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Raman/BOTDA Distributed Strain and Temperature Measurements. Yonas Seifu Muanenda, Mohammad Taki, Tiziano Nannipieri, Alessandro Signorini, Claudio ...

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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 34, NO. 2, JANUARY 15, 2016

Advanced Coding Techniques for Long-Range Raman/BOTDA Distributed Strain and Temperature Measurements Yonas Seifu Muanenda, Mohammad Taki, Tiziano Nannipieri, Alessandro Signorini, Claudio J. Oton, Farhan Zaidi, Iacopo Toccafondo, and Fabrizio Di Pasquale (Invited Paper)

Abstract—We provide an overview of the use of optical pulse coding to enhance the performances of long-range distributed optical fiber sensors for strain and temperature measurements. First, pulse coding techniques are introduced for distributed sensing, emphasizing in particular the advantages of advanced cyclic coding for fast distributed strain and simultaneous temperature measurements. Pulse coding techniques in Raman-based distributed temperature sensors employing multimode graded index fibers are introduced, also outlining their potential to achieve meter-scale long-distance sensing in standard single-mode fibers (SMF). The use of Simplex coding in Brillouin optical time-domain analysis (BOTDA) sensors is then described, showing accurate ultralongdistance sensing with meter-scale spatial resolution. Finally, the implementation of advanced cyclic coding in a hybrid Raman/BOTDA distributed sensor is presented, allowing fast long-distance strain and simultaneous temperature measurements over the same SMF, using a single narrowband laser with meter-scale spatial resolution. We show that advanced cyclic Simplex coding enables longdistance strain sensing with subsecond measurement times, also overcoming the typical temperature and strain cross-sensitivity issue affecting standard BOTDA sensors. Index Terms—BOTDA, Brillouin scattering, optical fiber sensors, Raman scattering, Simplex codes, source coding.

I. INTRODUCTION ISTRIBUTED optical fiber sensors are becoming increasingly attractive both in research and industrial applications. They offer highly competitive features not achievable using conventional electronic sensors and discrete multiplexed fiber optic sensors based on fiber Bragg gratings (FBGs). Among these, the most remarkable advantage is the capability to perform accurate distributed strain and/or temperature measurements over tens of kilometers of optical fiber, with meter-scale spatial resolution. Recent surveys show significant growth in the distributed fiber optic sensors market, in the oil and gas industry as well as in other strategic sectors, ranging from energy production and distribution to large infrastructure monitoring, industrial process control, transportation and military applications [1].

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Integration of different sensing physical mechanisms into the same sensor systems is becoming a key requirement for several fiber optic monitoring systems. For example, hybrid distributed acoustic and temperature measurement is becoming strategically important in the oil&gas industry for hydraulic fracture profiling, vertical seismic analysis, production flow monitoring as well as for pipeline integrity management and leakage detection [2], [3]. Moreover, hybrid Raman and Brillouin distributed sensing systems can find interesting applications in several strategic industrial fields [4]. On the other hand, hybrid Raman and FBG sensor systems [5] can be extremely attractive for temperature cross-sensitivity free quasi-distributed strain measurement using long FBG arrays [6]. As different sensing mechanisms typically require light sources with different bandwidth, power and coherence characteristics, it is typically difficult to develop highly integrated hybrid sensor systems operating on the same sensing fiber and using a common laser source and receiver block. A very interesting way to overcome this issue consists in using pulse coding, a widely used sensing technique which provides significant signal to noise ratio (SNR) improvement, allowing then to relax the required laser source specifications. In this paper we will focus our attention on distributed sensing; we will first review in Section II the use of standard pulse coding for optical time domain reflectometer (OTDR) [7] based distributed measurements, and then introduce advanced coding techniques, based on cyclic Simplex coding, outlining their potential impact for fast hybrid distributed sensing. In Sections III and IV, we specifically present the use of pulse coding and proposed systems in long-range Raman and BOTDA sensors, respectively. In Section V, we will show how cyclic pulse coding can be effectively implemented in hybrid Raman/BOTDA sensors for fast simultaneous measurement of strain and temperature using shared resources, while at the same time overcoming the cross-sensitivity issue in BOTDA sensors. II. CODING TECHNIQUES FOR DISTRIBUTED SENSING

Manuscript received May 29, 2015; revised September 8, 2015; accepted October 16, 2015. Date of publication October 25, 2015; date of current version February 5, 2016. (Corresponding author: Yonas Seifu Muanenda.) The authors are with the Scuola Superiore Sant’Anna, Pisa 56124, Italy (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected] it; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2015.2493438

Distributed measurement of physical parameters such as temperature and strain can be based on time and frequency domain techniques, which are characterized by their own benefits and limitations. In this paper, we will only consider time domain interrogation in which optical pump pulses are sent into the sensing fiber to induce spontaneous and/or stimulated scattering processes which can be exploited for sensing. In these

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MUANENDA et al.: ADVANCED CODING TECHNIQUES FOR LONG-RANGE RAMAN/BOTDA DISTRIBUTED STRAIN

kinds of systems, the spatial resolution of the measurement depends on the pulse width and there is a typical trade-off between sensing distance and spatial resolution which limits the sensing performance. Recently, various pulse coding techniques have been proposed to improve the performances of sensors based on OTDR, as well as to overcome the distance-resolution trade-off inherent in single pulse Raman DTS [8] and BOTDA sensors [9]. These coding techniques, which include standard correlation based, Simplex and advanced cyclic coding, are characterized by different features and advantages in terms of coding gain and dynamic performance. Standard pulse coding is based on the use of sequences of pulses that follow specific patterns according to particular codes. The optimum codes known for distributed optical fiber sensing are sequences based on Simplex codes [10], [44] and complementary-correlation Golay codes [7]. Those are linear codes in which the measured coded traces correspond to the linear combination of single-pulse traces, each of them delayed according to the bit pattern defined by the code itself. In order to retrieve the single-pulse fiber response, a linear decoding process has to be followed, which depends on the specific code used in the system. The single-pulse trace that is obtained after decoding is expected to have a significantly better SNR in comparison to the standard measurement method, with an SNR increment that is defined as the coding gain and depends on the number of bits used in the pulse sequence. Standard coding methods have two main disadvantages: the required sequences of short intensity-modulated pulses (∼10 ns per 1 m spatial resolution) at rather high modulation frequencies (∼100 MHz when using 10 ns pulses) are difficult to be implemented in practical systems, due to parasitic effects in high peak power directly modulated laser diodes and due the non-modulability of the fiber lasers. Also, as standard coding techniques require the use of several different sequences of pulses, they do not allow real-time decoding and then cannot be applied for fast distributed measurements, as required for example for distributed acoustic sensing [11] and dynamic strain measurements [12]. In order to overcome such limitations of standard coding, we have recently proposed an advanced technique, based on cyclic coding, which allows fast distributed measurement of strain [12] and vibrations [3] over several kilometers of standard singlemode fiber (SMF). The advanced coding technique is based on quasi-periodic Simplex bit sequences, allowing real-time decoding in less than one fiber transit time [13]. In particular the method consists in launching optical pulses into the fiber at a repetition rate higher than the inverse of the fiber round-trip time, so that the whole sensing fiber can be “filled” with a large number of intensity pulses. A suitable bit sequence is continuously generated by modulating the pump laser light, according to an M-bit binary pattern P = {p0 , . . . , pM −1 }, where pj = 0, 1 (with j = 0, . . . , M − 1), with a proper repetition rate to fill the fiber with M bits (spaced in M consecutive intervals). In such a scheme, the detected trace at a given sampling instant results from the sum of many contributions linked to the single-pulse fiber response and the used pulse pattern P [14]. The main advantages of advanced cyclic Simplex codes with respect to other standard coding schemes are their

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compatibility with low-repetition rate lasers, e.g., fiber lasers, and their dynamic capabilities. Note that real time decoding can be performed in less than one transit time, using for example multi-core architectures, in order to effectively implement the decoding algorithm, without any significant overhead time [15]. This feature is a key factor to implement fast distributed measurements over long distances; in fact the available coding gain can be effectively exploited to substantially reduce the required number of averages to guarantee acceptable SNR values, enabling long-distance sub-second measurement times. III. RAMAN BASED DISTRIBUTED TEMPERATURE SENSORS (RDTS) So far, RDTS have been intensively studied [16]–[18], due to their spatially-resolved measurement capabilities over long distances, and the consequent advantages over conventional electrical sensors. RDTS systems have found successful applications in a number of industrial areas including fire detection in tunnels and industrial plants, power cable monitoring in the energy distribution sector, down-hole well logging and pipeline leakage detection in the oil and gas industry. Since the silica fiber itself acts as the sensing element, RDTS can be safely and conveniently installed in all environments that are not compatible with traditional sensors, which exploit transfer of energy via electrical conductors, such as harsh and explosive environments. Most RDTS systems are based on the OTDR technique in which the temperature-dependent backscattered Raman anti-Stokes (AS) signal is acquired and then normalized to either the Raman Stokes (S) or Rayleigh backscattered component [18]–[20] to compensate for spurious losses along the sensing fiber. The most common solution, called single-ended RDTS, interrogates the fiber from one end only, and enables measurements across extended sensing distances [20]. However, single-ended RDTS systems are intrinsically affected even by slight variations of wavelength-dependent losses (WDL) [21] which can cause distortion in the measured temperature traces. WDLs are particularly relevant in harsh environments, such as in nuclear power plants or in geothermal wells, where the presence of ionizing radiation and/or high temperature and hydrogen concentration can strongly affect the optical fiber attenuation with time [21]–[23]. For all these reasons, a double-ended interrogation scheme (also called loop configuration technique) [17], in which both fiber ends are connected to the interrogation unit, is usually employed so that AS and S traces are alternately acquired in forward and backward directions and then properly averaged using geometric mean [21], leading to highly reliable systems for industrial applications. Raman DTS systems based on multi-mode graded-index fibers are finding widespread applications in strategic industrial sectors like transportation and energy. Multi-mode fibers (MMFs) are preferred to SMF ones due to the higher allowable peak power and higher backscattering coefficient, leading to longer sensing distances (up to a few tens of kilometers). However, intermodal dispersion in MMFs induces broadening of pulses along their propagation, ultimately limiting the achievable spatial resolution. Coding techniques have been developed

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Fig. 1. Scattering phenomena in SMF and their temperature and/or strain dependencies

to enhance the sensing capabilities on Raman DTS based on both MMFs and SMFs, providing cost-effective solutions with sensing distances up to several tens of kilometers and meter scale spatial resolutions. In this section, we will provide a brief description of the physical mechanisms in RDTS followed by a review of the use of standard and advanced pulse coding in Raman DTS systems. A. Raman DTS Basic Physical Mechanism Spontaneous Raman scattering (SpRS) is a temperaturedependent process caused by thermally driven molecular vibrations, in which two spectral components shifted from the incoming light are generated [24]. In particular, as schematically shown in Fig. 1, the intensity of the Raman upshifted frequency component (anti-Stokes light) exhibits a strong dependence on the temperature, while the downshifted frequency component (Stokes light) is only slightly temperature dependent [20]. The spontaneous anti-Stokes Raman scattering exhibits a quasi-linear relative sensitivity of 0.8%/K at around room temperature, a value that slightly changes to 1.2%/K at around −52 °C [17]. Fig. 1 also shows spontaneous Brillouin Stokes and Anti-Stokes components which are both strain and temperature dependent. In order to obtain the spatial information in RDTS systems, OTDR techniques are exploited [19]. This method is often called Raman-OTDR, and consists in launching short laser pulses into the sensing fiber and detecting the backscattered spontaneous Raman signal with high temporal resolution [17], [18]. As the WDL are not constant in time, and might change depending on external environmental conditions [17], [23] the ratio AS/S (or AS/Rayleigh) is expected to change during the sensor lifetime due to ageing or harsh environmental conditions, subsequently leading to significant errors in the temperature estimation [17], [25]. This makes the single-ended scheme, schematically described in Fig. 2(a), to be inappropriate for many applications since it would require a continuous calibration. To address the aforementioned problem, measurements can be performed in a double-ended configuration [21], as depicted in the scheme of Fig. 2(b). Even though the RDTS system requires access to both fiber-ends, this configuration has been demonstrated to provide a simple and effective solution to compensate differential attenuation issues [21]. It can be demonstrated that the use of a loop configuration cancels out

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Fig. 2. (a) Basic single-end interrogation scheme and (b) double-end (or loop) interrogation scheme in ODTR-based Raman DTS

all loss factors dependent on the fiber position z, leading to a self-calibrated temperature measurement which is robust to loss variations occurring during the sensor’s lifetime. An additional internal calibration fiber spool can also be added to compensate for variation in the temperature sensitivity and consequent offset with respect to the real temperature [25]. B. Optical Pulse Coding in Raman DTS Systems One of the main limiting factors in Raman DTS systems is related to the low intensity level of the SpRS component, which lowers the intensity of the signal at the receiver leading, to measurements with low SNR and consequently to poor sensing performances. The use of optical pulse coding techniques has been proposed in the past to improve the sensing capabilities of RDTS systems operating over MMFs [26] and SMFs [14], [27], [28]. Considering the same number of acquired traces for the single pulse and the optical pulse coding based systems, the noise reduction in the decoded traces is determined by the coding gain parameter G, which in case of standard Simplex coding is related to the number of bits √ N used in the pulse pattern according to G = (N + 1)/(2 N ) [26]. A better SNR clearly leads to RDTS measurements with better temperature resolutions and longer sensing distances in comparison to conventional single-pulse systems. Even though optical pulse coding enhances the SNR of RDTS measurements when MMF are used [26], the method does not solve the limitation imposed by modal dispersion, which degrades the sensor performance in terms of spatial resolution at long sensing ranges. On the other hand, when RDTS are operating over SMFs the low backscattering levels and the low maximum usable peak power level before the onset of nonlinearities (approximately 1–4 W) strongly limit their performance. Pulse coding can thus be highly beneficial in reaching long sensing distances over standard SMF achieving high spatial resolution even at long distances [14], [27], [28]. C. Advanced Cyclic Pulse Coding for Long-Distance Raman DTS Systems Over Standard SMFs In order to achieve meter-scale spatial resolution over tens of kilometers long sensing fibers, intermodal dispersion must

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respectively. Fig. 4 shows a temperature resolution of ∼4 °C at 55 km with 5–min measurement time; note that a single–pulse measurement would require at least 80 min in order to achieve the same resolution [14]. IV. BOTDA BASED STRAIN AND TEMPERATURE SENSORS A. BOTDA Sensing Mechanism

Fig. 3. Experimental setup for an RDTS system over a 58 km SMF, using 1023 bit Simplex coding

Fig. 4. Temperature resolution versus distance with 5 and 10 min measurement times, when 1023 bit cyclic-Simplex coding is used.

be avoided and standard SMF must be used. In order to overcome the difficulties of standard pulse coding in modulating high peak power laser diodes and fiber lasers at high repetition rates, advanced cyclic Simplex codes, as introduced in Section II, have been theoretically proposed and experimentally validated in [8] and [14], respectively. The main advantage of advanced cyclic Simplex codes for RDTS applications with respect to other coding schemes is their compatibility with low-repetition rate lasers, e.g., fiber lasers, which makes easier the generation of short pulse sequences, with constant high peak power levels. As described in Section II, they are based on a quasi-periodic intensity modulation pattern of the laser light, requiring only an additional external modulator to be used as a fast chopper when direct pulsed laser modulation cannot be implemented, obtaining an SNR enhancement corresponding to the coding gain similar to that in standard Simplex codes. It is worth noting that this coding method can also allow RDTS systems to have the spatial resolution enhanced down to sub-meter scale using conventional time-domain techniques. A schematic of the implementation of an RDTS using advanced cyclic Simplex coding and the obtained temperature resolution using a 1023–bit codeword are shown in Figs. 3 and 4,

BOTDA sensors are based on the distributed temperature and strain dependence of the Brillouin frequency shift (BFS) of the fiber, as schematically shown in Fig. 1, allowing for high performance distributed temperature and strain sensing over tens of kilometers, with meter-scale spatial resolution [29], [30]. BOTDA sensors exploit stimulated Brillouin scattering (SBS) which is a nonlinear process in which a pump beam and a counter–propagating probe beam interact through an existing pressure wave in the fiber which induces gain or loss of the probe beam. Originally, BOTDA technique was employed for measuring optical fiber attenuation along a SMF, as an alternative to the conventional OTDR technique [31], with significant improved performance. However, after a detailed understanding of the temperature and strain dependence of BFS, BOTDA has been the subject of intense research, both in academia and industry, leading to BOTDA-based schemes attaining extended sensing distances and improved measurement performances [32]–[34]. In BOTDA sensors, a continuous wave (CW) probe and a modulated pump are sent along the sensing fiber in counterpropagating directions in order to induce SBS and reconstruct the Brillouin gain/loss spectrum along the fiber. The BFS, i.e., the frequency difference between modulated pump and CW probe at which the maximum gain/loss occurs, can then be estimated. As the BFS is linearly dependent on the applied strain and temperature, it is then possible to reconstruct the strain and/or temperature distribution along the sensing fiber [30]. This position-dependent gain/loss is measured by a classical time-of-flight method detecting the received CW probe signal in time domain. If the probe wave is at the Stokes frequency, then energy flows from the pump to the Stokes wave providing Brillouin gain to the detected CW wave. If the probe wave is at the anti-Stokes frequency, it gives energy to the pump wave and the detected CW signal experiences a Brillouin loss [33]. Like all OTDR-based sensors, BOTDA sensors are affected by a trade-off between sensing distance and spatial resolution [9], with additional limitations due to acoustic phonon lifetime, nonlocal effects, modulation instability and self-phase modulation [34]–[37]. In the following section, we will investigate the use of pulse coding in standard and fast BOTDA sensors. B. BOTDA Sensors Using Optical Pulse Coding A state-of-the-art experimental setup used to evaluate the impact of optical pulse coding on a BOTDA sensor is shown in Fig. 5, where a single source is used to generate both coded pump pulses and probe signals. Fig. 6 compares the normalized BOTDA traces measured at the maximum Brillouin gain frequency of 10.986 GHz by using Simplex coding and conventional BOTDA schemes.

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Fig. 5. Coded BOTDA sensor scheme; PC: polarization controller, PS: polarization scrambler, VOA: variable optical attenuator, MZM: Mach–Zehnder modulator.

Fig. 6.

Comparison of BOTDA traces for Simplex coding and single pulse

The far fiber-end appears clearly visible using 511-bit Simplex coding, obtaining ∼5 dB of SNR at 50-km distance, while similar SNR is obtained only at 10 km distance in the conventional single pulse scheme. The significant 40-km distance enhancement is fully observed and is in full agreement with theoretical predictions [9]. Note that pulse coding combined with distributed amplification has proven to be an ultimate solution for sensing range enhancement without inserting active devices into the sensing loop, while ensuring high spatial resolution [39], [40]. Even though the sensing capabilities provided by BOTDA systems are very attractive in many applications, the relatively long measurement time makes this technology mainly suitable for static measurements, limiting its potential range of applications. In order to obtain the Brillouin gain spectrum (BGS) with a good accuracy, a scan of at least 100 MHz bandwidth for the probe frequency with a step of ∼2 MHz is typically required. Unfortunately, in normal operating conditions the level of Brillouin amplification is very low, and many traces have to be time-averaged to reduce the impact of Gaussian, zero-mean noise, for each of the ∼50 scanned frequencies [41]. This leads to a slow measurement process, which is typically of the order of a few minutes, limiting the system capabilities to static sensing only. For many application fields, such as for industrial plant monitoring, the distributed information provided by BOTDA sensing is crucial for static strain and temperature monitoring.

However, distributed or discrete dynamic strain measurements would be also required for structural health monitoring of civil and aerospace structures. Various techniques have been proposed to achieve distributed dynamic measurement in BOTDA systems. For example, Peled et al. presented two advanced methods, to allow dynamic Brillouin distributed sensing over hundreds of meters sensing range and sampling rates of tens hertz [42]. However, in case of long range distributed sensing, both techniques tend to become quasi-static due to the rather long optical fiber transit times. On the other hand, Bernini et al. proposed a slope assisted Brillouin based technique capable of dynamic strain measurements [43], which exploits the SBS interaction between two counterpropagating optical pulses and permits very high sampling frequencies to be achieved at specific sensing locations identified by the relative pulses delay. However, the proposed technique suffers from two limitations: first, it loses the distributed measurement capability; secondly, it is affected by a limited dynamic range intrinsic in the slope-assisted technique. To overcome the above drawbacks, we have used the advanced coding technique, based on Cyclic Simplex coding and introduced in section II, to perform accurate BOTDA measurements over several kilometers of standard SMF with sub-second measurement times and meter-scale spatial resolutions [12]. Real time decoding [15] can be performed in less than one transit time, providing a reduction factor in the measurement time, compared to standard BOTDA, which scales as the square of the coding gain in linear units. These features open the way to fast BOTDA sensors for vibration based monitoring of large civil infrastructures, operating over long sensing distances with meter-scale spatial resolutions. We have shown that, using cyclic pulse coding enables strain sensing with short measurement time, where the number of averages can be reduced by 100 times with respect to standard single pulse BOTDA systems; the BGS can then be computed in fraction of second, leading to 100-fold improvement in the measurement speed [12]. Fig. 7 shows the trend of the measurement time versus the codeword length. For a standard BOTDA, at least 10 k averages are needed to obtain acceptable performances over 10 km sensing distance, corresponding to a 40 s measurement time for a BFS measurement accuracy better than a few MHz. By exploiting the 10 dB coding gain which is experimentally achievable, the number of averages can be reduced by 100 times, leading to 0.4 s measurement time. Fig. 8 reports the measurement times versus code length, for different values of the fiber length and 40 scanned frequencies, assuming the switch time between two frequencies is negligible. It is evident that the proposed coding techniques can provide attractive dynamic performances over long sensing distances with meter scale spatial resolutions. V. HYBRID RAMAN/BOTDA SENSORS BASED ON CYCLIC PULSE CODING The suitability of BOTDA sensors for measuring strain or temperature over long distances with meter-scale spatial

MUANENDA et al.: ADVANCED CODING TECHNIQUES FOR LONG-RANGE RAMAN/BOTDA DISTRIBUTED STRAIN

Fig. 9.

Fig. 7. Theoretical-(blue line) and Experimental-(Red line) coding gains, and time required to reconstruct the BGS (green line) for each code length

Fig. 8. length.

BOTDA measurement times versus code length at different fiber

resolution has resulted in considerable attraction for their use in many industrial applications. However, such sensors exhibit cross-sensitivity between temperature and strain since the BFS is affected by both parameters. There are some situations in which it would be desirable to perform accurate, simultaneous measurement of both strain and temperature using hybrid systems and shared resources. This topic has been under investigation and some solutions exist to partially address it. For instance, sensors based on FBG and Raman/Brillouin scattering have been demonstrated in the last years [4], [45], addressing the cross-sensitivity only at discrete points where the FBGs are located. Simultaneously monitoring the BFS and the spontaneous Brillouin scattering (SpBS) intensity [46], and an implementation of a hybrid Raman/SpBS sensing scheme [47] have also been proposed. While the former is mainly limited by the low strain dependence of the SpBS intensity, the latter required very long averaging times and

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Experimental setup for hybrid Raman/BOTDA sensor.

coherent detection schemes due to the low signal levels achieved while using single pulses. An advanced coding scheme would be greatly beneficial in a hybrid Raman/BOTDA sensor, as it would allow decreasing the peak power needed by Raman sensing to values compatible with BOTDA requirements in SMFs, allowing at the same time fast strain measurement. In ref. [4] this scheme was implemented, which allowed an improvement in both Raman and Brillouin measurements, also overcoming the strain–temperature crosssensitivity affecting standard BOTDA sensors. The hybrid system shared the same laser, measuring fiber and codeword, and the same pump pulses were simultaneously used for the Raman and the BOTDA sensing. The Raman temperature profile was used in the BFS measurement to provide cross-sensitivity free strain distributions. The proposed system was experimentally validated by applying simultaneous strain and temperature variations along the same length of fiber, demonstrating successful quantitative measurements of both variables [48], [49]. The experimental setup of the Hybrid Raman/BOTDA sensor is shown in Fig. 9 with a small change in the receiver structure with respect to Fig. 5; a high isolation filter is added after the three-port optical circulator to separate the Raman Stokes/AntiStokes from the Rayleigh and Brillouin spectra. The Brillouin Stokes component is then selected using a narrowband FBG filter and an additional three-port optical circulator. Note that APDs are used for the Raman Stokes and AntiStokes components while a p-i-n photodiode is used for the Brillouin component. A section of the fiber is strained and heated simultaneously using a linear translation stage and an external thermojacket. The measurements of the BFS over a 80 m fiber section at the end of a 10 km SMF are reported in Fig. 10 at two different temperature values, clearly showing the BFS change due to temperature, with approximately 1 m spatial resolution. The maximum uncertainty of the BFS which occurs at the end of the fiber was found to be 1.7 MHz. The Raman based temperature profile at the end of the 10 km sensing fiber when the temperature was fixed at 42 °C over the strained fiber section, is reported in Fig. 11, which confirms the temperature increase above the room temperature at the thermojacket location. A temperature resolution of ∼2.6 °C was obtained at the end of the fiber. Finally, to determine the effectiveness of the hybrid technique in isolating the effect of temperature on the strain measurement,

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fiber optic sensors based on Raman and Brillouin scattering. We have proposed advanced solutions which address issues in conventional sensors including non-linearity and cross-sensitivity. Experimental results confirm the suitability of advanced coding techniques for the implementation of individual sensors and their inclusion in a hybrid scheme for simultaneous measurements of strain and temperature with shared resources and without temperature and strain cross-sensitivity.

Fig. 10. (a) BFS versus fiber length at 10 km for two different temperature values and (b) magnified view near strain location.

Fig. 11.

Temperature profile in the hybrid sensor using Raman technique.

Fig. 12.

Uncertainty of the strain resolution in the hybrid sensor.

the strain resolution across the whole fiber was estimated by propagating the uncertainties of the Raman temperature to that of the BFS change, assuming that the sources of noise in the two measurements are uncorrelated. Fig. 12 shows the result across the whole fiber, where a worst strain resolution of ∼62 με was obtained. VI. CONCLUSION In summary, we have presented the use of advanced pulse coding techniques for enhancing the performance of distributed

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MUANENDA et al.: ADVANCED CODING TECHNIQUES FOR LONG-RANGE RAMAN/BOTDA DISTRIBUTED STRAIN

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Yonas Seifu Muanenda received the B.Sc. degree in electrical engineering from Hawassa University, Hawassa, Ethiopia, in 2007, and the M.Sc. degree in computer science and networking, with emphasis on photonic technologies, from the University of Pisa and Scuola Superiore Sant’Anna, Pisa, Italy, in 2012. He is currently working toward the Ph.D. degree in emerging digital technologies, photonic technologies curriculum at the Institute of Information Communication and Perception Technologies, Scuola Superiore Sant’Anna, Pisa. His current research includes distributed optical fiber sensors based on Raman and Brillouin scattering, hybrid Brillouin/Raman sensors, and distributed acoustic sensing for temperature, strain, or vibration measurements with focus on advanced techniques for performance enhancement.

Mohammad Taki was born in Irkay, Lebanon, in 1984. He received the bachelor’s degree in information and telecommunication engineering from the Institut Universitaire de Technologie, Lebanese University, Beirut, Lebanon, in 2004, a second bachelor’s degree in electronic engineering and the master’s degree from the University of the Studies of Cagliari, Cagliari, Italy, in 2006 and 2008, respectively, and the Ph.D. degree from the Scuola Superiore di Studi Universitari e Perfezionamento Sant’Anna di Pisa, Pisa, Italy, in 2013. He is currently a Postdoctoral Research Fellow at Scuola Superiore di Studi Universitari e Perfezionamento Sant’Anna di Pisa, where he is involved in distributed and discrete fiber-optic sensors. He has authored and coauthored more than 20 scientific journals and conference papers in the areas of optical fiber sensors. He is a Scientific Reviewer of Optics Express, Optics Letters, Applied Optics, and Sensors.

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Tiziano Nannipieri received the B.S. (Laurea) and M.S. (Laurea Magistralis) degrees in telecommunications engineering from the University of Pisa, Pisa, Italy, in 2007 and 2008, respectively. He was a Consorzio Nazionale Interuniversitario per le Telecomunicazioni Scholarship Student with the National Laboratory of Photonic Networks, Pisa, from 2009 to 2010. He is currently at the Optical Fiber Sensors and Integrated Photonic Subsystems, Institute of Communication, Information and Perception Technologies, Scuola Superiore Sant’Anna, Pisa. His research interests include the areas of fiber-optic sensing, in particular, the design and development of distributed and discrete fiber-optic sensor systems for static and dynamic measurements based on spontaneous Raman and stimulated Brillouin scattering, and fiber Bragg gratings. He received the 2008 Best Italian Remote Sensing Thesis Award from the IEEE GRSS South Italy Chapter in 2009.

Alessandro Signorini received the M.S. degree in electronic engineering from the University of Pisa, Pisa, Italy. Since 2008, he has been at Scuola Superiore Sant’Anna, Pisa, where he is involved in the study, design, and development of a Raman-based distributed temperature sensor. His main research activities include fiber Bragg point sensors and distributed optical fiber sensors based on Brillouin and Raman scattering, in particular, the study of new measurement methods and new application techniques in the fields of safety, power, and environmental monitoring. He has authored and coauthored more than 20 international publications in journals, conferences, and patents in optical fiber sensing and their applications.

Claudio J. Oton received the Ph.D. degree from the University of La Laguna, San Crist´obal de La Laguna, Spain, in 2005. Then, he was with the Optoelectronics Research Center, Southampton, U.K., for four years where he worked on integrated optical amplifiers and lasers on silicon as a Marie Curie Postdoctoral Fellow. In 2009, he joined the Nanophotonics Research Center, Universidad Politecnica de Valencia, Valencia, Spain. Since 2012, he has been an Assistant Professor at TeCIP Institute, Scuola Superiore Sant’Anna, Pisa, Italy. He is an Author of more than 60 peer-reviewed scientific papers and several conference contributions. His current research interests include optical fiber sensors and silicon photonic integrated devices.

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Farhan Zaidi received two M.Sc. degrees, one in computer sciences and second in telecommunication engineering in 2006 and 2008, respectively, and the Ph.D. (summa cum laude) degree in 2014 for his research on dynamic interrogation of hybrid point/distributed fiber optic sensors. He is a Postdoctoral Researcher at the Scuola Superiore Sant’Anna, Pisa, Italy, having particular interests in the research and development of hybrid point and distributed fiber optic sensors. For more than one year, he has been working as a Research Scientist at BAM, Germany. He has been selected as a German DAAD and Erasmus scholar during his educational career. He is an Author/Coauthor of the OSA/IEEE journals, and currently, he is also serving as a Reviewer of the OSA journals.

Iacopo Toccafondo received the bachelors’ degree in physics from the University of Pisa, Pisa, Italy, in 2009, and the M.S. degree in information and communication technologies from the Electronics and Telecommunications Engineering Faculty, Technical University of Poznan, Poznan, Poland, in 2011. Since November 2011, he has been working toward the PhD degree at the TeCIP Institute, Scuola Superiore Sant’Anna, Pisa, and has been working on distributed and discrete optical fiber sensors for temperature and strain measurements. Also, he is currently a doctoral student at CERN where he is studying the effects of ionizing radiations on optical fibers and optical fiber dosimetry. He has published several papers both in scientific journals and conferences in the field of optical fiber sensors.

Fabrizio Di Pasquale received the degree in electronic engineering from the University of Bologna, Bologna, Italy, in 1989, and the Ph.D. degree in information technology from the University of Parma, Parma, Italy, in 1993. From 1993 to 1998, he was with the Department of Electrical and Electronic Engineering, University College London, London, U.K., as a Research Fellow, working on optical amplifiers, WDM optical communication systems, and liquid crystal displays. After two years with Pirelli Cavi e Sistemi and two years with Cisco Photonics Italy, he is currently a Full Professor of telecommunications at the Scuola Superiore Sant’Anna, Pisa, Italy. His current research interests include optical fiber sensors, silicon photonics, optical amplifiers, and WDM transmission systems and networks. He has filed 20 international patents and he is an Author and Coauthor of more than 180 scientific journals and conference papers. He is on the Board of Reviewers of the IEEE PHOTONICS TECHNOLOGY LETTERS, IEEE/OSA JOURNAL OF LIGHTWAVE TECHNOLOGY, IEEE SENSORS JOURNAL, Optics Communications, Optics Express, and Optics Letters.

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