Comparison of return-to-zero and non-return-to-zero coded pulses for BOTDA

The limitation of long distance sensing (>;20 km) based on BOTDA (Brillouin optical time doman analysis) with centimetre spatial resolution, and high strain or temperature resolution include 1) gain saturation of the Stokes signal; 2) pump depletion induced the Brillouin spectrum distortion. The coded pulse offers the best solution to reduce above limiting factors and to improve the signal to noise ratio (SNR). In this paper, two most commonly used pulse formats: non-return-to-zero (NRZ) and return-to-zero (RZ) are used for BOTDA, and it is found that RZ coded pulses offer minimum distortion in the time domain waveform and the Brillouin spectra while NRZ coded pulses introduce spatial broadening which has reduced spatial resolution. For SMF gain saturation occurs at much shorter length (<;20 km with 20 ns coded pulses) due to one peak Brillouin spectrum, while for 50 km LEAF fibre with 20 ns coded pulse, no gain saturation is observed due to three Brillouin peaks occurring and we have seen the lower and more uniform Brillouin gain across the fibre length. Using RZ coded pulses of differential Brillouin gain to realize DPP-BOTDA, we achieved 50 cm spatial resolution with the strain resolution of 12 με.