Pushing the limit of the distributed Brillouin sensors for the sensing length and the spatial resolution

Long distance sensing based on Brillouin scattering with centimeter spatial resolution, and yet high strain or temperature resolution requires the optimization of the optical and electronic system. In optical domain the limiting factors include gain saturation of the Stokes signal and pump depletion induced the Brillouin spectrum distortion, and thus a low gain is desired that requires low pump power, which sets a limit in the signal to noise ratio (SNR). The detection system must have high gain and narrow bandwidth to reduce electronic noise. The coded pulse offers the best solution as a low power solution of long distance sensing based on BOTDA to improve the signal to noise ratio (SNR), comparing two most common used formats: non-return-to-zero (NRZ) and return-to-zero (RZ), RZ coded pulses offer minimum distortion in the spatial resolution and the Brillouin spectrum, because the signal in RZ format returns to zero in very bit, while in NRZ coded pulse the signal returns to zero after continuous "1" s, which brings the higher gain and lower bandwidth comparing that in RZ coded pulse for BOTDA system. Hence NRZ coded pulse BOTDA would introduce spatial broadening and lower the spatial resolution. With minimum distortion of RZ signal we can use differential Brillouin gain to realize DPP-BOTDA technique for sub-meter spatial resolution. The minimum coded pulse width must be larger than the acoustic wave relaxation time to avoid the distorted Brillouin gain spectrum. Using LEAF fiber we achieved 50km sensing length and 50cm spatial resolution with the strain resolution of 8 mu epsilon which is equivalent to 0.7MHz Brillouin frequency shift, this is the 1(st) sub-meter spatial resolution for 50km sensing length combined with high strain resolution.