- and second-order pumps are used. In Fig. 1, the spectrum of the optical source is denoted by a dashed line. Because of the frequency shift (Δʋ) of the acoustic-optic modulator (AOM) used for achieving high extinction ratio (>40 dB) pulse, the frequency difference between the probe light (first-order pump) and the optical source should be ʋb − Δʋ (ʋb + Δʋ), respectively. The values of ʋb and Δʋ for the fiber under test (FUT) in our experiment are ~10,836 and 200 MHz, respectively.
Figure 2 describes the experimental setup of BOTDA using second-order DBA. The output of the distributed feedback laser diode at a wavelength of ~1549.53 nm is split by a 90:10 coupler. The 10% portion is modulated through an AOM, and the 90% portion is further split by a 50:50 coupler. The upper branch serves as a probe light through electro-optic modulator 1 (EOM1), whereas the lower branch behaves as a first-order Brillouin pump through EOM2. The optical-comb is formed by mixing the electronic-comb with a microwave generator at a fixed frequency of ʋb + Δʋ. The residual sideband behind EOM2 is filtered by a tunable band-pass filter (TBF1) with <0.1 nm bandwidth and >30 dB rejection ratio. Both EOM1 and EOM2 are biased at their carrier suppression points. The probe light and first-order Brillouin pump are then injected into the FUT situated after the first polarization scrambler (PS1), which is used to reduce the polarization-dependent gain fluctuation.
Fig. 2 Experimental setup of BOTDA using second-order DBA. DFB-LD: Distributed feedback laser diode; EOM: Electro-optic modulator; EDFA: Erbium-doped fiber amplifier; PS: Polarization scrambler; VOA: Variable optical attenuator; FUT: Fiber under test; AOM: Acoustic-optic modulator; TBF: Tunable bandpass filter; PD: photodetector; DAQ: Data acquisition system.
To generate the second-order pump, one of the 50% portions of the first-order optical-comb pump is further modulated by EOM3, biased at the carrier suppression point. Similarly, TBF2 is used to filter out low-frequency sideband. Another microwave source at the fixed frequency of ʋb is introduced to drive EOM3. The second-order pump and pulse light are combined to inject another end of the FUT behind PS2. All involved wavelength components are boosted by Erbium-doped fiber amplifiers (EDFAs) before being launched into the FUT.
At the receiver, TBF3 is applied to filter out the transmitted first-order pump, high-frequency sideband component generated from EOM1 and the Rayleigh backscattering noise resulting from the pulse and second-order pump. Because the probe light is not amplified by DBA along FUT, another pre-amplifier is used to compensate the loss of ~99 km FUT. The amplified spontaneous emission (ASE) noise and residual first-order pump is further eliminated by TBF4. Finally, the probe light is received by a photodetector with a 100 MHz bandwidth and a data acquisition card operating at 100 MSas−1 sampling rate.
Figure 3(a) shows the spectrum structures of various wavelength components before they enter the FUT. Owing to the resolution limitation (0.03 nm) of our optical spectrum analyzer, the optical-comb details could not be identified directly. This point was verified using a heterodyne detection technique and an electronic spectrum analyzer. The optical-comb pump that consists of −56, −40, −24, −8, 8, 24, 40, and 56 MHz components overcomes the pulse distortion caused by limited BGS bandwidth. The pulse output waveforms without and with DBA are shown in Fig. 3(b), showing the negligible pulse distortion. The spatial resolution uncertainty due to slow light is ~2 and ~3 m for first- and second-order DBA respectively. The injected powers of second- and first-order pumps in the experiment are ~6 and ~1.5 dBm, respectively. An optimized probe power of ~-15 dBm is utilized to overcome the well-known nonlocal effect [21]. The peak power of the injected pulse is ~10 dBm to suppress its depletion and BGS broadening due to MI and SPM. The impact of four-wave mixing (FWM) is insignificant due to lower pump power for each comb and probe input.
Fig. 3 (a) Optical spectrums of probe light, first- and second-order pumps before entering FUT. (b) Pulse output waveforms without and with DBA.
In the experiment, OPC [13] is used to further enhance SNR. For the transient gain saturation of pulse light due to limited phonon lifetime (~10 ns), the shorter length of a Simplex code (15 bits) has to be selected [19], corresponding to ~3 dB SNR improvement. The space of the coded pulse train (390 ns) is sufficiently long to minimize the gain transient effect. For a 50 ns pulse-width corresponding to 5 m spatial resolution, the measured transient gain saturation is ~13%, which is defined as the relative power variation of the transmitted pulse train [19].
3. Results and discussions
3.1 SNR improvement using second-order DBA
To explore the SNR improvement obtained using the proposed second-order DBA, we record the decoded Brillouin response for the cases with first- and second-order pumping and that without DBA (Fig. 4(a)). The frequency shift of the probe light is 10,836 MHz. The coded traces are pre-averaged by 300 times, equivalent to the total average times of 4500. For the first-order DBA, the pump power is ~2.5 dBm. The same averaging times and pulse peak power are utilized for a fair comparison. The figure shows that when the DBA is not introduced, the SNR decreases exponentially, and it was very low after ~60 km. Although the SNR can be enhanced considerably by the first-order DBA, it is low within the range of 60–80 km. After using the second-order DBA, the SNR is enhanced by ~3 dB within the same range. Here, the SNR is defined as the ratio of Brillouin response amplitude with respect to the standard deviation of noise [16]. The improved SNR is helpful for extending the sensing distance, or reducing the averaging times for achieving the similar sensing performance.
Fig. 4 (a) Decoded Brillouin response for different pumping schemes. (b) Simulated power and gain distributions. The measured pulse gain distribution is also shown in (b) for comparison.
To explore the underneath physical origin of the flattened gain distribution, the power distribution along FUT for the second-order DBA was simulated according to the following equations:
where α is the fiber loss coefficient; gB is the Brillouin gain coefficient; P1, P2, and Pp are the powers of first- and second-order pumps and pulse light, respectively. The pulse depletion from probe light with much lower power is omitted. The narrower pulse-width limits the interaction range within a few meters between the pulse light and first-order pump so that the third term on the right-hand side in Eq. (2) can be reasonably ignored. Figure 4(b) shows the simulated results, in which gB is 0.13 m−1W−1. The figure shows a good agreement between the measured and calculated gain distributions. The power of the second-order Brillouin pump decreases continuously along FUT, especially at the right end because of power depletion from the first-order pump. Besides, the power of the first-order pump injected from the right first decreased due to loss, and then increased when encountering the second-order pump at the left of FUT. Thus, a larger power fluctuation was displayed. For pulse light gain, two times of increase are undergone because of the amplified first-order pump near the two ends of FUT. Thus, we obtained more flattened gain distribution by using a second-order DBA.3.2 Detection of 5 m hotspot over ~99 km fiber
In the following experiment, a ~5 m segment of FUT at the right end is heated in a water bath. Further, the room temperature is kept at ~20°C using an air conditioner. The frequency shift of the probe light is gradually increased from 10,788 to 10,920 MHz, with a step size of 4 MHz. Figure 5(a) shows the measured BGS along FUT, and a clear hotspot is observed. A higher SNR is maintained along the entire fiber. Figure 5(b) indicates the peak BFS distribution obtained through Lorentz fitting. Because the FUT includes three segments, the BFS variation, represented by the dashed box, is caused by additional strains during the winding process. Although not shown here, the obtained full-width at half maximum (FWHM) of BGS is less than ~50 MHz along the entire FUT, implying that the nonlinear effects have been sufficiently controlled. Figure 5(c) displays the extracted temperature distribution. The maximal standard deviation within the range of 60–80 km is ~ ± 1.6 °C. Figure 5(d) shows the magnified view of the temperature distribution near the hotspot. The measured temperature variation agrees well with its actual value. The ~5 m spatial resolution is clearly visible from the FWHM of temperature distribution.
Fig. 5 (a) BGS, (b) peak BFS, and (c) extracted temperature distribution along FUT. (d) The magnified view of temperature distribution near hotspot.
4. Conclusions
In this study, we proposed and experimentally demonstrated a novel BOTDA configuration enhanced by a second-order DBA. A ~5 m hotspot was completely resolved at the end of the ~99 km FUT. We focused on the system’s high efficiency pumping and more flattened gain distribution that facilitates in considerably shortening the range with lower SNR for long-distance sensing. Our proposed configuration can also be applied for enhancing the performance of other distributed sensing such as Φ-OTDR.
Further sensing distances that extend far beyond 100 km would be limited by the shorter coding length for OPC because of transient gain saturation [19] and nonlocal effect that limits the maximal probe input power. However, we believe that by combining our proposed second-order DBA with the newly developed nonlocal effect suppression technique based on frequency modulation for probe light [20, 21], and SNR enhancement techniques, such as wavelet denoising [15] or image restoration [16], a considerably longer sensing distance could be achieved.
Acknowledgments
The authors greatly acknowledge the help of Han Wu and Zi-Nan Wang from the University of Electronic Science and Technology of China. This work was supported in part by the National Nature Science Foundation of China (NSFC) under Grants 61205079 and 61475105, in part by the Construction Plan for Scientific Research Innovation Teams of Universities in Sichuan Province under Grant 12TD008, and in part by the 251 Talents Program and Scientific Research Foundation of Sichuan Normal University under Grant 16ZP08.
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