Research On A New RBOTDA Sensor System

In this dissertation,the current research of distributed optical fiber sensing technology in recent years is firstly summarized.The principle of Brillouin Optical Time Domain Reflectometry?BOTDR?and Brillouin Optical Time Domain Analysis?BOTDA?based on distributed optical fiber sensing technology are introduced.In addition,the sensing principles and the characteristics of the traditional Rayleigh Brillouin optical time domain analysis?RBOTDA?is analyzed.The principle of EOM is analyzed and the intensity distribution of each order in the valley point and linear point of transmission curve is studied by simulation in the RBOTDA system.It is concluded that the signal-to-noise ratio of the first order optical wave is the largest when the bias port of EOM is locked at the valley point.Therefore,the parallel structure with two EOM is proposed to generate the pulsed light with continuous base light,which is required for RBOTDA sensing system.Besides,the probe light is provided by the Rayleigh scattering light of the base light.A new type of RBOTDA system which works on the single end and could overcome the shortcoming of dual end access is proposed.The new structure of RBOTDA system is described and the mechanism of non-local effect is analyzed in detail,then the method to solve the non-local effect is put forward.The dual sideband probe scheme used in the process of sensing turns the system to be highly robust against pump pulse depletion and the distortion caused by non-local effect.The balanced detector is used at the detection end to eliminate the common mode noise and improve the signal to noise ratio of the system based on analyzing the principle of the balanced detector to suppress the common mode noise.The RBOTDA experiment system is set up.The results show that the Brillouin gain spectra fit Lorenz Curve perfectly and non-local effect is suppressed effectively.Brillouin frequency shift is 10.867 GHz and the Brillouin line-width is about 40.21 MHz with 50 ns pulse width at a distance of 2.4 km in the fiber at room temperature 25?.A comparative experimental study and analysis of balance detection and non-balance detection is carried out in the system.It is concluded that the output noise power of balanced detection is 2^1/2 times that of Stokes or anti-Stokes,that is to say,the signal to noise ratio of the output signal of the balanced detection is increased by 2^1/2.The Brillouin spectrum of balance detection is more smooth and the noise is less than that of Brillouin gain or loss in the end of the fiber.