The Theoretical Analysis and Design of Coding BOTDR System With APD Detector

The signal-to-noise ratio and the coding gain of Brillouin optical time domain reflectometry system with APD detector are analyzed. In coding distributed optical fiber sensing system, there are a large number of multipath interferences (MPIs), which are proportional to the code length. The random shot noise caused by MPI will be generated before decoding and cannot be eliminated by decoding process. In addition, the shot noise power is proportional to the square of the code length; the thermal noise power increases linearly with the code length, then, the increase of the speed of the coding gain will be decreased with the increase of the code length. When the shot noise is much larger than the thermal noise, the coding gain will not be increased with the increase of the code length, but will maintain a constant value. This conclusion is in contradiction with the traditional conclusion. At the same time, the shot noise depends on the pulsewidth and power, but the thermal noise is independent on the pulsewidth and the power, so there exists an optimum code length for specific system.