Unbalanced Michelson's interferometer as a fiber optic distributed sensor of external signals

The subject of this work is a novel fibre optic distributed sensor system. The system uses a technique called multiplexed reflectometric interferometry to measure dynamic strain in a network of single mode optical fibre sensors. The sensor is constructed on unbalanced fibre optic Michelson's interferometer is activated by series of double pulses. The time interval between those pulses depends on the length of the section of sensor. Acousto-optical modulator acts as an optical frequency shifter. A change in a frequency of electric pulses exciting the modulator results in a frequency shift in each generated wave packet. Two pulses of coherent light, one at frequency F-1 and the other at F-2 are transmitted from a laser into each sensor string. The separation of the two transmit pulses is set such that the returning pulse at F-1 frequency from the far end of zone overlaps with the returning pulse at F2 frequency from the start of that zone. These optical pulses mix at the receiver photodiode to give an electric signal with the difference frequency (F-1 - F2) and a phase change caused by disturbance at the detecting zone of sensor. This article presents influence of wavelength change of radiation source on course of interference contrast function of unbalanced Michelson's interferometer. In the paper digital demodulation with application of fiber optic coupler 3x3 is also presented. The maximum number of individual sensors is currently limited by the optical power budget and will increase as new technology is implemented. The system can address a variety of sensor types for different physical parameters.