The location of a disturbance along an elongated optical waveguide is determined by measuring different propagation times for a disturbance induced phase variation to propagate from the disturbance up to opposite phase responsive receivers at ends of bidirectional signal paths. Each receiver (401, 402) has a coupler (120, 121) that functions as a beam combiner and also as a beam splitter for inserting the opposite signal. On the receiving end, the coupler provides two detectors with mutually independent phase related signal values. These values are processed and mapped to phase angles, from which relative phase angles versus time are derived for each opposite signal pair.
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