Abstract: Presents a sinusoidal phase modulation interferometer adopting an all-optical fiber light path and a new demodulation process in real time. The interferometer described has been employed in an optical fiber sensor for non -contact displacement measurements and has achieved a measurement accuracy of the nanometres order. It permits the measurement of arbitrary displacements and vibrations, with an operation range of 0.1-300Hz. A feature of the sensor is its ability in the measurement of quasi- instantaeous displacements. Principles of operation of the sensor and experimental results are given.