August 1, 2014 / Vol. 39, No. 15 / OPTICS LETTERS
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Phase-sensitive optical time-domain reflectometry with Brillouin amplification Z. N. Wang,* J. Li, M. Q. Fan, L. Zhang, F. Peng, H. Wu, J. J. Zeng, Y. Zhou, and Y. J. Rao Key Laboratory of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu 611731, China *Corresponding author: [email protected] Received March 31, 2014; revised June 10, 2014; accepted June 16, 2014; posted June 16, 2014 (Doc. ID 209074); published July 18, 2014 We propose a phase-sensitive optical time-domain reflectometry (Φ-OTDR) scheme with counterpumping fiber Brillouin amplification (FBA). High-sensitivity perturbation detection over 100 km is experimentally demonstrated as an example. FBA significantly enhances the probe pulse signal, especially at the second half of the sensing fiber, with only 6.4 dBm pump power. It is confirmed that its amplification efficiency is much higher than 28.0 dBm counterpumping fiber Raman amplification. The FBA Φ-OTDR scheme demonstrated in this work can also be incorporated into other distributed fiber-optic sensing systems for extension of sensing distance or enhancement of sensing signal level. © 2014 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (290.5870) Scattering, Rayleigh; (290.5900) Scattering, stimulated Brillouin; (190.4370) Nonlinear optics, fibers. http://dx.doi.org/10.1364/OL.39.004313
Distributed optical fiber sensing (DOFS) systems have been widely used for perimeter intrusion detection, pipeline safety monitoring [1], etc., owing to their large-scale monitoring range, low cost per monitored point, simple installation, and geometric versatility compared with point sensors [2]. Up to now, typical DOFS techniques have included phase-sensitive optical time-domain reflectometry (Φ-OTDR) [3] and polarization-sensitive optical time-domain reflectometry (P-OTDR) [4]. The perturbation of the fiber at any location can be detected by measuring the property variation of the Rayleigh backscattering light. Particularly, compared with P-OTDR, Φ-OTDR has several advantages, including much higher sensitivity, simultaneous multipoint detection ability, and better spatial resolution. A longer sensing range per system is generally preferred for many sensing applications, as long as the sensitivity and spatial resolution can be maintained. To extend the sensing distance of DOFS systems such as Φ-OTDR, valuable research work has been carried out using advanced signal amplification schemes. The erbium-doped fiber amplifier (EDFA) can be used to significantly boost the probe signal before launching into the sensing fiber, but signal power is monotonically attenuated along the fiber channel, leading to the fact that the remote-end intensity of the probe signal is much lower than the near-end intensity and too weak to be detected. Meanwhile, the sensing distance of EDFA-based Φ-OTDR is limited by the rapidly accumulated nonlinear effects along the fiber, which severely degrade the quality of the probe signal. Another well-known amplification technique is fiber Raman amplification (FRA). FRA has been proved to be an effective way to boost the signal, and it has been widely used in optical communication [5]. The distributed FRA provides a nice solution to extend the sensing distance of DOFS. Φ-OTDR based on FRA was reported to extend the sensing distance up to 62 km [6]. Recently, a Φ-OTDR with 128 km sensing and 15 m spatial resolution was demonstrated by utilizing bi-directional FRA [7]. However, due to the relatively low Raman gain 0146-9592/14/154313-04$15.00/0
coefficient, the required pump power is quite high (about 1 W). Also, the high-power Raman pump usually needs to be a Raman fiber laser with high relative intensity noise, which will deteriorate the quality of the sensing signal [8]. It is therefore useful to find an alternative solution to replace the expensive and noisy Raman fiber laser. Fiber Brillouin amplification (FBA), based on stimulated Brillouin scattering (SBS), can also provide fully distributed amplification [9,10,11]. More importantly, the gain coefficient of SBS is larger than that of stimulated Raman scattering (SRS) by about three orders of magnitude, which allows the use of much lower pump powers for FBA than FRA. When the Brillouin pump (BP) and the probe signal propagate in opposite directions, the weak probe signal can be amplified by tens of dB along the fiber. Note that the frequency difference between the pump and the probe should be close to the Brillouin frequency shift (BFS) of the sensing fiber. Although the Brillouin gain bandwidth is only around tens of megahertz in optical fibers, it is wide enough for most of the modern DOFS systems. In this Letter, we experimentally demonstrate a longdistance (100 km) fiber-optic Φ-OTDR distributed sensing system utilizing FBA. This technique enables optimal amplification for a small input signal (the peak power is 11.3 dBm) with a low BP power (6.4 dBm). We also compare the FBA scheme with a Raman counterpumping scheme, and the result shows that the FBA can amplify the sensing signal much more efficiently. As a result, the FBA-based Φ-OTDR DOFS system performs very well in terms of sensing range and signal quality. To analyze the performance of Brillouin amplification, we simulate the Rayleigh scattering curves of an OTDR using FBA and FRA, respectively. The two steady-state coupled equations of the FBA, Eqs. (1) and (2), modeled as quasi-CW SBS, are given by [12] dI p −gB I p I s − αI p ; dz © 2014 Optical Society of America
(1)
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dI s −gB I p I s αI s ; dz
(2)
where I p represents the BP intensity, I s represents the Stokes intensity (the probe light), α is the attenuation coefficient, and gB is the Brillouin gain coefficient. We set the FWHM of the Brillouin gain spectrum (BGS) to 35.8 MHz. To calculate the evolution of the pulse power I s z within the counterpumping FRA scheme, we use the bidirectional FRA equations presented in [13], setting the power of the forward Raman pump to zero. With FRA, both the signal and its Rayleigh scattering component will be amplified through SRS. Therefore, neglecting pump depletion, the Rayleigh scattering power observed at the detection point should be I zRS 0 εI s 0I s z∕ I s 02 εI 2s z∕I s 0 [13], where z is a given point along the fiber, ε is the Rayleigh backscatter coefficient, and essentially I s z∕I s 0 is the net gain acquired by the signal while traveling from length 0 to z (or vice versa). On the other hand, the Brillouin amplification can take effect only when the signal travels opposite to the pump; consequently, the Rayleigh scattering component of the signal cannot be amplified and only experiences the single-trip loss. Hence, within the FBA scheme, the Rayleigh scattering power observed at the detection point should be I zRS 0 εI s z exp−αz. The parameters used for simulation are presented in Tables 1 and 2 [14,15]. In the case of Brillouin amplification, we set the average BP power at 6.4 dBm and the peak pulse power at 11.3 dBm. As shown in Fig. 1, the probe pulse can acquire much higher gain from the BP than that from the Raman pump. As the ambient temperature changes, the BFS of the fiber may drift away from the preset frequency separation between the pump and the probe; as a result, the effective Brillouin gain will be lower than the maximum. Figure 2 shows the variation of the OTDR traces when the pump– probe frequency separation has drifted 10.0 and 17.9 MHz off from the optimal value. It can be seen that in either case, the probe pulse still acquires higher gain than the case with 25.2 dBm FRA. The experimental setup for the Φ-OTDR assisted by FBA is depicted in Fig. 3. An ultra-narrow-linewidth (100 Hz) laser operating at 1549.809 nm is used as the light source with polarization-maintaining (PM) output. The laser output is split into two branches by a 3 dB PM coupler (OC1). One branch is modulated by an acoustooptic modulator (AOM) to generate the pulsed probe
Fig. 1. Simulated Rayleigh scattering traces with Brillouin amplification/Raman amplification.
wave (with 100 ns pulse width, allowing for 10 m spatial resolution), and a 200 MHz frequency shift is induced at the same time. The other branch is further split into two divisions using a 10:90 optical coupler (OC2). The 10% division is used as the local oscillator (LO) for the heterodyne detection, while the 90% division is modulated by a Mach–Zehnder electro-optic modulator (EOM) to suppress the carrier and generate two sidebands. The frequency separation between the BP and the probe signal is set by tuning a microwave generator that drives the EOM. The high-frequency side-band is selected as the BP by a well-aligned fiber Bragg grating (FBG) with 0.08 nm bandwidth (BW) connected to a circulator (Cir.1). The lower-frequency sideband is discarded. The BP is amplified by an EDFA and then depolarized using a polarization scrambler (PS) in order to suppress the polarization dependent gain. A variable optical attenuator with maximal 60 dB isolation loss (VOA2) is inserted before the Circulator 3 (Cir. 3) in order to properly adjust the BP power launched into the sensing fiber. The probe pulse propagates counter to the BP and is amplified via SBS. The Rayleigh backscattering signal of the probe pulse is mixed with the LO in a 2 × 2 optical coupler (OC3) and detected by a 350 MHz balanced photodetector. The electrical signal is then sent to an electrical spectrum analyzer (ESA) working in the zero-span mode. The resolution bandwidth is set to 3 MHz, and the central frequency is set at 200 MHz. The ESA output signal is digitized by a 50 MS∕s analog-to-digital (A/D) converter and then analyzed by our own software. In our demonstration, the BP and the probe wave originate from the same laser. However, it should be noted that in principle the BP can also come from a different light source, provided that the frequency difference
Table 1. Parameters Used in the Brillouin Amplification Model Wavelength (nm) 1550 Table 2. Wavelength (nm) 1455 1550
α (dB/km)
gB (W−1 km−1 )
ε km−1
0.19
0.105 × 103
4.3 × 10−5
Parameters Used in the Raman Amplification Model α (dB/km) 0.23 0.19
gR (W−1 km−1 ) 0.42 ∼
ε km−1 −5
6 × 10 4.3 × 10−5
Fig. 2. Simulated Rayleigh scattering traces with different pump conditions. (The Brillouin gain variation is related to the pump–probe frequency offset with regard to the BFS; all other parameters are the same as those in Fig. 1.)
August 1, 2014 / Vol. 39, No. 15 / OPTICS LETTERS
Fig. 3. Experimental setup for Φ-OTDR with FBA. Arrangement A and B represent two schemes to amplify the probe pulse in different fiber coils by setting proper frequency shift. Arrangement A: FBA occurs in Section 1. Arrangement B: FBA occurs in Section 2.
between probe and pump can be kept constant regardless of the frequency changes of the lightwaves. Previously, many methods (such as the frequency counter) have been used to lock the output of different frequency light sources [16]. As in the simulation, the peak power of the probe pulse is adjusted to be 11.3 dBm, and the power of BP is set to be 6.4 dBm. This choice of the peak probe pulse power achieves sufficient backscattered intensity along the first 50 km of the sensing fiber while avoiding detrimental nonlinear effects [7]. The choice of BP power allows the maximal interference intensity near the fiber end to reach half of the maximum near the probe side. The 100 km sensing fiber consists of 50 km (Section 1), 25 km (Section 2), 12 km (Section 3), and 13 km (Section 4) segments. The BFSs of the four sections are 11.037, 10.927, 11.075, and 11.070 GHz, respectively, at room temperature. Mostly the sensing fiber coils are put in order as Arrangement A, and the frequency shift is set to be 11.037 GHz, in order to maximize the SBS in Section 1; Arrangement B is only used to obtain the results in Fig. 5. Figure 4 shows the original interference traces without further signal processing. Figure 4(b) clearly illustrates that the backscattered interference signal of the last half
Fig. 4. (a) Φ-OTDR trace without amplification. (b) Brillouin amplified trace with 6.4 dBm pump power. (c) Raman amplified trace with 25.2 dBm pump power. (d) Raman amplified trace with 28.0 dBm pump power.
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Fig. 5. Φ-OTDR trace with FBA applied to 50–75 km of the whole sensing range (Arrangement B is used).
of the sensing fiber is dramatically amplified by the 6.4 dBm BP. As a comparison, Figs. 4(c) and 4(d) show interference traces with different Raman pump sources. In Fig. 4(c), the pump is a Raman fiber laser providing 25.2 dBm power into the sensing fiber, and in Fig. 4(d), the counterpart is a semiconductor pump laser providing 28.0 dBm power. For each of the two Raman pumps, we used the highest output power available. Obviously, the stronger Raman fiber laser provides a higher gain than the semiconductor pump laser, yet still a far smaller gain than provided by FBA. The insets in Fig. 4 show the four enlarged traces. The portion before the vertical dashed line is within the round-trip time of a single probe pulse, while the portion after the dashed line is purely noise. To study the FBA effect when it is applied to different fiber coils that have different BFSs, we regroup the four fiber coils as Arrangement B in Fig. 3 and repeat the experiment by setting the pump–probe frequency separation to be 10.927 GHz, corresponding to the BFS of the 25 km Section 2 (50–75 km in the whole range). The original interference trace under this situation is shown in Fig. 5. Apparently, the probe signal power experiences a significant growth in Section 2. Therefore, the FBA can be tuned to emphasize a particular section of the fiber link, allowing the power evolution of the probe signal over distance to be adjusted. In real-life Φ-OTDR deployments, the BFS of the sensing fiber varies with temperature. For example, assuming a BFS drift of 1 MHz/°C and a diurnal temperature variation of 17°C, the optimal BFS may change by as much as 17 MHz. Figure 6 shows the interference traces for fiber Arrangement A with pump–probe frequency shifts of
Fig. 6. Φ-OTDR traces: (a) FBA pump–probe frequency shift set to 11.020 GHz versus 28.0 dBm Raman pumping. (b) FBA pump–probe frequency shift set to 11.055 GHz versus Raman pumping.
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100 km with counterpumping distributed FBA and achieved high SNR intrusion detection along a 100 km range. Owing to the high gain efficiency of SBS, the required Brillouin pump power is only 6.4 dBm. The system performance is significantly superior to counterpumping Raman amplification with 28.0 dBm pump power. Also, the effect of pump frequency variation is studied, and the system shows reasonable robustness. It should be noted that further extension of the sensing range is readily foreseeable by combining counterpumping Brillouin amplification with copumping Raman amplification (the first-order or higher-order Raman amplification). Our work shows great potential for FBA to be adopted in many other distributed fiber-optic sensing systems. This work is supported by Natural Science Foundation of China (61205048,61290312), Research Fund for the Doctoral Program of Higher Education of China (20120185120003), Fundamental Research Funds for the Central Universities (ZYGX2012J002), the PCSIRT project (IRT1218), and the 111 project (B14039). Fig. 7. Demodulated intrusion signal at different locations: (a) detected perturbation at 50.0 km and (b) detected perturbation at 98.8 km.
11.020 GHz and 11.055 GHz (a ∼17 MHz drift relative to the 11.037 GHz BFS of the fiber in Section 1). Though the amplification effect is still comparable with the FRA case in Fig. 4(d) (because the drift is within the 35 MHz FWHM bandwidth of the BGS), the degradation in FBA is obvious when compared with Fig. 4(b). Tracking signal intensity and then tuning the BP frequency to eliminate intensity fluctuations would minimize this problem. Alternatively, replacing a single frequency BP with a frequency comb (with ∼20 MHz frequency “tooth spacing”) would provide a wide gain spectrum and eliminate the need for adaptive tuning. To test the sensing performance, we made two monitoring points that are located at 50.0 and 98.8 km. At each point, a 10 m coil of bare fiber was glued into the flat surface of a metal pad. To indirectly perturb the attached fiber, we applied manual pressure to the surface of the metal pad. Wavelet denoising [17] was used in the signal processing to further improve the detection performance of Φ-OTDR. The perturbation experiment results obtained with Arrangement A are shown in Fig. 7. The pump–probe frequency shift was set at 11.037 GHz, maximizing the Brillouin amplification within Section 1. Distinct peaks are observed at the monitoring points where artificial disturbances are applied. The achieved SNR of the demodulated intrusion signals is 14.8 dB at 50.0 km and 16.6 dB at 98.8 km. (The SNR is calculated as the ratio between the signal peak intensity and the root-mean-square of the background noise intensity.) Accurate event identification of this system is assured by the high SNR of the signals. In our results, the averaging number is 32, and the pulse repetition rate is 1 kHz, so the maximum event-detection rate is 31.25 Hz. Higher rates are attainable with reduced SNR. In this Letter, we have experimentally demonstrated the extension of the sensing range of Φ-OTDR to
References 1. H. J. Wu, Z. N. Wang, F. Peng, Z. P. Peng, X. Y. Li, Y. Wu, and Y. J. Rao, “Field test of a fully distributed fiber-optic intrusion detection system for long-distance security monitoring of national borderline,” presented at the 23rd International Conference on Optical Fibre Sensors, Santander, Spain, 2014. 2. L. Thévenaz, in Optical Fiber Sensors, OSA Technical Digest (Optical Society of America, 2006), paper ThC1. 3. Y. L. Lu, T. Zhu, L. A. Chen, and X. Y. Bao, J. Lightwave Technol. 28, 3243 (2010). 4. F. Peng, Z. Wang, Y. Rao, and X. Jia, in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference (Optical Society of America, 2013), paper JW2 A.22. 5. J. Bromage, J. Lightwave Technol. 22, 79 (2004). 6. Y. J. Rao, J. Luo, Z. L. Ran, J. F. Yue, X. D. Luo, and Z. Zhou, Proc. SPIE 75031O, 1 (2009). 7. F. Peng, Z. P. Peng, X. H. Jia, Y. J. Rao, Z. N. Wang, and H. Wu, in Optical Fiber Communication Conference, OSA Technical Digest (Optical Society of America, 2014), paper M3 J.4. 8. C. Headley and G. P. Agrawal, Raman Amplification in Fiber Optical Communication Systems (Elsevier, 2005). 9. O. Terra, G. Grosche, and H. Schnatz, Opt. Express 18, 16102 (2010). 10. K. Predehl, G. Grosche, S. M. F. Raupach, S. Droste, O. Terra, J. Alnis, T. Legero, T. W. Hänsch, T. Udem, R. Holzwarth, and H. Schnatz, Science 336, 441 (2012). 11. A. Kobyakov, S. Darmanyan, M. Sauer, and D. Chowdhury, Opt. Lett. 31, 1960 (2006). 12. G. P. Agrawal, Nonlinear Fiber Optics (Springer, 2000). 13. H. F. Martins, S. Martín-López, P. Corredera, M. L. Filograno, O. Frazão, and M. Gonzalez-Herráez, J. Lightwave Technol. 32, 1510 (2014). 14. L. Chen and X. Bao, Opt. Commun. 152, 65 (1998). 15. Z. N. Wang, H. Wu, M. Q. Fan, Y. J. Rao, X. H. Jia, and W. L. Zhang, Opt. Express 21, 20090 (2013). 16. D. Williams, X. Bao, and L. Chen, Photon. Res. 2, 1 (2014). 17. Z. Qin, L. Chen, and X. Bao, IEEE Photon. Technol. Lett. 24, 542 (2012).
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Phase-sensitive optical time-domain reflectometry with Brillouin amplification Z. N. Wang,* J. Li, M. Q. Fan, L. Zhang, F. Peng, H. Wu, J. J. Zeng, Y. Zhou, and Y. J. Rao Key Laboratory of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu 611731, China *Corresponding author: [email protected] Received March 31, 2014; revised June 10, 2014; accepted June 16, 2014; posted June 16, 2014 (Doc. ID 209074); published July 18, 2014 We propose a phase-sensitive optical time-domain reflectometry (Φ-OTDR) scheme with counterpumping fiber Brillouin amplification (FBA). High-sensitivity perturbation detection over 100 km is experimentally demonstrated as an example. FBA significantly enhances the probe pulse signal, especially at the second half of the sensing fiber, with only 6.4 dBm pump power. It is confirmed that its amplification efficiency is much higher than 28.0 dBm counterpumping fiber Raman amplification. The FBA Φ-OTDR scheme demonstrated in this work can also be incorporated into other distributed fiber-optic sensing systems for extension of sensing distance or enhancement of sensing signal level. © 2014 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (290.5870) Scattering, Rayleigh; (290.5900) Scattering, stimulated Brillouin; (190.4370) Nonlinear optics, fibers. http://dx.doi.org/10.1364/OL.39.004313
Distributed optical fiber sensing (DOFS) systems have been widely used for perimeter intrusion detection, pipeline safety monitoring [1], etc., owing to their large-scale monitoring range, low cost per monitored point, simple installation, and geometric versatility compared with point sensors [2]. Up to now, typical DOFS techniques have included phase-sensitive optical time-domain reflectometry (Φ-OTDR) [3] and polarization-sensitive optical time-domain reflectometry (P-OTDR) [4]. The perturbation of the fiber at any location can be detected by measuring the property variation of the Rayleigh backscattering light. Particularly, compared with P-OTDR, Φ-OTDR has several advantages, including much higher sensitivity, simultaneous multipoint detection ability, and better spatial resolution. A longer sensing range per system is generally preferred for many sensing applications, as long as the sensitivity and spatial resolution can be maintained. To extend the sensing distance of DOFS systems such as Φ-OTDR, valuable research work has been carried out using advanced signal amplification schemes. The erbium-doped fiber amplifier (EDFA) can be used to significantly boost the probe signal before launching into the sensing fiber, but signal power is monotonically attenuated along the fiber channel, leading to the fact that the remote-end intensity of the probe signal is much lower than the near-end intensity and too weak to be detected. Meanwhile, the sensing distance of EDFA-based Φ-OTDR is limited by the rapidly accumulated nonlinear effects along the fiber, which severely degrade the quality of the probe signal. Another well-known amplification technique is fiber Raman amplification (FRA). FRA has been proved to be an effective way to boost the signal, and it has been widely used in optical communication [5]. The distributed FRA provides a nice solution to extend the sensing distance of DOFS. Φ-OTDR based on FRA was reported to extend the sensing distance up to 62 km [6]. Recently, a Φ-OTDR with 128 km sensing and 15 m spatial resolution was demonstrated by utilizing bi-directional FRA [7]. However, due to the relatively low Raman gain 0146-9592/14/154313-04$15.00/0
coefficient, the required pump power is quite high (about 1 W). Also, the high-power Raman pump usually needs to be a Raman fiber laser with high relative intensity noise, which will deteriorate the quality of the sensing signal [8]. It is therefore useful to find an alternative solution to replace the expensive and noisy Raman fiber laser. Fiber Brillouin amplification (FBA), based on stimulated Brillouin scattering (SBS), can also provide fully distributed amplification [9,10,11]. More importantly, the gain coefficient of SBS is larger than that of stimulated Raman scattering (SRS) by about three orders of magnitude, which allows the use of much lower pump powers for FBA than FRA. When the Brillouin pump (BP) and the probe signal propagate in opposite directions, the weak probe signal can be amplified by tens of dB along the fiber. Note that the frequency difference between the pump and the probe should be close to the Brillouin frequency shift (BFS) of the sensing fiber. Although the Brillouin gain bandwidth is only around tens of megahertz in optical fibers, it is wide enough for most of the modern DOFS systems. In this Letter, we experimentally demonstrate a longdistance (100 km) fiber-optic Φ-OTDR distributed sensing system utilizing FBA. This technique enables optimal amplification for a small input signal (the peak power is 11.3 dBm) with a low BP power (6.4 dBm). We also compare the FBA scheme with a Raman counterpumping scheme, and the result shows that the FBA can amplify the sensing signal much more efficiently. As a result, the FBA-based Φ-OTDR DOFS system performs very well in terms of sensing range and signal quality. To analyze the performance of Brillouin amplification, we simulate the Rayleigh scattering curves of an OTDR using FBA and FRA, respectively. The two steady-state coupled equations of the FBA, Eqs. (1) and (2), modeled as quasi-CW SBS, are given by [12] dI p −gB I p I s − αI p ; dz © 2014 Optical Society of America
(1)
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dI s −gB I p I s αI s ; dz
(2)
where I p represents the BP intensity, I s represents the Stokes intensity (the probe light), α is the attenuation coefficient, and gB is the Brillouin gain coefficient. We set the FWHM of the Brillouin gain spectrum (BGS) to 35.8 MHz. To calculate the evolution of the pulse power I s z within the counterpumping FRA scheme, we use the bidirectional FRA equations presented in [13], setting the power of the forward Raman pump to zero. With FRA, both the signal and its Rayleigh scattering component will be amplified through SRS. Therefore, neglecting pump depletion, the Rayleigh scattering power observed at the detection point should be I zRS 0 εI s 0I s z∕ I s 02 εI 2s z∕I s 0 [13], where z is a given point along the fiber, ε is the Rayleigh backscatter coefficient, and essentially I s z∕I s 0 is the net gain acquired by the signal while traveling from length 0 to z (or vice versa). On the other hand, the Brillouin amplification can take effect only when the signal travels opposite to the pump; consequently, the Rayleigh scattering component of the signal cannot be amplified and only experiences the single-trip loss. Hence, within the FBA scheme, the Rayleigh scattering power observed at the detection point should be I zRS 0 εI s z exp−αz. The parameters used for simulation are presented in Tables 1 and 2 [14,15]. In the case of Brillouin amplification, we set the average BP power at 6.4 dBm and the peak pulse power at 11.3 dBm. As shown in Fig. 1, the probe pulse can acquire much higher gain from the BP than that from the Raman pump. As the ambient temperature changes, the BFS of the fiber may drift away from the preset frequency separation between the pump and the probe; as a result, the effective Brillouin gain will be lower than the maximum. Figure 2 shows the variation of the OTDR traces when the pump– probe frequency separation has drifted 10.0 and 17.9 MHz off from the optimal value. It can be seen that in either case, the probe pulse still acquires higher gain than the case with 25.2 dBm FRA. The experimental setup for the Φ-OTDR assisted by FBA is depicted in Fig. 3. An ultra-narrow-linewidth (100 Hz) laser operating at 1549.809 nm is used as the light source with polarization-maintaining (PM) output. The laser output is split into two branches by a 3 dB PM coupler (OC1). One branch is modulated by an acoustooptic modulator (AOM) to generate the pulsed probe
Fig. 1. Simulated Rayleigh scattering traces with Brillouin amplification/Raman amplification.
wave (with 100 ns pulse width, allowing for 10 m spatial resolution), and a 200 MHz frequency shift is induced at the same time. The other branch is further split into two divisions using a 10:90 optical coupler (OC2). The 10% division is used as the local oscillator (LO) for the heterodyne detection, while the 90% division is modulated by a Mach–Zehnder electro-optic modulator (EOM) to suppress the carrier and generate two sidebands. The frequency separation between the BP and the probe signal is set by tuning a microwave generator that drives the EOM. The high-frequency side-band is selected as the BP by a well-aligned fiber Bragg grating (FBG) with 0.08 nm bandwidth (BW) connected to a circulator (Cir.1). The lower-frequency sideband is discarded. The BP is amplified by an EDFA and then depolarized using a polarization scrambler (PS) in order to suppress the polarization dependent gain. A variable optical attenuator with maximal 60 dB isolation loss (VOA2) is inserted before the Circulator 3 (Cir. 3) in order to properly adjust the BP power launched into the sensing fiber. The probe pulse propagates counter to the BP and is amplified via SBS. The Rayleigh backscattering signal of the probe pulse is mixed with the LO in a 2 × 2 optical coupler (OC3) and detected by a 350 MHz balanced photodetector. The electrical signal is then sent to an electrical spectrum analyzer (ESA) working in the zero-span mode. The resolution bandwidth is set to 3 MHz, and the central frequency is set at 200 MHz. The ESA output signal is digitized by a 50 MS∕s analog-to-digital (A/D) converter and then analyzed by our own software. In our demonstration, the BP and the probe wave originate from the same laser. However, it should be noted that in principle the BP can also come from a different light source, provided that the frequency difference
Table 1. Parameters Used in the Brillouin Amplification Model Wavelength (nm) 1550 Table 2. Wavelength (nm) 1455 1550
α (dB/km)
gB (W−1 km−1 )
ε km−1
0.19
0.105 × 103
4.3 × 10−5
Parameters Used in the Raman Amplification Model α (dB/km) 0.23 0.19
gR (W−1 km−1 ) 0.42 ∼
ε km−1 −5
6 × 10 4.3 × 10−5
Fig. 2. Simulated Rayleigh scattering traces with different pump conditions. (The Brillouin gain variation is related to the pump–probe frequency offset with regard to the BFS; all other parameters are the same as those in Fig. 1.)
August 1, 2014 / Vol. 39, No. 15 / OPTICS LETTERS
Fig. 3. Experimental setup for Φ-OTDR with FBA. Arrangement A and B represent two schemes to amplify the probe pulse in different fiber coils by setting proper frequency shift. Arrangement A: FBA occurs in Section 1. Arrangement B: FBA occurs in Section 2.
between probe and pump can be kept constant regardless of the frequency changes of the lightwaves. Previously, many methods (such as the frequency counter) have been used to lock the output of different frequency light sources [16]. As in the simulation, the peak power of the probe pulse is adjusted to be 11.3 dBm, and the power of BP is set to be 6.4 dBm. This choice of the peak probe pulse power achieves sufficient backscattered intensity along the first 50 km of the sensing fiber while avoiding detrimental nonlinear effects [7]. The choice of BP power allows the maximal interference intensity near the fiber end to reach half of the maximum near the probe side. The 100 km sensing fiber consists of 50 km (Section 1), 25 km (Section 2), 12 km (Section 3), and 13 km (Section 4) segments. The BFSs of the four sections are 11.037, 10.927, 11.075, and 11.070 GHz, respectively, at room temperature. Mostly the sensing fiber coils are put in order as Arrangement A, and the frequency shift is set to be 11.037 GHz, in order to maximize the SBS in Section 1; Arrangement B is only used to obtain the results in Fig. 5. Figure 4 shows the original interference traces without further signal processing. Figure 4(b) clearly illustrates that the backscattered interference signal of the last half
Fig. 4. (a) Φ-OTDR trace without amplification. (b) Brillouin amplified trace with 6.4 dBm pump power. (c) Raman amplified trace with 25.2 dBm pump power. (d) Raman amplified trace with 28.0 dBm pump power.
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Fig. 5. Φ-OTDR trace with FBA applied to 50–75 km of the whole sensing range (Arrangement B is used).
of the sensing fiber is dramatically amplified by the 6.4 dBm BP. As a comparison, Figs. 4(c) and 4(d) show interference traces with different Raman pump sources. In Fig. 4(c), the pump is a Raman fiber laser providing 25.2 dBm power into the sensing fiber, and in Fig. 4(d), the counterpart is a semiconductor pump laser providing 28.0 dBm power. For each of the two Raman pumps, we used the highest output power available. Obviously, the stronger Raman fiber laser provides a higher gain than the semiconductor pump laser, yet still a far smaller gain than provided by FBA. The insets in Fig. 4 show the four enlarged traces. The portion before the vertical dashed line is within the round-trip time of a single probe pulse, while the portion after the dashed line is purely noise. To study the FBA effect when it is applied to different fiber coils that have different BFSs, we regroup the four fiber coils as Arrangement B in Fig. 3 and repeat the experiment by setting the pump–probe frequency separation to be 10.927 GHz, corresponding to the BFS of the 25 km Section 2 (50–75 km in the whole range). The original interference trace under this situation is shown in Fig. 5. Apparently, the probe signal power experiences a significant growth in Section 2. Therefore, the FBA can be tuned to emphasize a particular section of the fiber link, allowing the power evolution of the probe signal over distance to be adjusted. In real-life Φ-OTDR deployments, the BFS of the sensing fiber varies with temperature. For example, assuming a BFS drift of 1 MHz/°C and a diurnal temperature variation of 17°C, the optimal BFS may change by as much as 17 MHz. Figure 6 shows the interference traces for fiber Arrangement A with pump–probe frequency shifts of
Fig. 6. Φ-OTDR traces: (a) FBA pump–probe frequency shift set to 11.020 GHz versus 28.0 dBm Raman pumping. (b) FBA pump–probe frequency shift set to 11.055 GHz versus Raman pumping.
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100 km with counterpumping distributed FBA and achieved high SNR intrusion detection along a 100 km range. Owing to the high gain efficiency of SBS, the required Brillouin pump power is only 6.4 dBm. The system performance is significantly superior to counterpumping Raman amplification with 28.0 dBm pump power. Also, the effect of pump frequency variation is studied, and the system shows reasonable robustness. It should be noted that further extension of the sensing range is readily foreseeable by combining counterpumping Brillouin amplification with copumping Raman amplification (the first-order or higher-order Raman amplification). Our work shows great potential for FBA to be adopted in many other distributed fiber-optic sensing systems. This work is supported by Natural Science Foundation of China (61205048,61290312), Research Fund for the Doctoral Program of Higher Education of China (20120185120003), Fundamental Research Funds for the Central Universities (ZYGX2012J002), the PCSIRT project (IRT1218), and the 111 project (B14039). Fig. 7. Demodulated intrusion signal at different locations: (a) detected perturbation at 50.0 km and (b) detected perturbation at 98.8 km.
11.020 GHz and 11.055 GHz (a ∼17 MHz drift relative to the 11.037 GHz BFS of the fiber in Section 1). Though the amplification effect is still comparable with the FRA case in Fig. 4(d) (because the drift is within the 35 MHz FWHM bandwidth of the BGS), the degradation in FBA is obvious when compared with Fig. 4(b). Tracking signal intensity and then tuning the BP frequency to eliminate intensity fluctuations would minimize this problem. Alternatively, replacing a single frequency BP with a frequency comb (with ∼20 MHz frequency “tooth spacing”) would provide a wide gain spectrum and eliminate the need for adaptive tuning. To test the sensing performance, we made two monitoring points that are located at 50.0 and 98.8 km. At each point, a 10 m coil of bare fiber was glued into the flat surface of a metal pad. To indirectly perturb the attached fiber, we applied manual pressure to the surface of the metal pad. Wavelet denoising [17] was used in the signal processing to further improve the detection performance of Φ-OTDR. The perturbation experiment results obtained with Arrangement A are shown in Fig. 7. The pump–probe frequency shift was set at 11.037 GHz, maximizing the Brillouin amplification within Section 1. Distinct peaks are observed at the monitoring points where artificial disturbances are applied. The achieved SNR of the demodulated intrusion signals is 14.8 dB at 50.0 km and 16.6 dB at 98.8 km. (The SNR is calculated as the ratio between the signal peak intensity and the root-mean-square of the background noise intensity.) Accurate event identification of this system is assured by the high SNR of the signals. In our results, the averaging number is 32, and the pulse repetition rate is 1 kHz, so the maximum event-detection rate is 31.25 Hz. Higher rates are attainable with reduced SNR. In this Letter, we have experimentally demonstrated the extension of the sensing range of Φ-OTDR to
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