| The Brillouin optical domain reflectometry(BOTDR)system is a full-distribution optical fiber sensing technology,which is based on the spontaneous Brillouin scattering.The change of strain or temperature in optical fiber is measured by measuring the frequency shift and intensity change of brillouin scattering spectrum.But the Brillouin scattering signal of the BOTDR system is weak,which limits the improvement of system performance.In this theis,the theory of pulse pre-pumped coding multi-wavelength BOTDR system are studied and simulated,wavelength scanning technology is introduced into heterodyne BOTDR,and theoretical analysis and experimental verification are carried out to improve the system signal-to-noise ratio and increase the sensing distance.This thesis first introduced the development status of BOTDR system,the basic theory of Brillouin scattering is studied,the relationship between Brillouin frequency shift and temperature and strain is analyzed,and the simultaneous measurement method of temperature and strain was discussed.The principle of pulse pre-pump coding is studied.The method of multi-wavelength laser generation is introduced,and the influence of different numbers of wavelengths on the signal-to-noise ratio of the system is analyzed.The system model based on local heterodyne and self-heterodyne detection is designed,respectively.The coding of pulse pre-pumped multi-wavelength BOTDR with self-heterodyne detection is theoretically analyzed,and the mathematical expression of the output signal of the photodetector after system decoding is obtained.The noise existing in the system is analyzed,and the three wavelengths is used as an example to derive the mathematical expression of the system signal-to-noise ratio and coding gain.The limitation of SBS threshold on system performance is studied.The pre-pump pulse width is 150 ns,the sensing pulse width is 8 ns for Golay coding,and the MATLAB simulation i s used to analyze the system signal-to-noise ratio,the relationship between coding gain and coding length,The optimal coding length of the system is 128 bits,and the relationship between signal-to-noise ratio,temperature resolution and strain resolution is analyzed.The results show that the signal-to-noise ratio is about 60.5 dB when using 128-bit pulse pre-pump coding from self heterodyne detection.,the temperature resolution is 0.65 °C,and the strain resolution is 12 ??.For the BOTDR system with wavelength sweep,the noise of the system is analyzed.The theoretical analysis of the number of scans of the wavelength sweep and the influence of the scan interval on the amplitude fluctuation of the signal is analyzed.The wavelength-scanning BOTDR system based on local heterodyne detection and self-heterodyne detection is designed and constructed.The experiment uses 130 ns pulse width modulation signal,the wavelength scanning range is 1550.12~1550.22 nm,and the scanning step is 0.01 nm.The experimental results show that the temperature resolution of single-wavelength is 1.53 °C in the local heterodyne,and after two times of wavelength scanning,the average value is reduced to 0.73 C.The system temperature measurement error is improved by 0.8 °C,and the signal amplitude fluctuation is reduced by about 0.992 dB.The signal-noise ratio is increased by 15.4 dB,and after wavelength scanning,the system's coherent Rayleigh noise can be effectively reduced,thereby improving the system signal-to-noise ratio.Wavelength-scanning single-wavelength BOTDR self-heterodyne test with a temperature measurement error of 0.56 °C,compared with a single-wavelength system at 2.15 °C,the temperature measurement error is improved by 1.59 °C,the signal amplitude fluctuation is reduced by about 0.7 dB,and the signal-to-noise ratio is improved by about 20 dB. |
PDF Full Text Request