The robustness of the BOTDA method based on dual-tone probe wave with fixed frequency separation is studied. It is verified that when the sensing fiber consists of two fiber segments with large Brillouin frequency shift difference (>100 MHz), the non-local effect would take place in the front fiber segment, which gives rise to frequency error on the determination of hotspot. Aiming at solving this problem, both the upper and lower probe sidebands are acquired simultaneously by using two photodiodes, and the average between the Brillouin gain and loss spectrum is calculated to eliminate the detrimental impact of the non-local effect.
A novel BOTDA method based on differential ∏-phase-shift-pulse pair is proposed for narrowing the Brillouin gain spectrum. Theoretical analysis and experimental results demonstrate that the proposal could achieve 17 MHz Brillouin gain spectrum linewidth with a fixed spatial resolution of 2.5 m. The Brillouin gain spectrum linewidth is 3 times narrower than that obtained using conventional single-pulse based BOTDA method with same spatial resolution, resulting in √3 times frequency accuracy improvement. Furthermore, the sharper rising/falling edge of the Brillouin frequency shift profile resulting from the narrowed Brillouin gain spectrum could obtain more precise temperature/strain information along the fiber.