| The interest in using fiber optics around the world is progressively changing and increasing due to its wonderful advanced technology.This phenomenon has been shown to have an impact on the development of daily human life directly or indirectly.To date,the use of optical fiber sensors becomes imperative,as they are easily available for humans,and economically sustainable,although some of them are expensive.They provide a good safety monitoring of big national infrastructures of the countries.It is clear that the world economy is increasing and at the same time,it increases demand for infrastructures such as construction(buildings,bridges),electricity,and transportation from time to time.Only fulfilling these demands is not the solution for society,there should be a safety monitoring of the infrastructure with accurate and minimum errors in the measurement.Therefore,there should be great and continuing work to be done to advance these technologies.Researchers are working hard in improving the novel methods and designs for improvement in fiber optics sensing technology and its efficiency.This thesis includes the studies of developing distributed optical fiber temperature/strain sensors and contributing to performance enhancement of the distributed fiber sensors based on Rayleigh backscattering(RBS).The Distributed fiber-optic sensors based on Rayleigh backscattering enable dynamic measurement of temperature and strain changes as well as vibration/acoustic on the optical fiber.As a key technology in optical fiber sensing,the phase-sensitive optical time domain reflectometer(ф-OTDR),which is a highly sensitive distributed sensing technology,has attracted much attention because of its simple system structure,strong robustness,and distributed sensing.This technology is mainly realized by the combination of frequency sweeping,chirped pulse,and coherent detection with different algorithms.These methods have different limitations in terms of measurement accuracy,response range,spatial resolution,and system complexity.Their applications in many fields that require a good,high accuracy measurement are very remarkable.Therefore,it is of tremendously important research significance and application prospects to study a reliable and straightforward dynamic distributed measurement system of ф-OTDR.A dual-frequency and multi-frequency ф-OTDR technology assisted by an image matching algorithm was proposed in this dissertation employing two-dimensional data to compensate for the phase change of scattering signal caused by temperature/strain change using the frequency difference between probe pulses.By selecting the appropriate frequency pair,the measurement of temperature and strain change rate is realized.This method has the advantages of a large dynamic measurement range,strong anti-fading noise ability,and low bandwidth requirement.The methods proposed in this dissertation incorporated the following main research concepts.This dissertation starts by investigating the scattering process that occurs when a light beam propagates in the optical fiber.Particularly,the Rayleigh backscattering has been described and characterized in detail.The simplified theoretical approaches allowing well apprehending the light scattering phenomena in the fiber optic have been reported.The fundamental theoretical equations governing the RBS process in the optical fiber have been presented.The main characteristics of the RBS which are the linewidth and the central frequency,have been defined.Moreover,the state-of-the-art on temperature and strain measurement for demodulation and measurement accuracy was proposed briefly describing the basic method for dual and multi-frequency ф-OTDR sensing technology.Secondly,a simulation model and the mechanism of the dual and multi-frequency ф-OTDR scheme based on the frequency compensation ф-OTDR technology principle is established and analyzed in detail.Then,the sensing principles of dual-frequency ф-OTDR and multi-frequency ф-OTDR are introduced respectively to verify their feasibility based on the image matching method.The relationship between the rate of change of the temperature and the time delay of dual-frequency pulse and the relationship between strain change and frequency shift of multi frequency pulse is simulated and analyzed.Then,the relationship between the temporal window error(reflecting the time delay)and the measurement error is studied,and the effect of the spatial window on demodulation error is studied.The results show that the phase of the Rayleigh scattering light field is related to temperature and strain even if the phase effect affects the linearity.Then,this thesis carries out simulation and temperature sensing experiments of the dual-frequency ф-OTDR based on the image matching method.By analyzing with simulation,the result shows that after the introduction of the image matching method for data processing,compared with the one-dimensional data matching process,the time resolution and sensing accuracy of the dual-frequency technique have been significantly improved.Compared with the conventional dual-frequency ф-OTDR,which retrieves data via curve matching,the proposed scheme can effectively improve the temporal resolution and measurement precision while keeping the spatial resolution without additional hardware.In the experiments,with a 10 s temporal window,the proposed scheme realized the same measurement precision as the conventional method,which used a 40 s window,suggesting a four times improvement of temporal resolution.While both were using the 10 s temporal window,the measurement error was suppressed from 21.4% to 1.2% in the sensing for the 2 m hot zone at the end of a 90 m fiber.The sensing precision is improved nearly five times and a distributed temperature sensor with 2 m spatial resolution is realized.Finally,in this work,ф-OTDR sensing method for strain measurement in polarizationmaintaining fiber was demonstrated.The simulation and experimental research of multifrequency ф-OTDR were carried out.Several pairs of pulses with frequency differences(35)?are used to monitor strain changes.The simulation results of distributed dynamic strain sensing experiments show that the sensing accuracy of frequency sweeping technology can be improved and the spatial resolution can be maintained at the level of 2 m using the proposed method.The results of vibration experiments confirmed that the frequencysweeping ф-OTDR based on the proposed method can measure dynamic nano-strain,and the minimum measurable strain can reach less than 10 nε.This is achieved by selectively detecting the signal from the given mode of PMF to enhance the ф-OTDR measurement.The strain was applied through a motorized positioning so that the level of accuracy for the applied strain was improved.This means that the ф-OTDR signal demonstrated nano-strain vibration,and the sensitivity is high.Thus,the calculated sensitivities based on the experimental results is-144.4 MHz/με for the small relative and accumulated large strain measurements of the optical fiber under test.In this range,the strain sensitivity close to single-mode fiber(150 MHz/με)was obtained.The obtained values are similar to the expected values.A high resolution distributed strain sensing based on a dual-mechanism sensing technique by combining the Brillouin and Rayleigh scattering has been proposed and experimentally demonstrated in this work.The scheme employs the same set of frequencyscanning optical pulses modulated through the frequency-agile technique for fast measurements.Brillouin sensing is used to provide absolute strain reference,and the Rayleigh sensing signal provides relative strain change with high strain resolution.On the40 m polarization-maintaining fiber,combined with the two sensing signals,the 6.6 nεhigh-resolution real dynamic strain measurement is realized.ANC technique was introduced to calibrate the Rayleigh sensing signal.The measurement range of the corrected signal is determined by Brillouin sensing,and the strain resolution depends on Rayleigh sensing.On the micro-strain order measurement benchmark,the real absolute dynamic strain sensing with both high resolution and large measurement range is realized.In general,through the research work of this dissertation,a feasible dual and multi frequency distributed optical fiber sensing proposed can accurately detect temperature and strain change.These solutions can provide technical support for improving the reliability of DOFs technology in field applications.Moreover,the multi-frequency DOFs proposed and demonstrated in this work can easily be applied to the existing DOFs based on Rayleigh scattering. |
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