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### Abstract

High spatial (~cm) and spectral (~MHz) resolution Brillouin sensing is realized with enhanced signal to noise ratio using a pre-activated acoustic field and an optical phase control over the interrogating pulse. Pre-activation of the acoustic field preserves the Brillouin natural linewidth and a differential gain technique extends the method to long ranges. Experimentally, fully resolved measurements of the Brillouin frequency shift of a 5 cm spot perturbation at the far end of a 5 km fibre have been performed with a frequency resolution of 3 MHz ( 2 sigma ), using a 500 ps (5 cm) pi phase shift pulse.