Eliminating the non-local effect in frequency-fixed probe wave based BOTDA sensor

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.