The Research Of The Optical Pulse Quality For The Distributed Brillouin Sensing System

Brillouin optical time domain reflectometer (BOTDR) is a type of distributed optical fiber sensing technology, which is based on spontaneous Brillouin scattering. It takes the optical fiber as the sensing element as well as the signal transmission medium. The technology can detect the physical quantities applied to the fiber, such as temperature or stress.Spontaneous Brillouin scattering is weak in the optical fiber, so BOTDR requires not only high-sensitivity photodetector but also the probe pulse with high extinction ratio and signal to noise ratio (SNR). In order to generate the probe pulse, the electro-optic modulator (EOM) is commonly used to modulate the continuous light from the narrow linewidth laser. The SNR of the probe pulse depends on the bias voltage of EOM. For the purpose of obtaining the probe pulse with SNR up to60dB, the system adopts the cascade approach with two EOM to modulate the continuous light. However, attributed to the inherent structure, EOM’s operating point drifts if it is affected by environmental factors. The drifting causes the fluctuation of the pulse’s SNR. Simultaneously, to ensure the long enough sensing distance of BOTDR, the probe pulse needs to be amplified by EDFA before entering the sensing fiber. Nevertheless, broadband noise is introduced into the signal after the pulse amplified by EDFA because of the amplified spontaneous emission (ASE) noise of EDFA, which reduces the SNR of the signal. It is necessary to filter the ASE noise. Fiber Fabry-Perot (FFP) tunable filter is a type of commonly used optical bandpass filter. Due to the temperature sensitivity of the FFP tunable filter, its center wavelength drifts in the course of filtering, which also causes the fluctuation of the pulse’s SNR.In this thesis, the work includes two aspects. First, the control scheme of EOM’s operating point is presented, which is specific to the high extinction ratio pulse generating module based on two cascaded EOM. The control system can make these two EOM work steadily for a long time and generate high extinction ratio pulse. Second, the control scheme of FFP tunable filter’s center wavelength is presented, which can make the center wavelength of the filter consistent with the wavelength of the probe pulse and achieves reliable bandpass filtering. These two aspects of work improve the measurement accuracy of the BOTDR system by enhancing the quality of the probe pulse.