Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A theoretical investigation of Brillouin optical-fiber time-domain analysis (BOTDA) is described. BOTDA uses Brillouin interaction in optical fibers to analyze the attenuation characteristics of the optical fibers nondestructively. The dynamic range performance of BOTDA is approximately 10-dB greater than that of conventional optical time-domain reflectometry. > <s> BIB001 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> This paper reviews the developments of a distributed strain and temperature sensing technique that uses Brillouin scattering in single-mode optical fibers. This technique is based on strain- and temperature-induced changes in the Brillouin frequency shift. Several approaches for measuring the weak Brillouin line are compared. > <s> BIB002 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> It is shown experimentally that the ratio of the intensities of Rayleigh and Brillouin backscattered light (Landau Placzek ratio) in an optical fibre has a temperature dependence which may be used for the basis of a distributed temperature sensor. This result, combined with the known frequency dependence of the Brillouin backscattering on temperature and strain, indicates that spontaneous Brillouin backscatter may be used for the unique determination of either temperature or strain in a distributed optical fibre sensing system. <s> BIB003 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We perform numerical simulations on a model describing a Brillouin-based temperature and strain sensor, testing its response when it is probed with relatively short pulses. Experimental results were recently published [e.g., Opt. Lett. 24, 510 (1999)] that showed a broadening of the Brillouin loss curve when the probe pulse duration is reduced, followed by a sudden and rather surprising reduction of the linewidth when the pulse duration gets shorter than the acoustic relaxation time. Our study reveals the processes responsible for this behavior. We give a clear physical insight into the problem, allowing us to define the best experimental conditions required for one to take the advantage of this effect. <s> BIB004 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> Methods are provided for analyzing multilayer structures. The multilayer structure is electrolytically anodized at constant current through a plurality of layers of the structure over a predetermined surface area. During the anodization, the change of the anodization voltage or a time derivative thereof as a function of time is monitored as the anodization through the layers occurs to obtain data. The data may be analyzed in several ways to determine whether degradation of the multilayer structure has occurred, to determine the number of layers present in the multilayer structure and to determine the layer thickness of the multilayer structure. The disclosed methods are accurate, inexpensive and rapid. <s> BIB005 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We present a novel technique for performing single-ended distributed fiber temperature measurements by use of microwave heterodyne detection of spontaneous Brillouin scattering. Brillouin frequency-shift measurements were obtained for a sensing length of 57 km, with a spatial resolution of 20 m. The rms error in frequency measurements at the far end of the sensing fiber was less than 3 MHz, and the overall frequency dependence on temperature was 1.07±0.06 MHz/K. <s> BIB006 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We investigated numerically the relationship between the temporal shape of an optical pulse launched into an optical fiber and the power spectrum of the Brillouin backscattered light it produces. We analyzed the measurement error of the peak-power frequency of the Brillouin backscattered light power spectra obtained from the launched light with various pulse shapes. In this investigation and analysis, we characterized the pulse shape by the width, leading-trailing time, and steepness. Regardless of the launched pulse shape, the peak-power frequency-measurement error increases as the pulse width shortens. For identical launched pulse widths, a triangular pulse generates the Brillouin backscattered-light power spectrum with both the narrowest profile and the largest peak power, and consequently provides the minimum error when we measure the peak-power frequency. This shows that a temporally triangular pulse is the best for the launched light. <s> BIB007 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We demonstrate enhanced performance of a single-ended spontaneous-Brillouin-intensity-based distributed-temperature sensor with a sensing length of 50 km and a spatial resolution of 15 m by use of Raman amplification of the probe pulse within the sensing fiber. The Raman amplification was achieved with a copropagating pump pulse at 1450 nm. The standard deviation error of the temperature resolution was 1 degree C at the front end and increased to less than 13 degrees C at 50 km with Raman pulse amplification. <s> BIB008 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> This paper presents a distributed fiber optic Brillouin scattering investigation, which involves measuring the Brillouin intensity versus time for real-time temperature measurements. This can be applied in practical fire detection schemes through the use of a threshold temperature that corresponds to the maximum Brillouin intensity. In the experiments, a 11 km fiber length was interrogated by 20 ns pulses (2 m spatial resolution) to monitor the temperature variations in a 10 m fiber region. The corresponding theory has confirmed the suitability of real-time intensity monitoring for temperature measurement. <s> BIB009 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A recently proposed method of measuring the two Brillouin frequencies in a multicompositional fiber core for unambiguously resolving temperature and strain in a distributed sensor is compared with the previously established technique of measuring the intensity and frequency of the single Brillouin peak in a standard single-mode fiber. <s> BIB010 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We report the longest sensing range of a distributed temperature change measurement along an optical sensing fibre with single-ended access. The technique is based on spontaneous Brillouin scattering and microwave heterodyne detection. The Brillouin frequency shift was used to obtain the distributed temperature change at a range of 100 km with a temperature error of less than 8 °C, and a spatial resolution of 50 m. <s> BIB011 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> The performance of distributed fiber sensors based on spontaneous Brillouin scattering is largely determined by the peak power governed by nonlinear thresholds that can be launched into the sensing fiber. Our investigations show that, in long-range (>20-km) sensors that use a standard single-mode fiber operating at 1.5 µm, modulation instability can limit the acceptable pulse power to below 100 mW. Using a nonzero dispersion-shifted fiber with negative dispersion we can avoid this problem and obtain a ninefold increase in launched power. <s> BIB012 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We report an extended range distributed temperature sensor based on coherent detection of the frequency shift of the spontaneous Brillouin backscatter combined with Raman amplification. The Raman amplification was achieved within the sensing fibre using either co- or counter-propagating Raman pump with respect to the probe pulse and experiments were conducted to investigate the optimum pump and probe power combination. With a co-propagating Raman pump a temperature resolution of 0.8C was achieved at a sensing range of 150km with 50m spatial resolution. <s> BIB013 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A novel dark-pulse-based technique has been used for the first time in a Brillouin scattering-based distributed fiber sensor. Experimentally obtained Brillouin spectra demonstrate that the dark-pulse configuration is as capable of strain and temperature measurement as conventional pulse-based systems but at much higher spatial resolution. A spatial resolution of 50 mm is reported with a strain measurement accuracy of 6 /spl mu//spl epsiv/ on a 100-m sensing fiber. <s> BIB014 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> Distributed fiber sensing based on Brillouin gain scattering (BGS) principle is a useful way to develop devices capable to measure temperature or/and strain in optical fibers. New effects or technologies that could achieve a larger distance and/or a better spatial resolution are a topic of special interest in this fiber sensing area. The influence of the probe-pulse shape in the interaction between the pulsed light and the continuous wave laser in a pump-probe system is presented. The purpose of this study is to improve the spatial resolution of the measurement without losing stability in the BGS. Also it is showed how the backscattering Brillouin gain is affected by inducing variations on the final value of the BGS intensity; this effect is illustrated by using an experimental set up based on the Brillouin optical time-domain analysis (BOTDA). Theoretical analysis of the probe pulse in the Brillouin shift and intensity value using triangular, sinc and saw tooth shapes around the medium phonon life time (~10ns) are presented; as well as the experimental results and possible applications are explained. <s> BIB015 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We propose Brillouin optical correlation-domain reflectometry (BOCDR), which can measure the strain and/or ::: temperature distribution along an optical fiber by controlling the interference of continuous lightwaves. In pulse-based ::: conventional Brillouin optical time-domain reflectometry (BOTDR), it is difficult to achieve a spatial resolution less than ::: 1 m in principle, and the measurement time is as long as 5-10 minutes. On the contrary, the continuous-wave-based ::: BOCDR can exceed the limit of 1-m resolution, and realize much faster measurement and random access to measuring ::: position. A 40-cm spatial resolution was experimentally demonstrated with 50-Hz sampling rate. <s> BIB016 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We discuss the effect of Fresnel reflection on BOTDR measurement. We show that the theoretical detection sensitivity with respect to Brillouin scattering is limited by the pulse modulation effect and phase noise of the light source that is incorporated into the Brillouin spectrum by way of the Fresnel reflection, when the Brillouin sensor is located near the Fresnel reflection point. We describe the detection sensitivity in the presence of Fresnel reflection in terms of the minimum detectable sensor length. In regard to the pulse modulation effect, we find that when the pulsewidth is short, the noise in the Brillouin spectrum is large and the detection sensitivity deteriorates. We derive quantitative equations and confirm the equations experimentally. When the pulsewidth is 10 ns with a 1-MHz linewidth laser, the boundary Fresnel reflection determining whether or not we can measure a 1-m sensor fiber near the Fresnel reflection is about 30 dB lower than that for a 100 ns pulsewidth. A 1-m sensor fiber near the Fresnel reflection could be measured with a 10-ns pulse only when the Fresnel reflection was very low at about . Moreover, the effect of the laser linewidth is also discussed analytically. The analysis suggests that a laser with a narrower linewidth will provide better sensitivity. <s> BIB017 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> The influence of the frequency chirp inherent to the generation of a pump pulse by a Mach-Zehnder intensity modulator is presented in the context of a distributed Brillouin sensor configuration. This frequency chirp depends on the pulse shape and its impact is related to the relative importance of the rising and falling edges with respect to the total pulse width. This results in a biased evaluation of the Brillouin frequency in regions where an abrupt change of this Brillouin frequency is observed. <s> BIB018 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A simple physical description of the nonlinear optical interaction based on Brillouin echoes is presented. This technique makes potentially possible distributed Brillouin sensing down to centimeter spatial resolution while preserving the narrowband feature of the natural Brillouin gain spectrum. Experimental conditions for the generation of Brillouin echoes are described and demonstrations of distributed measurements using a 1 ns (10 cm) pulse are presented. <s> BIB019 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A differential pulse-width pair Brillouin optical time domain analysis (DPP-BOTDA) for centimeter spatial resolution sensing using meter equivalent pulses is proposed. This scheme uses the time domain waveform subtraction at the same scanned Brillouin frequency obtained from pulse lights with different pulse-widths (e.g. 50ns and 49ns) to form the differential Brillouin gain spectrum (BGS) at each fiber location. The spatial resolution is defined by the average of the rise and fall time equivalent fiber length for a small stress section rather than the pulse-width difference equivalent length. The spatial resolution of 0.18m for the 50/49ns pulse pair and 0.15m for 20/19ns pulse pair over 1km sensing length with Brillouin frequency shift accuracy of 2.6MHz are demonstrated. <s> BIB020 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A theoretical and experimental analysis of optical pulse coding techniques applied to distributed optical fiber temperature sensors based on spontaneous Brillouin scattering using the Landau-Placzek ratio (LPR) scheme is presented, quantifying in particular the impact of Simplex coding on stimulated Brillouin and Raman power thresholds. The signal-to-noise ratio (SNR) enhancement and temperature resolution improvement provided by coding are also characterized. Experimental results confirm that, differently from Raman-based sensors, pulse coding affects the stimulated Brillouin threshold, resulting in lower optimal input power levels; these features allow one to achieve high sensing performance avoiding the use of high peak power pulses. <s> BIB021 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> For the first time to our knowledge, distributed impact waves due to the highway traffic on concrete slabs reinforced with FRP bars are monitored in real time using stimulated Brillouin scattering. The impact wave is caused by the traffic passing on the highway pavement at high speed (>100 km h−1), which induced pressure on the concrete slabs, and in turn created a local birefringence change, leading to variation of the local state of polarization change (SOP). The pump and probe waves of the stimulated Brillouin scattering 'see' the SOP change and react with a decrease of the Brillouin gain or loss signal, when the pump and probe waves have the same input polarization state. The frequency difference between the pump and probe waves are locked at the static-strain-related Brillouin frequency. Optical fiber was embedded throughout the concrete pavement continuously reinforced with FRP bars in Highway 40 East, Montreal, Quebec to detect impact waves caused by cars and trucks passing on these pavements at a sampling rate of 10 kHz. A spatial resolution of 2 m was used over a sensing length of 300 m. <s> BIB022 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We newly develop a temporal gating scheme to enlarge the measurement range of Brillouin optical correlation-domain reflectometry (BOCDR) for a fiber-optic distributed strain measurement. In this scheme, the trade-off problem between the measurement range and the spatial resolution can be dissolved. 66-cm spatial resolution and 1-km measurement range were simultaneously obtained with 50-Hz sampling rate. <s> BIB023 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We present a technique for dynamic strain measurements in optical fibers based on the stimulated Brillouin scattering interaction between two counterpropagating optical pulses. The technique allows for a high sampling rate and permits to addressing dynamically and randomly the position at which vibration is measured. Preliminary experimental results carried out with a perturbation frequency up to 98 Hz demonstrate the validity of the proposed technique. <s> BIB024 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> As an optical fiber sensing technology based on detecting spontaneous Brillouin scattering, Brillouin Optical Time Domain Reflectometry (BOTDR) is attracting significant attention due to its immense potential applications in temperature or strain monitoring. To enhance the signal to noise ratio (SNR) of BOTDR system, a new BOTDR based on Hadamard sequence probe pulse is proposed. In the new BOTDR system, the probe pulses are constructed from the Hadamard matrix. The experiment results show that the SNR of the proposed system can be greatly enhanced, and it is verified that the sensing range of the sensor can be improved without reducing the spatial resolution. <s> BIB025 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> High spatial ( cm) and spectral ( MHz) resolution Brillouin sensing is realized with enhanced signal to noise ratio using a pre-activated acoustic field and an optical phase control over the interrogating pulse. Pre-activation of the acoustic field preserves the Brillouin natural linewidth and a differential gain technique extends the method to long ranges. Experimentally, fully resolved measurements of the Brillouin frequency shift of a 5 cm spot perturbation at the far end of a 5 km fiber have been performed with a frequency resolution of 3 MHz (2) , using a 500 ps (5 cm) phase shift pulse. <s> BIB026 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A novel BOTDA technique for distributed sensing of the Brillouin frequency in optical fibers with cm-order spatial resolution is proposed. The technique is based upon a simple modulation scheme, requiring only a single long pump pulse for acoustic excitation, and no subsequent interrogating pulse. Instead, the desired spatial mapping of the Brillouin response is extracted by taking the derivative of the probe signal. As a result, the spatial resolution is limited by the fall-time of the pump modulation, and the phenomena of secondary “echo” signals, typically appearing in BOTDA sensing methods based upon pre-excitation, is mitigated. Experimental demonstration of the detection of a Brillouin frequency variation significantly smaller than the natural Brillouin linewidth, with a 2cm spatial resolution, is presented. <s> BIB027 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> Classical configurations of Brillouin distributed optical fibre sensors show a spatial resolution limited by the phonon lifetime to 1 meter. Novel approaches can substantially overcome this limit through the prior activation of the acoustic wave. <s> BIB028 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We report a high-performance 25km Brillouin-loss-based distributed fiber sensor through optimizing system parameters. First, the Brillouin spectrum distortion and measurement error induced by the excess amplification on probe pulse are investigated, and the results indicate that a low continuous-wave pump power is essential to decrease the measurement error. Then an optimal pulse pair is determined through the differential Brillouin gain evolution along the entire sensing fiber in a differential pulse-width pair Brillouin optical time domain analysis. Using dispersion-shifted fiber to allow a high-power probe pulse, we realize a 25km sensing range with a spatial resolution of 30cm and a strain accuracy of ±20μe, which we believe is the best performance in such a length, to the best of our knowledge. <s> BIB029 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We propose a novel concept for hybrid networks that combine point and distributed Brillouin sensors in a cost-effective architecture that also deploys remote distributed Raman amplification to extend the sensing range. A 46-km proof-of-concept network is experimentally demonstrated integrating point vibration sensors based on fiber Bragg gratings and tapers with distributed temperature sensing along the network bus. In this network the use of Raman amplification to compensate branching and fiber losses provides a temperature resolution of 0.7°C and 13 m. Moreover, it was possible to obtain good optical signal to noise ratio in the measurements from the four point vibration sensors that were remotely multiplexed in the network. These low-cost intensity sensors are able to measure vibrations in the 0.1 to 50 Hz frequency range, which are important in the monitoring of large infrastructures such as pipelines. <s> BIB030 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A pre-excitation pulse technique in Brillouin optical time domain analysis (PP-BOTDA) for enhancement of the spatial resolution is shown. The technique here exposed is based on the pre-excitation of the stimulated Brillouin scattering and the subtraction of the Brillouin scattering due to the intensity dc level present in the optical pulse. A main optical pulse with 3ns of duration followed by a pulse of 40ns and half the intensity of the main one are used for obtaining 30cm of spatial resolution. The spatial range is 3600m on a standard single mode optical fiber. <s> BIB031 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A general analytic solution for Brillouin distributed sensing in optical fibers with sub-meter spatial resolution is obtained by solving the acoustical-optical coupled wave equations by a perturbation method. The Brillouin interaction of a triad of square pump pulses with a continuous signal is described, covering a wide range of pumping schemes. The model predicts how the acoustic wave, the signal amplitude and the optical gain spectral profile depend upon the pumping scheme. Sub-meter spatial resolution is demonstrated for bright-, dark- and π-shifted interrogating pump pulses, together with disturbing echo effects, and the results compare favorably with experimental data. This analytic solution is an excellent tool not only for optimizing the pumping scheme but also for post-processing the measured data to remove resolution degrading features. <s> BIB032 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> Abstract: We propose and demonstrate a long-range Brillouin Optical Time-Domain Analysis (BOTDA) distributed sensing system making use of an unbalanced double sideband probe formed by a Stokes and an anti-Stokes line. In particular, we show that for each measuring condition an optimal Stokes /anti-Stokes input power ratio exists, allowing a larger suppression of nonlocal effects induced by pump depletion. Experiments on a 50 km single-mode sensing fiber with 5 meters spatial resolution are reported. <s> BIB033 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> In this paper we perform an optimization of Brillouin optical time-domain analysis (BOTDA) sensors for achieving high resolution over long sensing ranges using distributed Raman amplification. By employing an optimized first-order bi-directional Raman amplification scheme and combining high-power fiber-Raman lasers and Fabry-Pérot lasers with low relative-intensity-noise (RIN), we demonstrate distributed sensing over 120 km of standard single-mode fiber with 2 meter spatial resolution and with a strain/temperature accuracy of 45με/2.1°C respectively. <s> BIB034 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We present a novel method, based on stimulated Brillouin scattering (SBS), for the simultaneous distributed measurement of fast strain variations along the entire length of the sensing fiber. A specially synthesized and adaptable probe wave is used to place the Brillouin interaction always on the slope of the local Brillouin gain spectrum, allowing a single pump pulse to sample fast strain variations along the full length of a fiber with an arbitrary distribution of the Brillouin frequency shift. In this early demonstration of the method, strain vibrations of a few hundred Hz are demonstrated, simultaneously measured on two different sections of an 85m long fiber, having different static Brillouin shifts and with a spatial resolution of 1.5m. <s> BIB035 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A technique to detect real time variations of temperature or strain in Brillouin based distributed fiber sensors is proposed and is investigated in this paper. The technique is based on anomaly detection methods such as the RX-algorithm. Detection and isolation of dynamic events from the static ones are demonstrated by a proper processing of the Brillouin gain values obtained by using a standard BOTDA system. Results also suggest that better signal to noise ratio, dynamic range and spatial resolution can be obtained. For a pump pulse of 5 ns the spatial resolution is enhanced, (from 0.541 m obtained by direct gain measurement, to 0.418 m obtained with the technique here exposed) since the analysis is concentrated in the variation of the Brillouin gain and not only on the averaging of the signal along the time. <s> BIB036 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We report a high-spatial-resolution and long-range distributed temperature sensor through optimizing differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA). In DPP-BOTDA, the differential signal suffers from a signal-to-noise ratio (SNR) reduction with respect to the original signals, and for a fixed pulse-width difference the SNR reduction increases with the pulse width. Through reducing the pulse width to a transient regime (near to or less than the phonon lifetime) to decrease the SNR reduction after the differential process, the optimized 8/8.2 ns pulse pair is applied to realize a 2 cm spatial resolution, where a pulse generator with a 150 ps fall-time is used to ensure the effective resolution of DPP-BOTDA. In the experiment, a 2 cm spatial-resolution hot-spot detection with a 2 °C temperature accuracy is demonstrated over a 2 km sensing fiber. <s> BIB037 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> We derive an analytical solution for the differential gain in Brillouin optical time-domain analysis. The found solution gives insight into the mechanisms leading to the high-spatial and spectral resolution observed in differential schemes. An experimental test has been carried out to validate the analytical solution. <s> BIB038 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A novel processing algorithm is demonstrated for time-domain high-resolution Brillouin schemes based on acoustic field preactivation. An iterative method is employed to correct the measured data from spurious effects associated with acoustic wave transients. Based on a model of the stimulated Brillouin scattering in the frequency domain, the spatial resolution degrading features are computed and subtracted from the acquired data. Experimental tests with 20-cm spatial resolution and dark-pulse pumping are reported. <s> BIB039 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> Sub-meter distributed optical fiber sensing based on Brillouin optical time-domain analysis with differential pulse-width pairs (DPP-BOTDA) is combined with the use of optical pre-amplification and pulse coding. In order to provide significant measurement SNR enhancement and to avoid distortions in the Brillouin gain spectrum due to acoustic-wave pre-excitation, the pulse width and duty cycle of Simplex coding based on return-to-zero pulses are optimized through simulations. In addition, the use of linear optical pre-amplification increases the receiver sensitivity and the overall dynamic range of DPP-BOTDA measurements. Experimental results demonstrate for first time a spatial resolution of ~25 cm over a 60 km standard single-mode fiber (equivalent to ~240 k discrete sensing points) with temperature resolution of 1.2°C and strain resolution of 24 με. <s> BIB040 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> Rayleigh, Brillouin and Raman scatterings in fibers result from the interaction of photons with local material characteristic features like density, temperature and strain. For example an acoustic/mechanical wave generates a dynamic density variation; such a variation may be affected by local temperature, strain, vibration and birefringence. By detecting changes in the amplitude, frequency and phase of light scattered along a fiber, one can realize a distributed fiber sensor for measuring localized temperature, strain, vibration and birefringence over lengths ranging from meters to one hundred kilometers. Such a measurement can be made in the time domain or frequency domain to resolve location information. With coherent detection of the scattered light one can observe changes in birefringence and beat length for fibers and devices. The progress on state of the art technology for sensing performance, in terms of spatial resolution and limitations on sensing length is reviewed. These distributed sensors can be used for disaster prevention in the civil structural monitoring of pipelines, bridges, dams and railroads. A sensor with centimeter spatial resolution and high precision measurement of temperature, strain, vibration and birefringence can find applications in aerospace smart structures, material processing, and the characterization of optical materials and devices. <s> BIB041 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> A new technique for the fast implementation of Brillouin Optical Time Domain Analysis (BOTDA) is proposed and demonstrated, carrying the classical BOTDA method to the dynamic sensing domain. By using a digital signal generator which enables fast switching among 100 scanning frequencies, we demonstrate a truly distributed and dynamic measurement of a 100 m long fiber with a sampling rate of ~10 kHz, limited only by the fiber length and the frequency granularity. With 10 averages the standard deviation of the measured strain was ~5 µε. <s> BIB042 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Brillouin Scattering Distributed Fiber Sensors: Quick Overview <s> PART ONE: PRELIMINARY OVERVIEW Introduction to Fibre Optic Sensing Technology (J.M. Lopez--Higuera) The Commercialisation of Fibre Optic Sensors (S.D. Crossley) PART TWO: FUNDAMENTALS OF PHOTONICS AND COMPONENTS FOR SENSING Light and Waveguiding (J.L. Arce--Diego) Optical Waveguides and their Manufacture (J. Zubia and M. Lomer) Passive Bulk Optical Components for Sensing (C. Gomez--Reino, V. Perez and C. Bao) Fibre and Integrated Optic Components for Sensing (R. Willsch and W. Ecke) Semiconductor Optical Sources for Sensing Technology (I. Esquivias and J. Arias) Photodetectors for Sensing (J.M. Lopez--Higuera and J. Madruga) Optical Amplifiers (M.A. Rebolledo and M. Lopez--Amo) Superfluorescent Fibre Optic Sources (J.M. Lopez--Higuera) PART THREE: PRINCIPLES AND TECHNIQUES FOR SENSING Transduction Techniques based on Intensity Modulation of Light (A.C. Garcia and J. Echevarria Cuenca) Interferometry and Polarimetry for Optical Sensing (DC. Jones) Gas Spectroscopy Techniques for Optical Fibre Sensors (B. Culshaw) Distributed Optical--fibre Sensing (A. Rogers) Principles of Laser Doppler Velocimetry (D.A. Jackson and C.N. Pannell) Fibre Gyroscope Principles (S. Merlo, M. Norgia and S. Donati) Fibre Grating Technology: Theory, Photosensitivity, Fabrication and Characterization (R. Khasyap and J.M. Lopez--Higuera) Fibre Bragg Grating Interrogation Techniques (J.L. Santos and W.N. MacPherson) Discrimination Techniques for Optical Sensors (J.D.C. Jones and W.N. MacPherson) Optical Reliability of Fibre Gratings (S. Kannan and P. Lemaire) Passive Fibre Optic Sensor Networks (A. Dandridge and C. Kirkendall) Active Fibre Optic Sensor Networks (S. Abad, M. Lopez--Amo and I.R. Matias) PART FOUR: APPLICATIONS Optical Fibre Gratings Applications (S.T. Vohra) Laser Doppler Velocimetry Applications (D.A. Jackson and C.N. Pannell) Phototonic Sensing Technology in Civil Engineering Applications (D. Inaudi) Applications of Optical Fibre Sensors for the Nuclear Power Industry (P. Ferdinand and S. Magne) Optical Fibre Current and Voltage Sensors for the Electric Power Industry (A.H. Rose and G.W. Day) Fibre Optic Gyroscope for Industrial Applications (T. Kumagai and H. Kajioka) Optical Fibre Sensors for Fly--By--Light Aircraft (M. Kobayashi and K. Toyama) Optical Fibre Sensing of Electrical Discharges and Plasmas (G. Woolsey) Fibre Optic Sensors for Oilfield Services (R.J. Schroeder, R.T. Ramos, T. Yamate and E. Udd) Fibre Optic Biosensors (C.A. Rowe--Taitt and F.S. Ligler) Biomedical Fibre Optic Sensors (F. Baldini and A.G. Mignani) Fibre Optic Sensors for Environmental Applications (G. Holst and B. Mizaikoff) The Optical Nose (D. Walt and S. Stitzel) A New Approach to optical Fibre Sensing Techniques based on the Sensory Systems of Living Bodies (J.A. Martin--Pereda and A.P. Gonzalez--Marcos) Acronyms. Index. <s> BIB043
A device or a system capable of detecting, measuring, and reproducing faithfully a particular physical or chemical variable (measurand) in the electrical domain may be recognized as a sensor or a sensor system BIB043 . If light is used in such sensors, and the measurand changes some of the light properties, the device is known as photonic or optical sensor. These changes on the light properties usually happen in the transducer part of the sensor. Within the photonic sensors, the fiber sensors are those who are made up with optical fibers and the technology around them BIB043 . Distributed fiber optic sensors are capable of detecting and measuring variables along a fiber that acts both as a distributed transducer and as an optical channel. Some examples of sensor fibers are shown in Figure 2 , in this case three fibers, one for strain sensing, one for temperature sensing, and another for simultaneous strain and temperature sensing, are pictured (picture courtesy of the Photonic Engineering Group of the University of Cantabria). As it was previously mentioned, Brillouin scattering is a phenomenon that strongly depends on the temperature in the medium, its entropy, and the material density. In most of the cases, these variables affect the frequency shift or the Brillouin spectrum linewidth. Thus, this dependence can be used for indirectly determining the influence of external or intrinsic variations in the medium where the scattering takes place. Additionally, the Brillouin shift process can be accurately (∼cm) localized along the fiber, by time domain or frequency correlation techniques, among others. Hence, Brillouin scattering of light in optical fibers can be used as a basis to develop accurate distributed optical sensors and optical devices. In these cases, the optical fiber constitutes the medium where the interaction takes place, acting at the same time as distributed transducer and optical channel. These sensors are able to measure the change of a specific parameter along the entire fiber transducer within a given certitude. Prior to talk about the sensor schemes and ultimate ranges, it is important to clarify the meaning of some key parameters commonly used to characterize the distributed fiber sensors, such as spatial resolution, dynamic range, and distance range. The minimum change in the measurement, meaningfully detectable by the measurement system, is the resolution. In an optical fiber the minimum distance between two step transitions of the measurand is the spatial resolution. The maximum cumulated one-way (or two-way) loss in the optical link between the interrogator and the measurement point that makes possible a measurement within a specified performance is the dynamic range, which is measured as the ratio of the difference between the extremes of the measurement range to the resolution (in dB). The fiber length over which the measurement can be performed within the stated uncertainty and spatial resolution is denoted as distance range. From a general point of view Brillouin sensors can be classified in two main kinds: spontaneous Brillouin sensors and stimulated Brillouin sensors. The methods where only the incident light is launched into the optical fiber and there is not any additional stimulus on the phonon generation are denoted as spontaneous Brillouin sensing configurations BIB013 BIB010 BIB008 BIB006 BIB011 . Coherent detection of spontaneous Brillouin scattering gives greater dynamic range and allows simultaneous measurements of temperature and strain. However, nonlinear effects that limit the input probe pulse power, the weakness of the Brillouin signal, and the two-way fiber loss (0.4 dB/km) lead to a rapid reduction in performance as the sensing ranges are extended beyond 50 km. Brillouin optical time domain reflectometry (BOTDR)  BIB025 , the Landau-Placzek ratio BIB003 method, and Brillouin optical correlation domain reflectometry (BOCDR) BIB016 are part of this classification. On the other hand, Stimulated Brillouin sensors are based in the additional stimulus on the phonon generation given by an additional incident light, that is, the Brillouin scattering is enhanced or amplified. BOTDR is a coherent detection method that uses a pulsed light. This light is launched into the optical fiber to generate spontaneous Brillouin scattering. As illustrated in Figure 3 , the backward light is measured with a coherent receiver by mixing the scattering signal with that from a local oscillator  BIB025 . Since the power in the backscattered signal is small, fiber attenuation can induce a negative effect on the quality of the measurement. To compensate this drawback a coherent detection is currently used. In Brillouin optical time domain reflectometry the backscattered light, which is generated by the pump light launched in a fiber, is combined with a local oscillator. Nonetheless its dynamic range decreases with the length of the fiber. More disadvantages on this method are the fact that the spatial resolution cannot be less than one meter, the frequency shift is simultaneously dependent on both the temperature and the longitudinal strain, and that is necessary to introduce electrical filtering to eliminate Rayleigh signal. Since the Brillouin frequency shift is a function of temperature and strain simultaneously, there is no way, uniquely measure each variable unless their effects can be separated. Nonetheless, the ratio between the intensities of the Rayleigh and Brillouin backscattered light (Landau-Placzek ratio (LPR)) is only dependent on temperature, and it can be used for obtaining a distributed temperature profile from a fiber regardless of the strain distribution BIB003 . The standard OTDR technique is used for determining the fiber loss profile as a function of position, later this is subtracted from the intensity measurements made with the BOTDR system and the intensity measurements are cleaned out of fiber attenuation and any form of fiber losses. The Brillouin shift on BOCDR is measured by controlling the interference of the continuous probe and pump waves within the optical fiber, and the position is determined by scanning along the fiber. This technique allows the spatial range to be below the one meter range along one kilometer of optical fiber BIB023 . Since BOCDR is not based on an optical pulse but on a continuous wave, long integration time for the reflected signal is not needed, hence it is faster than BOTDR. However, the number of sample points on the fiber is limited by the spatial resolution and the spatial range. An experimental setup is depicted in Figure 4 . BOTDA uses Brillouin gain (or loss) spectroscopy in which a pulsed-optical wave (pump) and a counterpropagating light (probe/Stokes), normally a continuous wave, are injected into an optical fiber; the two most frequently implementaed ways are shown in Figure 5 . When the frequency difference between the pulsed and the continuous light (Δν = ν − ν B ) is tuned to the Brillouin frequency ν B of the fiber, the continuous light is amplified through the stimulated Brillouin scattering process; that is, the increasing continuous light is measured as function of time, like with OTDR BIB001 ( Figure 6 ). The gathered signal using the BOTDA technique is higher than the Rayleigh backscattering power, for example, when they launched continuous light power into standard fibers which is around 0.1 mW, the backscattering can be a hundred times higher than the Rayleigh BIB002 . The pump and probe signals are launched into opposite ends of the fiber in order to enhance the scattering process. Thus the Brillouin gain spectrum is depicted by each value of the frequency difference, Δν, and is given by (2a) and (2b). Effects on the pump-probe technique have been widely studied, such as the Fresnel reflection BIB017 , electrooptical modulation effects BIB018 BIB004 , or pulse shape effects BIB015 BIB007 . In BOTDA based on pulse pumping, spatial resolution is limited to ∼1 m due to the finite phonon lifetime   . To overcome this constraint, several techniques that include variations on the probe and handling the pumping (or probe) intensity have been proposed. In the timedomain pumping schemes, the more significant methods are (1) prepumping  BIB031 , (2) differential pulse-width pairs (DPP-BOTDA) with or without phase shift BIB031 BIB019 BIB020 BIB026 , (3) dark pulses BIB014 (by these methods, cm or even mm scale spatial resolution has been demonstrated BIB037 BIB027 ), and (4) dynamic Brillouin gratings (DBGs) BIB005 . The mentioned methods achieve the goal of simultaneously high spatial and spectral resolution thanks to an acoustic field preactivated by a low intensity prepump pulse BIB031 BIB031 BIB019 BIB020 BIB026 BIB014 BIB037 BIB027 BIB005 BIB038 BIB028 or a continuous pump background BIB032 (see Figure 7 ). Even, when the pump is restored to a nonnull intensity after the pulse, it interacts with the decaying acoustic wave, producing "echoes" in the acquired signal BIB026 . Although the acoustic wave partially decays during the pulse duration, and a second attenuated response appears beyond the acoustic lifetime, the measurement of submeter perturbations on the fiber may be hidden. One way to suppress or attenuate the impact of this background Brillouin response is based on the subtraction of two Brillouin gain spectra obtained from pulses shifting in time or pulses with different widths BIB020 ; in this technique, the rising and the falling times of the pulses define the spatial resolution (see Figure 8 ). Another proposal to overcome the appearance of ghost peaks in the Brillouin gain spectra, which makes ambiguous the determination of the Brillouin frequency shift, is addressed to the numerical correction algorithms applied to BOTDA measurements, such as the correction method based on the deconvolution of the time traces, in which each trace is inverse filtered by use of the fiber response evaluated in the resonance case BIB026 or the iterative numerical approaching, using the analytical model of the stimulated Brillouin scattering in the frequency domain BIB039 . BOTDA systems require a uniform signal-to-noise ratio (SNR) to avoid uneven Brillouin gain along the optical fiber. Thus, a low Brillouin gain across a long sensing system is needed. One compromise of low gain system over a long sensing fiber causes an increment of the spatial resolution. In order to maintain the SNR high and a low Brillouin gain, the pump power should be kept minimized and the probe power maximized, but below the influence if the modulation instability BIB012 . The modulation instability is a process induced mostly by the amplitude and phase modulation part, which is included in BOTDA systems to generate the probe from the same coherent pump source. To avoid the effect of the modulation instability, a dispersion shifted fiber with normal dispersion can be used BIB029 , by coding the pulse BIB021 6 or an unbalanced double sideband probe BIB033 . The coding technique could reduce the spatial resolution, 25 cm or 2 m over a long distance range 60 km and 120 km, respectively BIB040 BIB034 . Additionally to these techniques, an interesting solution to enhance the spatial range without compromising the SNR ratio is addressed to the combination of techniques, or hybrid sensors, such as the Brillouin sensing with Raman amplification BIB030 to minimize the modulation instability. The measurement time required by a traditional BOTDA system is on the order of minutes BIB041 , time that also depends on the total length range and constitutes a serious drawback for dynamic detection in health monitoring structures. Dynamic variations of temperature or strain can be measured using Brillouin sensors by modifying the sensor technique or the system. The simplest method consists of the direct measurement of the intensity of the Brillouin peak gain/loss signal versus time, allowing the use of a BOTDA system BIB009 BIB035 , which drastically reduces the measurement time, but it has a threshold detection and requires high averaging sampling as the length range is enlarged. Similar to this idea is the technique that uses two counter-propagating optical pulses with a fixed optical frequency difference; this difference is set to a spectral distance from the local Brillouin frequency shift approximately equal to half the Brillouin gain spectrum linewidth, then any vibration-induced modulation of the local Brillouin frequency shift will be measured as an intensity variation of the Stokes pulse peak intensity BIB024 ; important changes in the setup are required in this technique and nonperiodic dynamic variations are not detected. Another method uses the polarization dependence of the Brillouin gain to avoid the need for scanning the pumpprobe frequency shift BIB022 . To reduce the measurement time, a technique based on a multitude of pump signals in the form of a frequency-domain comb in a complex BOTDA based system is also proposed  . Further to these techniques, a technique for measuring dynamic variations of temperature or strain based on the well-known anomaly detection method referred to as the RX algorithm to process the data gathered from a BOTDA system BIB036 was presented. This technique exploits the Brillouin sensing advantages without punishing complexity or performance limitations. Most recently a method to enhance the time for measuring the Brillouin frequency detection by generating a comb probe was proposed BIB042 ; this method uses an arbitrary waveform generator (AWG) in the intensity modulation stage.