An optical perturbation sensing system includes a probing beam incident on a medium with perturbations and a sensing beam redirected from the medium and incident on a surface area of a photodetector. A reference beam directed onto the photodetector surface forms, with the sensing beam, an interference pattern on the photodetector surface and a phase patterner with at least two phase regions across its section, generates different phases in different regions of the interference pattern. An array of photodetector elements detects each phase region of the interference pattern and a constructive combiner subtract pairs of the detected signals, squares the subtracted signal squares, and sums the squared signals to form a stronger detected signal with reduced intensity noise, reduced background noise, and reduced sensitivity to phase drifts.