A Brillouin Optical Time-Domain Analysis (BOTDA) distributed sensor system and method use a continuous wave (cw) Stokes wave interrupted with a dark pulse for improved spatial resolution. The cw Stokes wave causes a continuous depletion of the pump wave. The dark pulse causes the depletion to stop for the duration of the pulse. Brillouin interactions are measured during the dark pulse. Very narrow dark pulses can be used because sufficient Stokes wave energy is maintained. The system produces a stronger time-domain signal and narrower linewidth Brillouin spectra than traditional techniques using a bright Stokes pulse. Narrower measurement pulses can be used leading to improved spatial resolution. A quasi-cw Stokes wave can be used to reduce the effect of stimulated Brillouin scattering (SBS) in long measurement fibers. The system can be used for distributed strain or temperature measurements.