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Coherent Brillouin optical time domain analysis (BOTDA) sensing system with intensity modulated local (IML) light and fast IQ demodulation is proposed and demonstrated. IML light instead of phase modulated local (PML) light is utilized to reduce the coherent and multiple sidebands induced noises. A spatial resolution of 3-m and ± 1.8°C temperature accuracy at the far end of the fiber are obtained over 40-km sensing distance.
© 2015 Optical Society of America
1. Introduction
Brillouin optical time domain analysis (BOTDA) sensing is a well-developed technology for distributed temperature and strain sensing, which has multiple applications such as structural health monitoring, geotechnical engineering and leakage detection along pipelines [1–3 ]. The temperature/strain distribution along the fiber turns out to be linearly related to the local Brillouin frequency shift (BFS), which can be obtained by utilizing a pulsed pump light and a frequency-sweeping counter-propagating continuous-wave (CW) probe light [4–6 ]. In conventional BOTDA configurations, direct detection is most commonly used and may suffer from baseband noise perturbations (BNP) [2–10 ]. Recently, in order to reduce the measurement uncertainty, coherent detection BOTDA sensor with phase modulated light as the probe has been proposed to reduce BNP and increase signal-to-noise ratio (SNR) [11]. Such configuration utilizes sub-GHz carriers to carry the Brillouin gain spectrum (BGS) information, resulting in ~10dB SNR improvement. Similar structures are reported in [12–15 ]. Although these schemes demonstrated the effectiveness of SNR improvement, non-local effect on the BGS may exist since one sideband of the laser source is used as the pump light, subsequently the depletion of the pump power can’t be compensated dynamically [16]. In [17] uses two symmetric sidebands of the laser source as the probe, thus the depletion of the pump light caused by one sideband can be compensated by the other one. However, its performance may be limited by the coherent noises since same spectrum lines are used for both the probe and the pump light, ending with low carrier-suppression-ratio (CSR) of phase modulation. On the other hand, multiple sidebands generated by carrier-suppressed phase modulation (i.e. large signal modulation) may also degrade the demodulation accuracy [18]. Therefore the spatial resolution achieved in [17] is only 20-meter over 100-km distance (aided by Raman amplification).
In this paper, we propose a coherent BOTDA sensing scheme to improve the overall accuracy by minimizing above limitations. The phase modulated probe (PMP) light is amplitude modulated by the BGS and then coherently detected with the local light by a photo-detector (PD) to generate GHz amplitude modulated (AM) signal, which is decoded by a fast In-phase/Quadrature (IQ) demodulator. The local light is generated by carrier-suppressed intensity modulation (i.e. IML), instead of carrier-suppressed phase modulation (i.e. PML) in [17]. The BGS information can also be amplitude modulated on the GHz beat-note signal between the PMP light and IML light. Experimental demonstration shows that a 3-meter spatial resolution with ± 1.8°C temperature accuracy at the far end of the 40-km fiber is achieved. Moreover, we also investigate the effect of the chromatic dispersion (CD) on the shape of BGS.
2. Operation principle
The operation principle of [17] is indicated in Fig. 1(a) . The laser source light is split into two branches, one branch is intensity modulated to generate the pulsed pump light, the other branch is modulated to generate local light and CW probe light by two phase electro-optic modulators (EOMs) sequentially. The first phase EOM operated in carrier-suppressed mode and driven by a microwave frequency fLO is employed to create multiple sidebands as the local light, and fLO is less than the expected BFS. The local light is further phase modulated by a tunable frequency fS, which is chosen so that the spectrum lines fP-fLO-fS and fP + fLO + fS (i.e. probe light) can sweep the Brillouin gain and loss regions of the pump light, respectively.
Fig. 1 Schematic diagram of the coherent BOTDA sensor based on (a) phase-modulated local (PML) light and (b) intensity-modulated local (IML) light. FUT: fiber under test; PM-IM: phase modulation to intensity modulation; PMP: phase modulated probe.
Due to the phase modulation, no signal should be detected at frequency fS in the absence of pump gain and loss (by ignoring CD). However, in the presence of simulated Brillouin scattering (SBS) between the pump and probe light, fP-fLO-fS component will be amplified and fP + fLO + fS component will be depleted, ending with a PM-IM conversion at the frequency fS. However, due to coherent noises and multiple sidebands induced noises (MSIN), the BGS reconstructed through the PM-IM conversion would be distorted. In order to solve these problems, the first phase EOM may be replaced by an intensity EOM, which has higher carrier suppression ratio (~30-40dB) and could generate fewer sidebands (typically only 1st and −1st sidebands). Note that, the CSR in phase modulation is decided by the power of the microwave signal, i.e. higher power (less than 1W) corresponds to a higher CSR. However, higher power can generate more sidebands, which results in a bigger MSIN. Thus, it’s a trade-off between CSR and MSIN, which leads to a low CSR. Therefore, the power of the fP component in IML case is much less than it in PML case, so the coherent noises can be greatly reduced. The operation principle of our proposal is illustrated in Fig. 1(b). The 1st and −1st sidebands generated by the intensity EOM work as the IML light, while their 1st and −1st sidebands generated by the phase EOM work as the PMP light. When the PMP light interacts with the pump light, the optical field before the PD is [19]:
where E0, ELO and ES are the complex amplitudes of the laser source light, local light and probe light, respectively. fD = fS + fLO-fB is the detuning frequency between the probe light frequency and the peak frequency of Brillouin gain. fB is BFS. gSBS(f) and lSBS(f) correspond to the amounts of Brillouin gain and loss centered at frequency fD = 0, respectively. φg(f) and φl(f) are the corresponding phase shifts of Brillouin gain and loss. Due to the real impulse response of Brillouin gain and loss spectra, gSBS(f), lSBS(f), φg(f) and φl(f) obey to the relationships [9]: gSBS(f) = lSBS(-f) and φg(f) = -φl(-f). Then the output current of the PD as a function of fS can be expressed as [19]where RC is the sensitivity of the PD, g0 is local Brillouin gain, and ϕ is the phase of the PD output current. Equation (2) indicates that the detected intensity at frequency fS is modulated by the BGS, which can be easily demodulated by many methods (e.g. envelope detection and IQ demodulation). On the other hand, it is also modulated by the amplitude of the local light, which can be set to be much bigger than Es*sqrt(gSBS(fD = 0)), so the SNR of the demodulated BGS signal can be dramatically improved.3. Experimental setup
The experimental setup is shown in Fig. 2 . A CW light with a central wavelength of 1500.12nm and a peak power of 10dBm from a tunable laser source (TLS) is split into two branches by a 50:50 coupler. One branch is modulated by cascaded intensity EOM1 and phase EOM. EOM1 is operated in carrier-suppressed mode and driven by a microwave source with a frequency of 9.6-GHz (i.e. fLO) and an output power of 18dBm. The phase EOM is driven by a tunable radio frequency (RF) signal (i.e. fS). The output light of the phase EOM is amplified by an erbium-doped fiber amplifier (EDFA1) followed by an optical polarization scrambler and an isolator before launching into to the sensing fiber, the peak powers of local light and probe light are −5dBm and −25dBm, respectively. The other branch is modulated by another intensity modulator (EOM2) which is driven by a pulsed signal generator (PSG) in order to obtain 30-dB extinction ratio (ER) single pulse light with 1-ms period and 30-ns duty cycle (corresponding to 3-meter spatial resolution). The output light of EOM2 is amplified by EDFA2 to obtain a peak power of 18dBm and then sent to the sensing fiber as the pulsed pump light through a circulator.
Fig. 2 Experimental setup of the proposed coherent BOTDA sensor. TLS: tunable laser source; PC: polarization controller; EOM: electro-optic modulator; EDFA: erbium-dropped fiber amplifier; CW: continue wave; PD: photo-detector; BPF: band pass filter; LNA: low noise amplifier; ESA: electrical spectrum analyzer; OSC: oscilloscope.
In our implementation, RF signal from the signal generator is swept from 1.1GHz to 1.3GHz to reconstruct the BGS. After interacting with the pulsed pump light, the probe light is detected by a PD with 10-GHz bandwidth. The recovered RF signal is selected by the band pass filter (BPF) and amplified by the low noise amplifier (LNA) to increase SNR. Since the useful information is contained in the amplitude of the recovered RF signal, a direct frequency down-conversion IQ demodulator is utilized to convert the amplitude into the baseband signal, which is further sampled and stored by an oscilloscope with 100-MHz sampling rate. High demodulation bandwidth (i.e. 3-dB 500MHz) of the IQ demodulator makes it fast enough to decode the amplitude modulation. In addition, the polarization state of the phase EOM’s input light is carefully adjusted through the PC to ensure that the BGS measured by the electrical spectrum analyzer (ESA) is symmetric. To achieve best system performance, the optical powers of the probe light, local light, and pump light can be independently adjusted by controlling the power of the RF signal, the gain of the EDFA1 and EDFA2, respectively.
4. Results
About 40-km standard single mode fiber (SSMF) with 10.8-GHz BFS at room temperature is employed as the sensing fiber in the experiment. About 25-m testing fiber near the far-fiber end is heated up to 45°C by the oven and the remaining 39.975-km fiber is kept at room temperature (25°C). The temperature difference of 20°C results in ~20MHz frequency shift on the BFS. Figure 3 shows the measured BOTDA trace 3(a) along the whole 40-km fiber and 3(b) from 39.94km to 40.02km when the sweep frequency is 1.2GHz (i.e. corresponding to BFS of the fiber at room temperature) and 1.22GHz (i.e. corresponding to BFS of the fiber heated), respectively. At the far end of the fiber, we can clearly observe the induced intensity transition corresponding to the fiber heating with a SNR of 23.5dB. In order to check the performance enhancement through our proposal in contrast to the approach shown in [17], the intensity EOM1 is replaced by a phase EOM. The powers of the local light and probe light equal to them in the case of IML by adjusting the gain of EDFA1. Figure 3 shows the measured BOTDA trace 3(a) along the whole 40-km fiber and 3(b) from 39.94km to 40.02km when the sweep frequency is 1.2GHz and 1.22GHz, respectively. The fiber heating induced intensity transition can’t been observed no longer due to a low SNR of 13.7dB. Results in Fig. 3 and Fig. 4 illustrate the effectiveness of our approach with ~10dB SNR improvement.
Fig. 3 Measured BOTDA trace: (a) along the whole fiber and (b) near the far end of the fiber based on PML + PMP.
Fig. 4 Measured BOTDA trace: (a) along the whole fiber and (b) near the far end of the fiber based on IML + PMP.
By tuning the RF signal over a frequency span from 1.1GHz to 1.3GHz with 4-MHz step, the shape of the BGS is measured after 1000-time average at each sweep frequency point. Figure 5 shows the top view of the decoded BGS along the whole fiber (as shown in Fig. 5(a)) and from 39.95km to 40km (as shown in Fig. 5(b)), where we can clearly observe the induced BGS shift corresponding to the fiber heating.
Figure 6 shows the corresponding decoded BFS 6(a) along the whole fiber and 6(b) from 39.95km to 40km through Lorentz fitting of the measured BGS, the ~20MHz frequency transition between the heated and unheated segment is observed obviously as shown in Fig. 6(b). The oscillation of the decoded BFS is introduced by the additional strain mainly due to fiber coiling and system noises. About 3-m spatial resolution (from 10% to 90%) is measured as shown in Fig. 6(b), which agrees well with 30-ns pulse width. The achieved temperature accuracy is calculated through the BFS trace, resulting in ± 1.8MHz along the heated segment, which corresponds to a temperature accuracy of ± 1.8°C.
5. Effect of CD
As indicated in [17], the PM-IM conversion is not only introduced by the Brillouin gain/loss, but also by the CD of the sensing fiber. When considering CD, the optical field before the PD is
where ϕ(f) is the CD induced optical phase shift corresponds to the term of E with a frequency of f. Corresponding output current of the PD at frequency fs can be expressed aswhere D and L are the chromatic dispersion coefficient and the length of the sensing fiber, respectively. λc is optical wavelength of local light, c is the speed of light in vacuum, and ϕd is the phase of the PD output current. Thus, the PD output current without Brillouin gain at frequency fs can be written aswhere Jn(⋅) is the nth-order Bessel function of the first kind, m is the modulation index of the phase modulator, ϕdw is the phase of the PD output current, and E01 is the power of local light before phase modulation. The Brillouin gain induced current can be calculated byCD induced PM-IM conversion as shown in Eq. (5) (i.e. corresponding to sin(2π2β2Lfs2)) gets bigger with the increase of the sweep frequency fs. On the other hand, PM-IM conversion caused by Brillouin gain/loss is symmetrically centered at the BFS as indicated in Eq. (2). Subsequently, the total PM-IM conversion as indicated in Eq. (4) is asymmetric centered at the BFS and may lead to asymmetry of the measured BGS that obtained through Eq. (6).In order to investigate the effect of CD on the BGS, we measure the BGS at the far fiber end in two cases: the polarization state is aligned to i) the Z-axis of the phase modulator, i.e. corresponding to a modulation index of 0.6 when RF signal with 10-dBm power is applied, ii) the X-axis of the phase EOM, i.e. corresponding to a modulation index of 0.2 calculated by the inherent properties of the phase EOM (electro-optic coefficient is 30.8*10−12mv−1 at Z-axis and 9.6*10−12mv−1 at X-axis). Figures 7(a) and 7(b) show the measured PM-IM conversions versus fS in the absence of pulsed pump (blue lines) by turning off the pulse generator, total PM-IM conversion versus fS in the presence of pulsed pump (red lines) and calculated BGS through Eq. (6) (purple lines) in these two cases, respectively. The data at each sweep frequency is measured by the OSC through 1000-time average. The PM-IM conversion without pulsed pump increases fast versus fS as shown in Fig. 7(a), which leads to the asymmetry of the measured BGS and ~6MHz fitting error as shown in Fig. 8(a) . On the other hand, it increases slowly versus fS as shown in Fig. 7(b), thus a symmetric BGS is measured and no fitting error is observed as shown in Fig. 8(b).
Fig. 7 Measured PM-IM conversions versus fS in the absence of pulsed pump (blue lines), total PM-IM conversions versus fS in the presence of pulsed pump (red lines) and calculated BGS (purple lines) when the polarization state is aligned to (a) Z-axis and (b) X-axis of the phase modulator, respectively.
Fig. 8 Measured BGS and their Lorentz fittings when the polarization state is aligned to the (a) Z-axis and (b) X-axis of the phase modulator, respectively.
It’s obvious that the PM-IM conversion without pulsed pump leads to the asymmetry of the BGS, in order to check it’s indeed mainly due to CD, a comparative study between the theoretical model in Eq. (5) and the results in Fig. 7 is implemented. Figure 9(a) shows the simulated PM-IM conversions according to the theoretical model in Eq. (5) under different modulation index (i.e. m = 0.6, 0.5, 0.4, 0.3, 0.2 and 0.1). In the simulation, D = 17ps/nm/km, λc = 1550nm, L = 40km, fS changes from 1.1GHz to 1.3GHz and assuming that 8RCE012 = 1. When m equals to 0.6 and 0.2 respectively, the slopes of the simulated PM-IM conversion curves are 0.0107 (i.e. p1) and 0.0040 (i.e. p2), respectively. Figure 9(b) shows the measured PM-IM conversions without pulsed pump (i.e. blue lines as shown in Fig. 7) with their fitting curves versus fS (i.e. red lines). When the polarization state is aligned to the Z-axis, the slope of the fitting line is 0.3695 (i.e. k1). On the other hand, when the polarization state is aligned to the X-axis, we suppose that the slope of the fitting curve is k2 if the PM-IM conversion is caused by CD. Even system induced loss/gain can change the values of p1, p2, k1 and k2, an equation defined as p1/p2 = k1/k2 should always be satisfied, so the ideal PM-IM conversion curve when m = 0.2 should be the purple dotted line shown in Fig. 9(b). The slight difference between the purple dotted line and the fitted red line when m = 0.2 can be ignored, so we can draw a conclusion that the measured PM-IM conversions without pulsed pump are mainly caused by CD. The oscillation of the measured PM-IM conversion curves is mainly caused by the power jitter of the RF signal generator. Note that, when m = 0.6, the CW optical light leakage due to the finite extinction ratio of the intensity EOM1 introduces a Brillouin gain centered at 1.2GHz as shown in Fig. 9(b), which also contributes part of the asymmetry of the BGS.
Fig. 9 (a) Simulated PM-IM conversion according to the theoretical model in Eq. (5) under different m values and (b) measured PM-IM conversions without pulsed pump when the polarization state is aligned to the Z-axis (m = 0.6) and X-axis (m = 0.2) of the phase modulator, respectively.
Furthermore, different polarization states of the input of the phase EOM may lead to different fitting errors along the whole fiber, so BOTDA sensor based on the PMP light requires high stable polarization state control to achieve long-term stability. On the other band, it’s possible to further reduce CD induced BGS distortion by using dispersion shifted fiber, CD compensation algorithm, and etc.
6. Conclusion
Coherent BOTDA sensor with IML light and IQ demodulation has been proposed and experimentally demonstrated. Compared with coherent BOTDA sensors with PML light, it offers the advantages of coherent and multiple sidebands induced noises reduction by using carrier-frequency-suppressed intensity modulation. About 3-meter resolution with ± 1.8°C temperature accuracy is achieved for a sensing distance of 40-km. Further efforts should be focused on the reduction of the CD induced BGS distortion.
Acknowledgments
The research is supported in part by the International Science and Technology Cooperation Program of China (2014DFA11170), the National Natural Science Foundation of China (No. 61325023), the Key Grant Project of Chinese Ministry of Education (No.313049) and the key project of Sichuan Province of China (2011GZ0239).
References and links
1. T. Horiguchi, K. Shimizu, T. Kurashima, M. Tateda, and Y. Koyamada, “Development of a distributed sensing technique using Brillouin scattering,” J. Lightwave Technol. 13(7), 1296–1302 (1995). [CrossRef]
2. M. A. Soto, M. Taki, G. Bolognini, and F. D. Pasquale, “Simplex-coded BOTDA sensor over 120-km SMF with 1-m spatial resolution assisted by optimized bidirectional Raman amplification,” IEEE Photon. Technol. Lett. 24(20), 1823–1826 (2012). [CrossRef]
3. R. Bernini, A. Minardo, and L. Zeni, “Long-range distributed Brillouin fiber sensors by use of an unbalanced double sideband probe,” Opt. Express 19(24), 23845–23856 (2011). [CrossRef] [PubMed]
4. T. Sperber, A. Eyal, M. Tur, and L. Thévenaz, “High spatial resolution distributed sensing in optical fibers by Brillouin gain-profile tracing,” Opt. Express 18(8), 8671–8679 (2010). [CrossRef] [PubMed]
5. Y. Li, L. Chen, Y. Dong, and X. Bao, “A novel distributed Brillouin sensor based on optical differential parametric amplification,” J. Lightwave Technol. 28(18), 2621–2626 (2010). [CrossRef]
6. W. Li, X. Bao, Y. Li, and L. Chen, “Differential pulse-width pair BOTDA for high spatial resolution sensing,” Opt. Express 16(26), 21616–21625 (2008). [CrossRef] [PubMed]
7. M. A. Soto, G. Bolognini, and F. Di Pasquale, “Analysis of pulse modulation format in coded BOTDA sensors,” Opt. Express 18(14), 14878–14892 (2010). [CrossRef] [PubMed]
8. M. A. Soto, S. Le Floch, and L. Thévenaz, “Bipolar optical pulse coding for performance enhancement in BOTDA sensors,” Opt. Express 21(14), 16390–16397 (2013). [CrossRef] [PubMed]
9. Y. Dong, L. Chen, and X. Bao, “Extending the sensing range of Brillouin optical time-domain analysis combining frequency- division multiplexing and in-line EDFAs,” J. Lightwave Technol. 30(8), 1161–1167 (2012). [CrossRef]
10. Y. Dong, L. Chen, and X. Bao, “Time-division multiplexing-based BOTDA over 100 km sensing length,” Opt. Lett. 36(2), 277–279 (2011). [CrossRef] [PubMed]
11. A. Zornoza, M. Saguesand, and A. Loayssa, “Self-heterodyne detection for SNR improvement and distributed phase-shift measurements in BOTDA,” J. Lightwave Technol. 30(8), 1066–1072 (2012). [CrossRef]
12. X. Lu, M. A. Soto, M. González-Herraez, and L. Thévenaz, “Brillouin distributed fibre sensing using phase modulated probe,” in Proc. SPIE, Fifth European Workshop on Optical Fibre Sensors 8794, 87943P–1-87943P–4 (2013). [CrossRef]
13. J. Urricelqui, M. Sagues, and A. Loayssa, “Synthesis of Brillouin frequency shift profiles to compensate non-local effects and Brillouin induced noise in BOTDA sensors,” Opt. Express 22(15), 18195–18202 (2014). [CrossRef] [PubMed]
14. J. Urricelqui, M. Sagues, and A. Loayssa, “Phasorial differential pulse-width pair technique for long-range Brillouin optical time-domain analysis sensors,” Opt. Express 22(14), 17403–17408 (2014). [CrossRef] [PubMed]
15. J. Urricelqui, M. Sagues, and A. Loayssa, “BOTDA measurements tolerant to non-local effects by using a phase-modulated probe wave and RF demodulation,” Opt. Express 21(14), 17186–17194 (2013). [CrossRef] [PubMed]
16. A. Minardo, R. Bernini, and L. Zeni, “A Simple technique for reducing pump depletion in long-range distributed Brillouin fiber sensors,” IEEE Sens. J. 9(3), 633–634 (2009). [CrossRef]
17. X. Angulo-Vinuesa, D. Bacquet, S. Martin-Lopez, P. Corredera, P. Szriftgiser, and M. Gonzalez-Herraezet, “Relative intensity noise transfer reduction in Raman-assisted BOTDA systems,” IEEE Photon. Technol. Lett. 26(3), 271–274 (2014). [CrossRef]
18. X. Tu, Q. Sun, W. Chen, M. Chen, and Z. Meng, “Vector Brillouin optical time-domain analysis with heterodyne detection and IQ demodulation algorithm,” IEEE Photon. J. 6(2), 6800908 (2014). [CrossRef]
19. J. Urricelqui, A. Zornoza, M. Sagues, and A. Loayssa, “Dynamic BOTDA measurements based on Brillouin phase-shift and RF demodulation,” Opt. Express 20(24), 26942–26949 (2012). [CrossRef] [PubMed]
