Reverse Peak of Brillouin Spectrum in BOTDA sensor

Using a short optical pulse with finite extinction ration (ER) in Brillouin optical time domain analysis (BOTDA) system can achieve high spatial resolution and frequency resolution at the same time. However, the fluctuation after the pulse generated by the pulse generator can result in a same fluctuation to the optical pulse modulated by electro-optic modulator (EOM). Theoretical and experimental results demonstrate that this fluctuation can result in a reverse peak which has a 60MHz dip in the detected Brillouin spectrum. This makes it impossible to perform the sensing successfully when the Brillouin frequency shift (BFS) induced by the strain or temperature is within the dip. By adjust the DC bias of the EOM and the power of the electrical pulse, the influence of the fluctuation is eliminated successfully. A 15MHz BFS induced by strain on 20cm section over a 50m fiber is successfully detected with a frequency error of 0.8MHz.