| In recent years, taking the scattered light in the optical fiber as the sensor carrier, the distributed optical fiber sensor gets the favor of many researchers in the sensing field with its unique advantages, such as optical fiber combining sensing and transmission. With this advantage, multi-parameters such as temperature and stress can be monitored real-time at the same time, and all-round intelligent monitoring can be realized to overcome the disadvantages of traditional point monitoring and improve the success rate of the monitor. BOTDR technologies with the feature of single-ended measurement which is convenient for the practical application, is one of the hot spot of research at home and abroad. Among the distributed optical fiber sensing technology based on brillouin optical time domain reflection technology, coherent detection has the highest sensing accuracy. BOTDR has wide application prospects in large projects such as maintenance of oil and gas pipeline and health monitoring of large concrete structures (dam, tunnel, buildings, etc.) and geological disasters such as landslides. BOTDR is still not mature enough and mass commercialization for the difficult of detecting weak brillouin scattering light. It is particularly important to research signal demodulation method and how to raise the accuracy to improve the precision and reliability of the system.This paper presents a set of feasible solution of distributed temperature and strain sensing system based on spontaneous brillouin scattering, under the analysis and comparison of the hotspot in the study of distributed optical fiber sensing. Combining with research focus of the issue, a series of experiments and tests have been performed to research and develop the system step by step.Heterodyne coherent detection is used in the system. The CW light is coupled by the first optical coupler (90:10ratio), and then90%CW light modulated by electro-optical modulator to form100ns optical pulse. After the EDFA amplifier, optical circulator and optical filter, the pulsed light enters into25km sensing fiber, then the backscattered light including Rayleigh and Brillouin components enter the second optical coupler (50:50ratio). The output light in another arm of the first coupler enter EOM modulated by microwave signal and DC voltage as reference light with brillouin frequency shift, enters the second optical coupler too, after a scrambler, the above two beams of light are coherent and detected by an APD at the same time, after frequency mixer and low pass filter, then the analog electronic signal is changed to digital electronic signal by data acquisition card with100MHz sampling frequency, finally the data will be analyzed by the pre-defined software for noise reduction and analysis. Then the distributed temperature and strain of sensing fiber can be acquired. This system is convenient for the practical application as Brillouin scattering signal is acquired by single-ended measurement.In this paper, a series of measures are used to improve the system accuracy.(1) This system adopts two1order sidebands generated by EOM as the reference light with brillouin frequency shift. To obtain reference light with high intensity difference between1-order sideband and0-order sideband (1-0intensity ratio) is the key to the signal demodulation system.First, how polarization acts on intensity of sideband generated by EOM is studied in detail. In this paper, the method of polarization controller combined with DC bias voltage is proposed to gain reference light with high1-0intensity ratio. Second, automatic microwave frequency sweep with1MHz Step for the reference signal with best frequency is used to improve accuracy of signal demodulation system.(2) Amplifier with low background noise is used to improve the signal. Then, a wavelength tunable fiber grating filter controlled by Temperature control box (the filter accuracy reaches O.OOlnm/℃) is used to improve the system accuracy of signal demodulation.(3) Heterodyne coherent detection method is used to signal demodulation. That the terahertz order of magnitude of high frequency brillouin signal fell to the intermediate frequency signal of hundred MHZ order of magnitude which is easy to detect and deal with, to improve the detection precision. And Lab VIEW analysis and a series of measures are used to reduce noise to improve the precision, stability and reliability of the system, and so on, then reduce the cost of the system at the same time.Finally, a series of experimental research and system test are performed and analyzed. The main contents of the paper are as following:(a)The general situation of distributed optical fiber sensing technology has been introduced. Including the characteristics of the distributed optical fiber sensing technology, the classification of the optical fiber sensing technology, and the optical fiber sensing technologies based on brillouin scattering(BOTDR、BOTDA、BOFDA).Then Present situation and the research significance of distributed optical fiber sensing technology based on spontaneous brillouin scattering have been summarized. Finally this section introduces the main work done in this paper.(b)This section laid a theoretical basis for design of BOTDR system. Scattering phenomenon and three kinds of scattering spectrum are introduced. The relationship between strain (or temperature) and the brillouin frequency shift and scattering intensity are analyzed theoretically after the mechanisms of spontaneous and stimulated brillouin scattering are expounded. Distributed temperature and strain sensing simultaneous measurement model has established basing on the mechanism of the frequency shift and the intensity of spontaneous brillouin scattering affected by temperature and strain.(c) How polarization impacts on the intensity of sidebands generated by EOM is investigated theoretically and demonstrated experimentally in this paper. That intensity of sidebands change with polarization presents a cos2x function development is proposed and verified by experiment. The method of polarization controller combined with DC bias voltage is proposed to gain reference light with high1-0intensity difference.The reference light with1-0intensity difference of24dB, which is18dB higher than that gained by only adjusted DC voltage, improves signal-to-noise ratio and accuracy of the system significantly.(d) System test and the analysis of experimental results are the core contents in this paper. Including framework of the whole system, the selection of devices and experimental parameters in each subsystem, experimental research, and the system temperature sensing experiment result discuss as well as Lab VIEW analysis. Amplifier with low background noise, wavelength tunable fiber grating filter with precision of0.001nm/℃, automatic microwave frequency sweep are used to improve accuracy of the system.The frequency shift is10.853GHz acquired by a spectrum analyzer in experiments, which is consistent with the theoretical value of brillouin frequency shift. The temperature coefficient is1.0843MHz/℃in the system temperature sensing experiment, which is close to the reported data1.2MHz/℃, the difference between them is mainly due to the instrument error. The brillouin scattering signal intensity is analyzed by LabVIEW. The cumulative average denoising, wavelet and low-pass filtering denoising principle are expounded in detail, the effect of three methods of denoising are compared.(e)This section summarizes and analyzes the deficiencies existing in the system, then points out the next step of studies. |
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