A sensor is provided, with a sensing element (8) whereby a measurand induces a relative phase shift between two waves, a phase modulator (5) adding a modulation to the relative phase shift, at least two detectors (10-1,10-2), with a first detector (10-1) detecting an interference signal responsive to a relative phase shift not including the relative phase shift as induced by the measurand, and with a second detector (10-2) detecting an interference signal responsive to a relative phase shift including the relative phase shift as induced by the measurand, and further comprising a signal processing unit (11) adapted to analyze the two detected interference signals and to derive a measurand value therefrom.