Two mechanisms of phase modulation in multimode fiber-optic interferometers

The mechanisms of phase modulation of optical radiation in multimode light guides are treated, namely, the first mechanism with equal relative phase increments for all modes and the second mechanism including the mode interaction and characterized by different relative phase increments for each mode. It is demonstrated theoretically that, in the case of several modulating effects on the light guide, the behavior of signals of modal interference from each one of those effects upon variation of the parameters of the medium surrounding the fiber depends on the type of effect. The variations of signals from the effects of the first type are in full agreement. Signals from effects of the second type or of different types may vary differently. In experiments with an intermode fiber-optic interferometer 500 m long, modulators associated with the mechanism of the first type exhibited a high correlation (97%) of the behavior of signals upon drift of the light guide temperature. The variations of signals from modulators of the second type and of different types were almost uncorrelated (6 and 1%). (C) 2000 MAIK "Nauka/Interperiodica".