| This thesis is a contribution to performance enhancement of the distributed fiber sensors based on stimulated Brillouin scattering(SBS).The Distributed fiber-optic sensors based on Brillouin scattering enable to measure changes in both temperature and strain along the fiber cable.The present study especially focused on improving the spatial resolution of long-range Brillouin optical time-domain analysis(BOTDA)sensor.A new high spatial resolution Brillouin based interrogator enabling detecting a cm-scale disturbance over long sensing distances was developed.The novel sensing method called “double-peak spectrum” based on the differential pulse-width pair DPP-BOTDA technique has been proposed and investigated theoretically,demonstrated numerically and implemented experimentally.In standard BOTDA sensing,the spatial resolution is an important parameter because it represents the minimum detectable perturbation length of the sensor,which is determined by the width of the pump pulse.However,this important parameter is limited unfortunately to 1m by the acoustic phonons lifetime(10-ns).For pump pulse durations shorter than 10-ns the BGS experiences broadening.The latter feature turns to be harmful to the measurement accuracy because it leads to a poor frequency resolution when estimating the central frequency of the gain spectrum.Meanwhile,for pulses much longer than 10-ns,the gain spectral width tends toward the Brillouin natural linewidth(30 MHz).For traditional BOTDA sensor when the spatial resolution is high enough to resolve a given perturbation the resulting BGS has a single peak.The estimation of the BGS peak frequency representing the mean Brillouin frequency shift(BFS)of the corresponding sensing fiber location is simply done by using Lorentzian or Gaussian fitting function.Elsewhere,if the length of the perturbation to be detected is much shorter than the spatial resolution of the sensor that is defined by the pump pulse width,the measured BGS experiences two or even multiple peaks.The main peak stands for a signal component of the sensing fiber,while the secondary peak represents the signal component of the section under perturbation(temperature or strain).This feature leading to broadening the BGS has been considered for long as a factor limiting the sensor's spatial resolution due to the fact that it makes difficult the determination of the center frequency of the spectrum.By investigating on the reasons for those spectral distortions causing many frequency errors,we found that,since the spatial resolution is defined by the length of the pump pulse,every sample of the Brillouin signal contains local spatial information about sensing fiber segment lengths which are equivalents to the nominal spatial resolution of the sensor.Therefore,the appearance of several peaks in the BGS indicates that the corresponding signal sample involves several frequency components.However,this thesis was only focused on the case of double-peak BGS.It was discovery thanks to numerical simulation,and demonstrated experimentally that when the peak frequency difference between the two peaks is quite larger than the bandwidth of the main BGS,a section much shorter than the spatial resolution defined by the pulse width can also be detected.So,the double-peak BGS based on DPP-BOTDA technique was proposed for improving the spatial resolution of distributed Brillouin based-sensor.Two types of single mode fiber(SMF)with a large Brillouin frequency shift(BFS)difference have been employed.Our technique allowed designing a temperature and strain Brillouin sensor that has sub-spatial resolution events detection ability.That particular and unique feature of our sensor has been illustrated through achieving the detection of a 5cm perturbed segment over a 16 m single mode fiber by using a 50 cm spatial resolution DPP-BOTDA sensor through numerical simulation in a first time.Then,the concept has been experimentally proved by detecting 4 short segments of 50 cm,20cm,10 cm and 5cm of another type of SMF inserted at the remote end of a 40m-long SMF fiber.The mean BFS of the main fiber was 10.83 GHz and that of the inserted fiber was 10.68 GHz which corresponds to 152 MHz frequency difference with respect to the main sensing fiber.The long-range detection capability of the novel developed distributed sensing method have been also demonstrated.By using a 50 cm spatial resolution DPP-BOTDA sensor,perturbation lengths of 20 cm,10-cm and 5cm located at the far end of a 6km-long SMF fiber have been successfully detected simultaneously.Moreover,a 5cm hot spot has been detected at the far end of a 24km-long single-mode fiber with 0.5 °C temperature accuracy and only 1 GS/s sample rate which corresponds to 10cm/point.Also,the new proposed Brillouin interrogator has the advantage of alleviating the well-known trade-off existing between the spatial resolution and the width of the pump pulse.Therefore,some features of this technique make it a good candidate for the detection of events that induce big frequency shift.However,by employing another type of fiber having a large BFS separation with respect to the main fiber in critical sensing locations,the system becomes thus sensitive even to perturbation events inducing a small frequency shift.The doublepeak BGS technique is simple and cost-effective because its implementation is based on the traditional BOTDA scheme.It should be noted that two successive small-scale perturbations have to be separated by a minimum length corresponding to the nominal spatial resolution for being detected.Also,this dissertation investigates the different forms of scattering that occur when a light beam propagates in the optical fiber.All of them have been described and characterized in detail.In addition,simplified theoretical approaches allowing well apprehending the light scattering phenomena in the fiber optic have been reported.The fundamental theoretical equations governing the SBS process in the optical fiber have been presented.The main characteristics of the BGS that are the linewidth and the central frequency have been defined.Finally,the state-ofthe-art on the long-range and high spatial resolution sensing is proposed by briefly describing the main available methods for both extended range and high spatial resolution sensing. |
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