There is provided an optical perturbation sensing system, including a probing beam incident on a medium, a sensing beam redirected from the medium and incident on a surface area of a photodetector, an array of photodetector elements, and a constructive combiner for constructively combining detected signals from the elements, to form a stronger detected signal. A method for detection of perturbations in a medium is also provided. The invention avoids the need for detection of the phase or the frequency variations of the scattered signal as used in the more complex hitherto-proposed approaches.