A distributed acoustic sensing system for use in a wellbore can include a shutter positioned between an amplifier and an optical-wave flow controller of the distributed acoustic sensing system for reducing optical noise in the distributed acoustic sensing system. The distributed acoustic sensing system can also include a controller operable to transmit a signal to the shutter to (i) open the shutter for allowing an optical pulse to transmit through the shutter, and (ii) close the shutter for preventing an extraneous optical waveform, that generates at least a portion of the optical noise, from transmitting through the shutter.