3.3 Basic setup of BOTDA

As illustrated in Figure 6(a), a typical conventional BOTDA setup mainly comprises three parts. A highly coherent laser is split via a coupler into the probe (upper) and pump (lower) branch. A microwave signal, which can be scanned over the range of the BFS (�11 GHz � 150 MHz), is applied on a Mach-Zehnder modulator (MZM 1) in the probe branch and biased in the carrier suppression regime. The generated lower frequency sideband serves as the probe wave, while the upper one is blocked by a narrow band-pass filter (FBG 1). The probe power is controlled by an erbium-doped fiber amplifier (EDFA 1) and launched into one end of the FUT. In the pump branch, the pump pulse with a required duration is formed by another MZM, amplified by EDFA 2 and launched into the other end of the fiber via a circulator (Cir 1). In order to mitigate the influence of the SBS polarization effect [50], the polarization of either pump or probe wave is scrambled via a polarization scrambler (Pol.S.) before launched into the fiber.

The Brillouin amplified probe wave is detected by a fast photodiode (PD) whose minimum bandwidth should correspond to the inverse of the pump pulse duration, to avoid any trace distortions. Another narrow band-pass filter (FBG 2) blocks the residual reflected pump wave. The electrical signal from the PD, which represents the evolution of the probe power, is processed digitally with a large number of averaging by a signal processor, that is, an oscilloscope or a digitizer. The Brillouin gain, which is essential for BFS estimation, can be derived by dividing the output of the PD by the probe power before the pump pulse is launched into the fiber [51].
