Performance Enhancement Methods For Phase-sensitive Optical Time Domain Reflectometry Based On Frequency Division Multiplexing

There is an increasing demand for distributed acoustic sensors(DAS)that can reconstruct an acoustic field's frequency,amplitude,and phase.These kinds of sensors typically realize by phase-sensitive optical time-domain reflectometry(?-OTDR).Yet,there are some common issues in ?-OTDR systems such as fading effect,optical path difference(OPD),poor spatial resolution,and low backscattering intensity.In this thesis,we suggested new techniques and solutions in terms of frequency division multiplexing(FDM)to mitigate these problems and enhance the whole system performance.Here,we focused on some of ?-OTDR current challenges.Then,we offered an FDM based technique to enhance the quality of the phase signal against fading phenomena in a?-OTDR system,which is one of the main concerns about phase-sensitive distributed sensors.By injecting several probe frequencies into a sensing fiber,we provided enough elbow room to choose the one that provides the best phase signal at any moment.We designed an algorithm that indicates the best beat frequency in a real-time system.This algorithm can predict distortion in the phase signal and chooses another beat signal to switch on immediately.We experimentally attained a 98.85% distortion-free phase signal on a real-time basis and over a very long running time.This method is promising for online and in-field purposes owing to its unique features of agility,repeatability,and functionality.Moreover,Conventional ?-OTDR systems suffer from the effects of laser frequency drift and OPD between the detection and reference arms,which induces noise in the measurements.For this purpose,a novel technique was proposed in our work to compensate OPD and thereby improve the signal-to-noise ratio.We believe this is the first-ever study that reports a very high level of OPD compensation with an FDM method without the need for sophisticated equipment.In the present work,this influence is counteracted by providing a loop path in the reference arm.Within this technique,using a laser source with low coherence length is facilitated.In comparison to a conventional?-OTDR system,SNR incremented to 5d B in an optimal system.Despite OPD compensation attributes,generating multiple reference beams increases the possibility of obtaining a better output.Besides,a multiple spatial resolutions method suggested promoting the functionality,as well as the use of fiber bragg gratings(FBGs)to boost the intensity of the backscattered lights.These multiple spatial resolutions came from using modulators with various frequencies.We achieved 3d B enhancement in SNR of the sensing system in comparison with the average SNR,with various spatial resolutions.Also,comparing to the conventional systems,the detection frequency range of 0?440KHz over 330 m fiber rooted with FBGs,was improved 3 times in this demonstration structure.These FDM based methods simplify the system requirements,which provide an auspicious DAS solution for practical online objectives.The performance of distributed optical fiber sensing devices based on ?-OTDR has been improved by FDM,which is more suitable for practical situations.Related sensing device developed by research group has been applied in applications such as seismic wave detection and high-speed rail online monitoring,which obtained good results.This will play an important role in promoting?-OTDR in the application of the structural health monitoring.