Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> It is demonstrated that stimulated Brillouin scattering (SBS) occurring in the core of an optical fibre may be used in the construction of a distributed temperature sensor. Initial experiments with a length of fibre have shown that the sensitivity of the Stokes frequency to temperature is approximately 5.5MHz/°C and that different Stokes frequencies may be observed according to the temperature distribution along the fibre. <s> BIB001 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> Temperature distribution in silica single-mode optical fibers, from −30 to +60°C, is successfully measured by using Brillouin optical-fiber time-domain analysis. A temperature measurement accuracy of 3°C with a spatial resolution of 100 m is attained over a fiber length of 1.2 km. <s> BIB002 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <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> BIB003 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> A new sensing technique for the distributed measurement of temperature and strain, based on Brillouin optical frequency-domain analysis, is presented. Theoretical investigations and first experimental results of distributed measurements demonstrate the feasibility of this new concept. <s> BIB004 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> A novel method for Brillouin gain spectrum measurements in optical fibers is presented. It is based on the pump and probe technique with the specificity to use a single laser source together with an external modulator to generate the interacting lightwaves. The high accuracy and inherent stability of the technique makes it suitable for calibration and reference measurements. Different fibers with different refractive index profiles have been tested and characterized. The problem of the evolution of the polarization of the interacting waves is addressed in the article and a polarization insensitive determination of the actual Brillouin gain coefficient is made possible through two successive measurements with different polarizations. The effects of strain and temperature on the Brillouin gain spectrum are also fully characterized. <s> BIB005 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> Brillouin optical-fiber frequency-domain analysis is a new sensing technique for the distributed measurement of temperature and strain. Extensive theoretical investigations and experimental results of distributed temperature and strain measurements demonstrate the feasibility of this new concept. In an experimental demonstration, a spatial resolution of 3 m was achieved for a 1 km-long single-mode fiber. <s> BIB006 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <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> BIB007 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> Distributed strain sensing with millimeter-order spatial resolution is demonstrated in optical fibers based on Brillouin optical correlation domain analysis. A novel beat lock-in detection scheme is introduced to suppress background noises coming from the reflection of Brillouin pump waves. The Brillouin frequency shifts of 3 mm fiber sections are successfully measured with a theoretical spatial resolution of 1.6 mm. <s> BIB008 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> The viability of fiber-optic frequency-domain Brillouin strain sensing for accurate high-resolution structural health monitoring, is demonstrated. High performances have been achieved by applying an iterative reconstruction algorithm which correctly takes into account the influence of the acoustic wave involved in Brillouin scattering. The quality of the reconstructions was confirmed by finite-element method (FEM) numerical simulations. A number of experiments with an aluminum beam under different load conditions are presented, demonstrating the potential benefits resulting from the use of SBS-based sensors in a wide range of structural health monitoring applications. <s> BIB009 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> In real sensors, the crosstalk or undesirable crossed sensitivities must be minimized. Distributed Brillouin sensing is a ::: very useful technique to measure fluctuations of temperature along an optical fiber. However, the later measurement can ::: be influenced by the humidity on the fiber; therefore its effect must be minimized. Because the aforementioned, the ::: Brillouin frequency changes with the humidity. Thus, for a given temperature on a distributed fiber sensor such ::: variations have been investigated. The experimental results obtained using three different types of single mode fibers ::: with 1000m length, at 25°C are reported in this paper. <s> BIB010 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> Distributed sensing based on the Brillouin frequency shift is a useful technique to measure temperature along an optical fiber. However, the measurement can be influenced by the humidity on the fiber and its history. Experimental results using three different types of single-mode fibers with 1 km length, at 25degC and 30degC are reported. Depending of the type of fiber, a displacement on Brillouin frequency from 0.4 up to 2.8 MHz due to changes of humidity is obtained. <s> BIB011 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> We report a novel kind of all-optical dynamic grating based on Brillouin scattering in a polarization maintaining fiber (PMF). A moving acoustic grating is generated by stimulated Brillouin scattering between writing beams in one polarization and used to reflect an orthogonally polarized reading beam at different wavelengths. The center wavelength of the grating is controllable by detuning the writing beams, and the 3dB bandwidth of ~80MHz is observed with the tunable reflectance of up to 4% in a 30m PMF. <s> BIB012 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> We study the principle of correlation-based distributed generation and readout of dynamic grating spectrum generated in stimulated Brillouin scattering in a polarization-maintaining optical fiber. The experimental validation is demonstrated by applying synchronous sinusoidal frequency modulations to two orthogonally polarized laser sources that serve as the Brillouin pump-probe light and readout light of the dynamic grating, respectively. Temperature-induced opposite changes in the Brillouin frequency shift and in the birefringence-determined optical frequency deviation are clearly observed with 1.2 m spatial resolution and a 110 m measurement range. <s> BIB013 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> This paper presents a novel method that realizes simultaneous and completely discriminative measurement of strain and temperature using one piece of Panda-type polarization-maintaining fibre. Two independent optical parameters in the fiber, the Brillouin frequency shift and the birefringence, are measured by evaluating the spectrum of stimulated Brillouin scattering (SBS) and that of the dynamic acoustic grating generated in SBS to get two independent responses to strain and temperature. We found that the Brillouin frequency shift and the birefringence have the same signs for strain-dependence but opposite signs for temperature-dependence. In experiment, the birefringence in the PMF is characterized with a precision of ~10-8 by detecting the diffraction spectrum of the dynamic acoustic grating. A reproducible accuracy of discriminating strain and temperature as fine as 3 micro-strains and 0.08 degrees Celsius is demonstrated. <s> BIB014 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> We demonstrate a novel kind of tunable optical delays based on dynamic grating generated by Brillouin scattering in an optical fiber. An axial strain gradient is applied to a 15 m section of a polarizationmaintaining fiber, and the Brillouin reflection grating is generated position-selectively by controlling the optical frequencies of Brillouin pump waves. Tunable time delays of up to 132 ns are achieved with an 82 ns Gaussian pulse. <s> BIB015 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> We demonstrate the distributed measurement of Brillouin dynamic grating spectra in a polarization-maintaining fiber based on time-domain analysis. Local reflection spectra of the Brillouin dynamic grating are acquired by synchronized propagation of the pump and the probe pulses based on the map of the Brillouin frequency distribution. Large temperature sensitivity as high as −50.9 MHz/°C is observed with 2 m spatial resolution in 100 m range. <s> BIB016 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> The dependence of the Brillouin frequency shift (BFS) on UV-cured acrylate coating and uncoated fibers for media that have different water vapor concentrations is experimentally investigated. The BFS is proportional to the temperature within the fiber, but it also depends on the water vapor contained in the surroundings of the fiber. A hypothesis based on the efficiency of the heat transfer due to the different humidity concentration in the media is proposed, and the temperature difference that depends on the heat transfer is quantified in standard fibers. A shift of ~0.22 MHz for relative humidity change between 60% and 98% at 20°C is measured. <s> BIB017 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> We report on what we believe to be the first truly distributed birefringence measurement of polarization-maintaining fibers (PMFs) based on transient Brillouin grating (TBG). A TBG is created by two short pump pulses in the slow axis of the PMF, and then the birefringence-related TBG spectrum is mapped by scanning a probe pulse launched in the fast axis, where the local birefringence can be calculated using the birefringence induced frequency shift. Two types of widely used PMFs, bow-tie and panda, with a length of 8 m were measured at a spatial resolution of 20 cm, and the results show that the birefringence features a periodic variation, and their variation ranges are ~2.4×10−6 and 1.3×10−6 along the test fibers, respectively. <s> BIB018 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> We demonstrate a scheme which allows for temperature corrected distributed strain measurements under environments involving simultaneous application of strain and temperature, with enhanced spatial (5 cms), strain (66 µ.epsilon) and temperature resolutions (1.9°C). The technique utilizes the combination of frequency based BOCDA with Brillouin intensity measurements. <s> BIB019 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Journal of Sensors <s> The first (to our knowledge) observation of Brillouin dynamic grating in conventional single-mode fibers is reported, and the characterization is demonstrated with respect to the external parameters for the grating generation. When a 100 m single-mode fiber is used, a reflectance of 8% with a spectral bandwidth as low as 2.4 MHz is achieved, which is less than 10% of ordinary Brillouin gain bandwidth. <s> BIB020
Spontaneous scattering of light is mostly caused by thermal excitation of the medium, and it is proportional to the incident light intensity. On the other hand, the scattering process becomes stimulated if fluctuations in the medium are stimulated by the presence of another electromagnetic wave that reinforces the spontaneous scattering. The scattering process is in the stimulated regime provided that the intensity of the input light has a value above a level known as the threshold, which is lower than the threshold of the spontaneous regime. The stimulated scattering process is readily observed when the light intensity reaches a range between 10 6 and 10 9 Wcm −2  and is capable of modifying the optical properties of the material medium. Stimulated Brillouin Scattering (SBS) can be achieved by using two optical light waves. In addition to the optical pulse, usually called the pump, a continuous wave (CW), the socalled probe signal, is used to probe the Brillouin frequency profile of the fiber BIB005 . A stimulation of the Brillouin scattering process occurs when the frequency difference of the pulse and the CW signal corresponds to the Brillouin shift and provided that both optical signals are counterpropagating in the fiber. The interaction leads to a larger scattering efficiency, resulting in an energy transfer from the pulse to the probe signal and an amplification of the probe signal. The stimulated Brillouin interaction in single mode fibers can be modeled by the three-wave transient equations for the pump (subscript p) and Stokes (subscript s) waves with the field amplitudes E p,s (z, t) interacting with the acoustic wave E a (z, t) (in time t and position z along the fiber)  : where n f is the fiber refractive index, c is the velocity of light in vacuum, and g B is the Brillouin gain factor. Γ is the damping constant of the acoustic wave, Γ 1 is the damping time of the phonon, Γ 2 is the detuning angular frequency given by Γ 2 = 2π(ν − ν B ) and ν is the beat frequency between the probe wave and the pump wave. The sensor system focuss on measuring the Brillouin backscattered light, which gives information about changes in temperature or strain experienced by the fiber. Considering pump pulses larger (time-width) than the phonon lifetime and no pump depletion, the Brillouin backscattered light power P B (z, ν) detected at the receiver can be given by   P s (z = 0, ν, t) = |E s | 2 , where P(t) is the total power of the launched pulsed light, α is the attenuation coefficient of the fiber, p 12 is the electooptic coefficient, υ is the acoustic velocity, λ p is the pump wavelength, ρ 0 is the material density, and the Brillouin gain spectrum g B in this case is given by a Lorentzian function and is assumed not to depend on z. The parameter ν B is the frequency at which g B has a peak value g 0 , and Δν is the full width at half-maximum (FWHM). Also, it is important to remark that there is an exponential relationship between the Brillouin power gain and the Brillouin gain spectrum, which is maximized at ν B value. According to the mathematical description presented in (1), Brillouin scattering is strongly dependent on thermodynamical variables BIB003 BIB002 BIB010 BIB011 BIB017 . The dielectric constant varies according to the pressure wave that is generated and which travels along the medium. Then, the Brillouin shift frequency is a function of the acoustic phonon, as well as the medium structure and its constituents. The material structure is clearly perturbed by changes on environmental temperature or by strong alterations on its density distribution; such is the case when a longitudinal force or a stress is applied. Those transversal or longitudinal forces relay on a shrinking or an enlargement of the original size of the material. The Brillouin frequency shift has a linear dependence (for values of strain and temperature within its tolerance ranges) on the applied strain ε and the temperature variation ΔT (at a reference temperature T 0 ) that can be written as BIB003 BIB002  where C ε is the strain coefficient (MHz/με), C T is the temperature coefficient (MHz/ • C) and reference strain ε 0 . These values are mostly determined by the fiber composition, pump wavelength, fiber coatings, and jackets. Brillouin scattering was proposed for the first time to measure temperature in 1989 BIB001 , and, currently, it is widely used for distributed temperature and strain sensing because the Stokes side-lobe is temperature and strain dependent. One reason of this success is that Brillouin effect can be used in long transducers (hundreds of kilometers) made of standard monomode telecommunication optical fibers, since the Brillouin frequency shift is about 10-11 GHz at 1550 nm  . An example of this dependence for a standard single mode fiber (SMF) is shown in Figure 1 .  Proposed by Hotate BIB007 , the BOCDA is a technique that controls Brillouin scattering using the synthesis of optical coherence function (SOCF). This technique involves phase modulation of the continuous pump wave and the probe wave. Brillouin interaction only takes place in positions where the phase of these two signals is highly correlated (Figure 9 ). As the phase difference between the pump and probe is changed, the position along the fiber changes where the Brillouin interaction occurs; thus providing a way to scan the length of the fiber. In conventional BOTDA techniques, if the spectral width of the pulse exceeds the Brillouin linewidth, the gain spectrum broadens and the measurement deteriorates. Nonetheless, in BOCDA if the spectral width of the pump and probe increases, the resolution of correlation technique also increases. Spatial resolution up to 1.6 mm, as well as improvements on the sampling rate of 1 kHz, and measurement range of 1 km BIB008 BIB019 were achieved. A common drawback of the BOCDA system is its limited measurement range due to the periodic nature of sensing position and that the transducer length is shorter than other Brillouin-based sensors; additionally the transducer is much more complex and the postprocessing is more intense, which may affect negatively the measurement time. Brillouin optical frequency domain analysis (BOFDA) is based on the measurement of a complex transfer function that relates the amplitudes of counter-propagating pump and probe waves along a fiber BIB004 . The continuous probe wave is modulated in intensity with a sinusoidal signal over a range of frequencies, whilst in the pump wave an intensity modulation is induced. This induced signal has (Figure 10 ). Thus the temperature or strain can be determined from the Brillouin profile along the fiber BIB006 BIB009 . Recently, the concept of Brillouin dynamic grating (BDG) has been newly implemented in polarization maintaining fibers (PMFs) BIB012 and single mode fibers BIB020 , since Figure 9 : Experimental setup for BOCDA technique. the operation principle strongly depends on the local birefringence of the medium. Acoustic waves generated during the process of stimulated Brillouin scattering in one polarization are used to reflect an orthogonally polarized wave (probe wave) at a different optical frequency from the pump. Figure 11 shows the conceptual scheme of the BDG; the frequency separation Δ f between the pump and the probe waves in the BDG operation is determined by the local birefringence of the fiber and lies in the several tens of GHz in usual cases. BDG has been used as strain and temperature sensors BIB013 BIB014 , birefringence sensor BIB018 or as tunable delay lines BIB015 . BDG can also be applied to enhance the spatial resolution of an ordinary Brillouin optical timedomain analysis (BOTDA) system by replacing the Brillouin probe with the reflection from the BDG BIB016 .