The use of Walsh code in modulating the pump light of high spatial resolution phase-shift-pulse Brillouin optical time domain analysis with non-return-to-zero pulses

We have previously reported phase modulation of pump light of phase-shift pulse Brillouin optical time domain analysis with non-return-to-zero (NRZ)-formatted Golay codes. However, the Brillouin signal has been distorted when the duration of the coded pulse used exceeds the time constant of the amplitude in the acoustic wave that causes stimulated Brillouin scattering. Therefore, we propose pump modulation with Walsh codes to overcome this problem. We have shown that the use of Walsh codes in the NRZ-formatted pump induces little error in the spatial resolution, contrary to the previous results for Golay codes. The maximum signal-to-noise ratio improvement for 8-bit Walsh codes recorded was 3 dB and the spatial resolution obtained was 10 cm, being consistent with analytical and numerical calculations.