4162
OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
Hybrid Raman/Brillouin-optical-time-domainanalysis-distributed optical fiber sensors based on cyclic pulse coding M. Taki,* A. Signorini, C. J. Oton, T. Nannipieri, and F. Di Pasquale Scuola Superiore Sant’Anna, Via G. Moruzzi 1, 56124 Pisa, Italy *Corresponding author: [email protected] Received August 1, 2013; revised September 13, 2013; accepted September 16, 2013; posted September 17, 2013 (Doc. ID 195069); published October 10, 2013 We experimentally demonstrate the use of cyclic pulse coding for distributed strain and temperature measurements in hybrid Raman/Brillouin optical time-domain analysis (BOTDA) optical fiber sensors. The highly integrated proposed solution effectively addresses the strain/temperature cross-sensitivity issue affecting standard BOTDA sensors, allowing for simultaneous meter-scale strain and temperature measurements over 10 km of standard single mode fiber using a single narrowband laser source only. © 2013 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (060.4370) Nonlinear optics, fibers; (290.5900) Scattering, stimulated Brillouin; (280.1350) Backscattering; (280.4788) Optical sensing and sensors. http://dx.doi.org/10.1364/OL.38.004162
Distributed fiber optic sensors, for simultaneous temperature and strain measurements, find several applications in a wide range of industrial sectors, ranging from security and safety, to energy and transportation. Most distributed fiber optic sensors for simultaneous strain and temperature measurements are currently based on Brillouin scattering measurements [1,2]. One of the main issues to be addressed in such systems is related to the strain and temperature cross-sensitivity of the Brillouin frequency shift (BFS), which is often overcome in practical systems by using two different sensing fibers, only one of them embedded into the structure to be monitored. Simultaneous strain and temperature sensing using a single optical fiber without cross-sensitivity is, however, attractive and it has already been implemented using either hybrid spontaneous Raman–Brillouin scattering detection [3], or by exploiting simultaneous detection of both Brillouin intensity and BFS [4]. Although both schemes allow, in principle, for distinguishing between strain and temperature, several drawbacks limit their practical applications. In particular, in hybrid optical-time-domain-reflectometry (OTDR)-based Raman–Brillouin schemes [3], the spontaneous Ramanscattered anti-Stokes line is strain independent and is used to estimate the fiber temperature as well as to correct for the temperature-dependence of the spontaneous BFS parameter. The noise in the antiStokes trace has been, however, identified as the major inaccuracy source in simultaneous temperature–strain measurements. On the other hand, the performance of distributed sensors based on spontaneous Brillouin backscatter coherent detection is strongly limited by the low strain sensitivity of the Brillouin intensity [4]. In order to overcome these performance limitations, in this Letter we propose and demonstrate, for the first time to our knowledge, a hybrid Raman/Brillouin optical time-domain analysis (BOTDA)-distributed optical fiber sensor by using cyclic pulse coding and a single narrowband laser source. 0146-9592/13/204162-04$15.00/0
Although BOTDA can provide extremely long sensing distances even with submeter spatial resolution [5], it is, however, well known that the ideal optical sources to be used in BOTDA sensors are not really effective for Raman-based temperature-sensing. The narrow Brillouin gain bandwidth (few tens of MHz in silica fibers) requires, in fact, the use of narrowband optical sources for accurate BFS measurement, such as distributed feedback lasers (DFB) or external cavity lasers (ECL). Such lasers are characterized by a typical output power below a few tens of milliwatt, which is not ideal for Ramanbased temperature sensing. In order to find a reliable and practical solution to the strain and temperature cross-sensitivity of the BFS, also overcoming the intrinsic limitation related to the required narrowband laser source characteristics, we propose here the use of cyclic pulse coding in hybrid Raman/ BOTDA-distributed optical fiber sensor systems. The proposed scheme, schematically shown in Fig. 1, has the great advantage of using a single narrowband laser source, which is exploited to simultaneously generate both Raman and Brillouin pumps as well as the CW probe for BOTDA measurements. The use of pulse coding allows us to improve the accuracy of both Raman antiStokes and BFS measurements, leading to a highly integrated solution for meter–scale strain and temperature
Fig. 1. Experimental setup. © 2013 Optical Society of America
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS
sensing. The proposed scheme uses one standard single mode optical fiber only, intrinsically allowing for temperature-independent strain measurements. More specifically, the use of cyclic simplex coding [6], provides additional benefits due to the possibility to perform Raman-based static temperature measurements and a simultaneous fast BOTDA implementation [7]. This important feature is related to the use of quasi-periodic bit sequences, which in principle allows for real-time decoding in less than one fiber transit time, opening the way to fast BOTDA measurements over long distances [7]. As the temperature variations in most applications are intrinsically slow or quasi-static, the proposed hybrid Raman/BOTDA sensor system allows us to perform static temperature measurements with a high number of averages to further improve the temperature resolution, in addition to the coding gain performance enhancement. The proposed technique can then play a key role not only for static structural health monitoring (SHM), but also for distributed temperature-independent strain and vibration measurements along large civil infrastructures at a few hertz or tens of hertz, with important applications for vibration-based monitoring [8]. Figure 1 shows a schematic structure of the hybrid Raman/BOTDA sensor system. Note that a single distributed-feedback (DFB) laser is used for both Raman and BOTDA measurements. The CW beam from the DFB laser at 1550 nm is split into two branches using a 3 dB optical coupler. In the Raman/Brillouin-pump branch, a high extinction ratio Mach–Zehnder modulator (MZM), which is controlled by a programmable waveform generator (WFG), and an Erbium-doped fiber amplifier (EDFA), are used to generate a 511 bit cyclic simplex code word to be launched into the fiber acting as a pump source for both Raman and BOTDA measurements. The used cyclic coding technique is based on the RZ pump signal modulation format with a 10 ns pulse width to obtain a 1 m spatial resolution. Note that a 511 bit code word implies a 200 ns bit duration, which ensures enough time for the acoustic wave to vanish between two consecutive pulses [9], avoiding the detrimental distortion of the acquired traces. In the second branch, a doublesideband probe signal is generated by intensitymodulating the CW light with a second MZM, which is driven by a microwave (RF) generator. A variable optical attenuator (VOA) is used to adjust the optical power launched into the fiber, while a polarization scrambler (PS) is employed to reduce polarization-induced gain oscillations. By sweeping the frequency of the RF signal, the optical frequency of the probe can be conveniently scanned to sample the BGS along the fiber. Probe and pump signals are launched in counter-propagating directions into ∼10 km of standard single-mode fiber. The coded optical pulses launched into the optical fiber with a peak power of 19 dBm are used for both Raman-and Brillouin-distributed temperature and strain measurements. The spontaneous Raman scattering (SpRS) is measured by OTDR techniques [10]. In particular, the intensity trace of the temperature-dependent anti-Stokes component is normalized by the temperatureindependent Stokes trace, so that the intensity variations due to the local fiber losses are canceled out
4 P AS λS hΔν ; ∝ exp pS λAS kB T
4163
(1)
where P S P AS and λS λAS are the power and wavelength for the Stokes (anti-Stokes) light respectively, h is Planck’s constant, Δν is the Raman frequency shift, and kB is the Boltzmann constant. As the intensity of the anti-Stokes SpRS is about 60 dB below the used peak power levels in the fiber, the backscattered optical power reaching the receiver is very low, constituting the main factor limiting the sensing performance of RDTS systems and consequently also of hybrid Raman/BOTDA sensors. Note that pulse coding and averaging can effectively overcome this limitation. With a 511 bit cyclic simplex code word the measured coding gain is ∼10 dB, allowing for significant performance enhancement in the temperature measurement [10], also considering that the quasi-static feature of the temperature change allows for rather long measurement times. In the proposed cyclic coding technique, the same bit sequence with a peak power of 19 dBm, continuously generated for Raman measurements by modulating the pump laser light, according to an M-bit binary pattern P fp0 ; …; pM−1 g, where pj 0 or 1 (with j 0; …, M-1), is also used to simultaneously perform the BOTDA measurements. The BGS is reconstructed along the sensing fiber by sweeping the frequency offset between the counter-propagating pulsed pump and the continuous wave signal (−22 dBm) around the BFS. The intensity variations of the CW probe signal (ΔI CW ) are measured at the near end of the fiber (z 0) as a function of time t and the optical frequency difference (Δν), obtaining ΔI CW t;Δν I CWL exp−αL Z vg t∕2Δz gB ξ; ΔνIpξ; Δνdξ − 1 ; × exp vg t∕2
(2) where I CWL is the input probe intensity at the far end of the fiber z L, α is the fiber loss coefficient, L is the fiber length, vg is the group velocity, Δz is the spatial resolution (proportional to the pulse length), and gB (ξ; Δν) and I P (ξ; Δν) are the Brillouin gain coefficient and the pump intensity at a distance ξ, respectively. The proposed coding technique is based on a linearization of Eq. (2), assuming that the intensity contrast of the measured CW probe signal (ΔI CW ) depends linearly on the pump intensity, according to Z ΔI CW t; Δν ∝
vg t∕2Δz vg t∕2
gB ξ; ΔνIpξ; Δνdξ:
(3)
Note that the linearity is actually one of the fundamental requirements to effectively apply coding techniques. The specific use of cyclic simplex pulse coding offers a theoretical p SNR enhancement which is equal to L 1∕2 L, where L is the code length. Real-time decoding can be performed in less than one transit time, using for example a field-programmable gate
OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
ΔνB C νB ε · Δε C νB T · ΔT;
(4)
where C νB ε (1 MHz∕20 με) and C νB T (1 MHz∕°C) are the strain and temperature coefficients for BFS in silica fibers, respectively. The temperature contribution in Eq. (4) is obtained from the Raman measurement based on Eq. (1), thus providing a temperature-independent estimation of the strain variation Δε. In order to investigate our hybrid sensor performance, 15 m of fiber at ∼10 km distance have been placed inside a temperature-controlled chamber (TCC) at 60°C, while the rest of the fiber was kept at room temperature (32°C). In the proposed sensor, both Raman and Brillouin measurements were performed simultaneously. In our experiment we are interested in overcoming the temperature/strain cross-sensitivity and since the fast BOTDA measurements have been already demonstrated by using cyclic codes [7], the BGS shown in Fig. 2 has been measured, with 10 k averages (40 s measurement time), with no distortion, along the whole length of the fiber. The BGS measured at the far end of the fiber is reported in Fig. 3, where we can note a BFS of 28 MHz in the heated 15 m. Note that the BFS induced in the BGS can be obtained due to a change in the temperature and/or the strain. However, in the proposed scheme the Raman measurement can be used to overcome the
Fig. 2. Decoded BGS along the sensing fiber.
Fig. 3. Decoded BGS near the fiber end. 0.003 Anti-Stokes Stokes
0.002 0.001 0 -0.001 0
Fig. 4.
2
4
6 Distance (km)
8
10
Stokes and anti-Stokes traces along the sensing fiber.
temperature/strain cross-sensitivity. Figure 4 shows the measured decoded Stokes/anti-Stokes traces, which can be used in Eq. (1) to extract the temperature profile along the sensing fiber. The temperature profile, estimated near the fiber end and reported in Fig. 5, clearly shows a 28°C temperature step in the heated zone. This 80 70 Temperature (°C)
array (FPGA), or an application-specific integrated circuit (ASIC), to effectively implement the decoding algorithm, without any significant overhead time [7]. These features open the way to fast BOTDA measurement systems operating over long sensing distances with meter-scale spatial resolutions [7] and without temperature–strain cross sensitivity. In this work, the benefits provided by cyclic simplex pulse coding to hybrid Raman/BOTDA fiber sensors have been experimentally investigated, achieving what we believe is the first demonstration of a 10 km long-range hybrid Raman/BOTDA sensor with 1 m spatial resolution on the same SMF fiber. As shown in Fig. 1, the coded probe signal and the backscattered pump and Raman lights at the receiver side are coupled into a 3-port optical circulator followed by a Raman filter which separates, with a high extinction ratio, the Raman/Stokes, Raman/anti-Stokes, and Rayleigh probe components into three different ports. A narrow band fiber Bragg grating (FBG, 6 GHz) is used on the Rayleigh port to filter out the residual suppressed carrier, the Brillouin/anti-Stokes line and the Rayleigh signal. A low noise InGaAs 125 MHz photo-receiver is connected to an oscilloscope which is controlled by a computer for BOTDA trace acquisition and decoding. The two receivers on the Raman/Stokes and Raman/ anti-Stokes ports are composed of InGaAs avalanche photo-detectors (125 MHz bandwidth) with small dark current (I D 7 nA) and low excess noise, a high-gain transimpedance amplification stage, and a custom FPGAcontrolled analog-to-digital converter (ADC) which is then connected to a computer for data-logging. The strain variation Δε is derived from the BFS parameter ΔνB , which linearly depends on both strain and temperature according to the formula
Amplitude (a.u.)
4164
60 50 40 30 20 10 10
10.05
10.1 Distance (km)
10.15
Fig. 5. Temperature profile calculated using Raman scattering.
Temperature resolution °C
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS
8
Calculated resolution Fitting
6 4 2 0 0
5 Distance (km)
10
Fig. 6. Temperature resolution along the sensing fiber from the Raman measurement.
4165
Finally, the strain resolution has been estimated from Eq. (4) by propagating the uncertainty of the Raman temperature (ΔT) and the uncertainty of the BFS (ΔνB ), assuming that both sources of noise are uncorrelated. Therefore, it has been calculated as the square root of the sum of the squares of the uncertainties. The result is shown in Fig. 7, where the worst resolution of 80 με was observed at the fiber end. In conclusion, we have experimentally demonstrated the use of cyclic pulse coding for hybrid Raman/BOTDAdistributed sensing. The proposed technique allows for temperature and strain measurement implementation with meter-scale spatial resolution over 10 km of standard single mode fiber.
160 Calculated strain resolution Fitting
Strain resolution (µε)
140 120 100 80 60 40 20 0 0
2.5
5 Distance (km)
7.5
10
Fig. 7. Strain resolution along the sensing fiber.
confirms that the 28-MHz-induced BFS in the BGS in Fig. 3 is due to the temperature change. The corresponding Raman temperature resolution has been calculated from the standard deviation of the measured temperature along a window of 20 m, and is shown in Fig. 6. The worst temperature resolution (at the fiber end) is calculated to be ∼3.4°C with 3 million averages, corresponding to a 5 min measurement time.
References 1. T. Horiguchi, K. Shimizu, T. Kurashima, M. Tateda, and Y. Koyamada, J. Lightwave Technol. 13, 1296 (1995). 2. M. Nikles, L. Thevenaz, and P. A. Robert, J. Lightwave Technol. 15, 1842 (1997). 3. M. N. Alahbabi, Y. T. Cho, and T. P. Newson, Opt. Lett. 30, 1276 (2005). 4. S. M. Maughan, H. H. Kee, and T. P. Newson, Meas. Sci. Technol. 12, 834 (2001). 5. H. Liang, W. Li, N. Linze, L. Chen, and X. Bao, Opt. Lett. 35, 1503 (2010). 6. M. Taki, F. Zaidi, I. Toccafondo, T. Nannipieri, A. Signorini, S. Faralli, and F. Di Pasquale, Opt. Lett. 38, 471 (2013). 7. M. Taki, Y. Muanenda, C. J. Oton, T. Nannipieri, A. Signorini, and F. Di Pasquale, Opt. Lett. 38, 2877 (2013). 8. J. M. W. Brownjohn, A. De Stefano, Y.-L. Xu, H. Wenzel, and A. Emin Aktan, J. Civ. Struct. Health Monit. 1, 79 (2011). 9. M. A. Soto, G. Bolognini, and F. Di Pasquale, Opt. Express 18, 14878 (2010). 10. M. A. Soto, T. Nannipieri, A. Signorini, A. Lazzeri, F. Baronti, R. Roncella, G. Bolognini, and F. Di Pasquale, Opt. Lett. 36, 2557 (2011).
OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
Hybrid Raman/Brillouin-optical-time-domainanalysis-distributed optical fiber sensors based on cyclic pulse coding M. Taki,* A. Signorini, C. J. Oton, T. Nannipieri, and F. Di Pasquale Scuola Superiore Sant’Anna, Via G. Moruzzi 1, 56124 Pisa, Italy *Corresponding author: [email protected] Received August 1, 2013; revised September 13, 2013; accepted September 16, 2013; posted September 17, 2013 (Doc. ID 195069); published October 10, 2013 We experimentally demonstrate the use of cyclic pulse coding for distributed strain and temperature measurements in hybrid Raman/Brillouin optical time-domain analysis (BOTDA) optical fiber sensors. The highly integrated proposed solution effectively addresses the strain/temperature cross-sensitivity issue affecting standard BOTDA sensors, allowing for simultaneous meter-scale strain and temperature measurements over 10 km of standard single mode fiber using a single narrowband laser source only. © 2013 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (060.4370) Nonlinear optics, fibers; (290.5900) Scattering, stimulated Brillouin; (280.1350) Backscattering; (280.4788) Optical sensing and sensors. http://dx.doi.org/10.1364/OL.38.004162
Distributed fiber optic sensors, for simultaneous temperature and strain measurements, find several applications in a wide range of industrial sectors, ranging from security and safety, to energy and transportation. Most distributed fiber optic sensors for simultaneous strain and temperature measurements are currently based on Brillouin scattering measurements [1,2]. One of the main issues to be addressed in such systems is related to the strain and temperature cross-sensitivity of the Brillouin frequency shift (BFS), which is often overcome in practical systems by using two different sensing fibers, only one of them embedded into the structure to be monitored. Simultaneous strain and temperature sensing using a single optical fiber without cross-sensitivity is, however, attractive and it has already been implemented using either hybrid spontaneous Raman–Brillouin scattering detection [3], or by exploiting simultaneous detection of both Brillouin intensity and BFS [4]. Although both schemes allow, in principle, for distinguishing between strain and temperature, several drawbacks limit their practical applications. In particular, in hybrid optical-time-domain-reflectometry (OTDR)-based Raman–Brillouin schemes [3], the spontaneous Ramanscattered anti-Stokes line is strain independent and is used to estimate the fiber temperature as well as to correct for the temperature-dependence of the spontaneous BFS parameter. The noise in the antiStokes trace has been, however, identified as the major inaccuracy source in simultaneous temperature–strain measurements. On the other hand, the performance of distributed sensors based on spontaneous Brillouin backscatter coherent detection is strongly limited by the low strain sensitivity of the Brillouin intensity [4]. In order to overcome these performance limitations, in this Letter we propose and demonstrate, for the first time to our knowledge, a hybrid Raman/Brillouin optical time-domain analysis (BOTDA)-distributed optical fiber sensor by using cyclic pulse coding and a single narrowband laser source. 0146-9592/13/204162-04$15.00/0
Although BOTDA can provide extremely long sensing distances even with submeter spatial resolution [5], it is, however, well known that the ideal optical sources to be used in BOTDA sensors are not really effective for Raman-based temperature-sensing. The narrow Brillouin gain bandwidth (few tens of MHz in silica fibers) requires, in fact, the use of narrowband optical sources for accurate BFS measurement, such as distributed feedback lasers (DFB) or external cavity lasers (ECL). Such lasers are characterized by a typical output power below a few tens of milliwatt, which is not ideal for Ramanbased temperature sensing. In order to find a reliable and practical solution to the strain and temperature cross-sensitivity of the BFS, also overcoming the intrinsic limitation related to the required narrowband laser source characteristics, we propose here the use of cyclic pulse coding in hybrid Raman/ BOTDA-distributed optical fiber sensor systems. The proposed scheme, schematically shown in Fig. 1, has the great advantage of using a single narrowband laser source, which is exploited to simultaneously generate both Raman and Brillouin pumps as well as the CW probe for BOTDA measurements. The use of pulse coding allows us to improve the accuracy of both Raman antiStokes and BFS measurements, leading to a highly integrated solution for meter–scale strain and temperature
Fig. 1. Experimental setup. © 2013 Optical Society of America
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS
sensing. The proposed scheme uses one standard single mode optical fiber only, intrinsically allowing for temperature-independent strain measurements. More specifically, the use of cyclic simplex coding [6], provides additional benefits due to the possibility to perform Raman-based static temperature measurements and a simultaneous fast BOTDA implementation [7]. This important feature is related to the use of quasi-periodic bit sequences, which in principle allows for real-time decoding in less than one fiber transit time, opening the way to fast BOTDA measurements over long distances [7]. As the temperature variations in most applications are intrinsically slow or quasi-static, the proposed hybrid Raman/BOTDA sensor system allows us to perform static temperature measurements with a high number of averages to further improve the temperature resolution, in addition to the coding gain performance enhancement. The proposed technique can then play a key role not only for static structural health monitoring (SHM), but also for distributed temperature-independent strain and vibration measurements along large civil infrastructures at a few hertz or tens of hertz, with important applications for vibration-based monitoring [8]. Figure 1 shows a schematic structure of the hybrid Raman/BOTDA sensor system. Note that a single distributed-feedback (DFB) laser is used for both Raman and BOTDA measurements. The CW beam from the DFB laser at 1550 nm is split into two branches using a 3 dB optical coupler. In the Raman/Brillouin-pump branch, a high extinction ratio Mach–Zehnder modulator (MZM), which is controlled by a programmable waveform generator (WFG), and an Erbium-doped fiber amplifier (EDFA), are used to generate a 511 bit cyclic simplex code word to be launched into the fiber acting as a pump source for both Raman and BOTDA measurements. The used cyclic coding technique is based on the RZ pump signal modulation format with a 10 ns pulse width to obtain a 1 m spatial resolution. Note that a 511 bit code word implies a 200 ns bit duration, which ensures enough time for the acoustic wave to vanish between two consecutive pulses [9], avoiding the detrimental distortion of the acquired traces. In the second branch, a doublesideband probe signal is generated by intensitymodulating the CW light with a second MZM, which is driven by a microwave (RF) generator. A variable optical attenuator (VOA) is used to adjust the optical power launched into the fiber, while a polarization scrambler (PS) is employed to reduce polarization-induced gain oscillations. By sweeping the frequency of the RF signal, the optical frequency of the probe can be conveniently scanned to sample the BGS along the fiber. Probe and pump signals are launched in counter-propagating directions into ∼10 km of standard single-mode fiber. The coded optical pulses launched into the optical fiber with a peak power of 19 dBm are used for both Raman-and Brillouin-distributed temperature and strain measurements. The spontaneous Raman scattering (SpRS) is measured by OTDR techniques [10]. In particular, the intensity trace of the temperature-dependent anti-Stokes component is normalized by the temperatureindependent Stokes trace, so that the intensity variations due to the local fiber losses are canceled out
4 P AS λS hΔν ; ∝ exp pS λAS kB T
4163
(1)
where P S P AS and λS λAS are the power and wavelength for the Stokes (anti-Stokes) light respectively, h is Planck’s constant, Δν is the Raman frequency shift, and kB is the Boltzmann constant. As the intensity of the anti-Stokes SpRS is about 60 dB below the used peak power levels in the fiber, the backscattered optical power reaching the receiver is very low, constituting the main factor limiting the sensing performance of RDTS systems and consequently also of hybrid Raman/BOTDA sensors. Note that pulse coding and averaging can effectively overcome this limitation. With a 511 bit cyclic simplex code word the measured coding gain is ∼10 dB, allowing for significant performance enhancement in the temperature measurement [10], also considering that the quasi-static feature of the temperature change allows for rather long measurement times. In the proposed cyclic coding technique, the same bit sequence with a peak power of 19 dBm, continuously generated for Raman measurements by modulating the pump laser light, according to an M-bit binary pattern P fp0 ; …; pM−1 g, where pj 0 or 1 (with j 0; …, M-1), is also used to simultaneously perform the BOTDA measurements. The BGS is reconstructed along the sensing fiber by sweeping the frequency offset between the counter-propagating pulsed pump and the continuous wave signal (−22 dBm) around the BFS. The intensity variations of the CW probe signal (ΔI CW ) are measured at the near end of the fiber (z 0) as a function of time t and the optical frequency difference (Δν), obtaining ΔI CW t;Δν I CWL exp−αL Z vg t∕2Δz gB ξ; ΔνIpξ; Δνdξ − 1 ; × exp vg t∕2
(2) where I CWL is the input probe intensity at the far end of the fiber z L, α is the fiber loss coefficient, L is the fiber length, vg is the group velocity, Δz is the spatial resolution (proportional to the pulse length), and gB (ξ; Δν) and I P (ξ; Δν) are the Brillouin gain coefficient and the pump intensity at a distance ξ, respectively. The proposed coding technique is based on a linearization of Eq. (2), assuming that the intensity contrast of the measured CW probe signal (ΔI CW ) depends linearly on the pump intensity, according to Z ΔI CW t; Δν ∝
vg t∕2Δz vg t∕2
gB ξ; ΔνIpξ; Δνdξ:
(3)
Note that the linearity is actually one of the fundamental requirements to effectively apply coding techniques. The specific use of cyclic simplex pulse coding offers a theoretical p SNR enhancement which is equal to L 1∕2 L, where L is the code length. Real-time decoding can be performed in less than one transit time, using for example a field-programmable gate
OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
ΔνB C νB ε · Δε C νB T · ΔT;
(4)
where C νB ε (1 MHz∕20 με) and C νB T (1 MHz∕°C) are the strain and temperature coefficients for BFS in silica fibers, respectively. The temperature contribution in Eq. (4) is obtained from the Raman measurement based on Eq. (1), thus providing a temperature-independent estimation of the strain variation Δε. In order to investigate our hybrid sensor performance, 15 m of fiber at ∼10 km distance have been placed inside a temperature-controlled chamber (TCC) at 60°C, while the rest of the fiber was kept at room temperature (32°C). In the proposed sensor, both Raman and Brillouin measurements were performed simultaneously. In our experiment we are interested in overcoming the temperature/strain cross-sensitivity and since the fast BOTDA measurements have been already demonstrated by using cyclic codes [7], the BGS shown in Fig. 2 has been measured, with 10 k averages (40 s measurement time), with no distortion, along the whole length of the fiber. The BGS measured at the far end of the fiber is reported in Fig. 3, where we can note a BFS of 28 MHz in the heated 15 m. Note that the BFS induced in the BGS can be obtained due to a change in the temperature and/or the strain. However, in the proposed scheme the Raman measurement can be used to overcome the
Fig. 2. Decoded BGS along the sensing fiber.
Fig. 3. Decoded BGS near the fiber end. 0.003 Anti-Stokes Stokes
0.002 0.001 0 -0.001 0
Fig. 4.
2
4
6 Distance (km)
8
10
Stokes and anti-Stokes traces along the sensing fiber.
temperature/strain cross-sensitivity. Figure 4 shows the measured decoded Stokes/anti-Stokes traces, which can be used in Eq. (1) to extract the temperature profile along the sensing fiber. The temperature profile, estimated near the fiber end and reported in Fig. 5, clearly shows a 28°C temperature step in the heated zone. This 80 70 Temperature (°C)
array (FPGA), or an application-specific integrated circuit (ASIC), to effectively implement the decoding algorithm, without any significant overhead time [7]. These features open the way to fast BOTDA measurement systems operating over long sensing distances with meter-scale spatial resolutions [7] and without temperature–strain cross sensitivity. In this work, the benefits provided by cyclic simplex pulse coding to hybrid Raman/BOTDA fiber sensors have been experimentally investigated, achieving what we believe is the first demonstration of a 10 km long-range hybrid Raman/BOTDA sensor with 1 m spatial resolution on the same SMF fiber. As shown in Fig. 1, the coded probe signal and the backscattered pump and Raman lights at the receiver side are coupled into a 3-port optical circulator followed by a Raman filter which separates, with a high extinction ratio, the Raman/Stokes, Raman/anti-Stokes, and Rayleigh probe components into three different ports. A narrow band fiber Bragg grating (FBG, 6 GHz) is used on the Rayleigh port to filter out the residual suppressed carrier, the Brillouin/anti-Stokes line and the Rayleigh signal. A low noise InGaAs 125 MHz photo-receiver is connected to an oscilloscope which is controlled by a computer for BOTDA trace acquisition and decoding. The two receivers on the Raman/Stokes and Raman/ anti-Stokes ports are composed of InGaAs avalanche photo-detectors (125 MHz bandwidth) with small dark current (I D 7 nA) and low excess noise, a high-gain transimpedance amplification stage, and a custom FPGAcontrolled analog-to-digital converter (ADC) which is then connected to a computer for data-logging. The strain variation Δε is derived from the BFS parameter ΔνB , which linearly depends on both strain and temperature according to the formula
Amplitude (a.u.)
4164
60 50 40 30 20 10 10
10.05
10.1 Distance (km)
10.15
Fig. 5. Temperature profile calculated using Raman scattering.
Temperature resolution °C
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS
8
Calculated resolution Fitting
6 4 2 0 0
5 Distance (km)
10
Fig. 6. Temperature resolution along the sensing fiber from the Raman measurement.
4165
Finally, the strain resolution has been estimated from Eq. (4) by propagating the uncertainty of the Raman temperature (ΔT) and the uncertainty of the BFS (ΔνB ), assuming that both sources of noise are uncorrelated. Therefore, it has been calculated as the square root of the sum of the squares of the uncertainties. The result is shown in Fig. 7, where the worst resolution of 80 με was observed at the fiber end. In conclusion, we have experimentally demonstrated the use of cyclic pulse coding for hybrid Raman/BOTDAdistributed sensing. The proposed technique allows for temperature and strain measurement implementation with meter-scale spatial resolution over 10 km of standard single mode fiber.
160 Calculated strain resolution Fitting
Strain resolution (µε)
140 120 100 80 60 40 20 0 0
2.5
5 Distance (km)
7.5
10
Fig. 7. Strain resolution along the sensing fiber.
confirms that the 28-MHz-induced BFS in the BGS in Fig. 3 is due to the temperature change. The corresponding Raman temperature resolution has been calculated from the standard deviation of the measured temperature along a window of 20 m, and is shown in Fig. 6. The worst temperature resolution (at the fiber end) is calculated to be ∼3.4°C with 3 million averages, corresponding to a 5 min measurement time.
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