Spectrally selective investigation on optical losses in multimode fibers with loop resonators

A technique for the selective characterization of the losses of the transverse modes propagating in a multimode fiber is presented. The method is based on the implementation of a fiber ring cavity which introduces a frequency discrimination among the resonances of the different modes, thus allowing to investigate them separately. Each mode is excited by locking the laser frequency on the corresponding cavity resonance, and its losses are measured from the average photon lifetime with a ring-down technique. Two different distributions of transverse modes, obtained by controlling the injection of the multimode fiber at the cavity input port, are investigated. The technique is proposed as a complementary approach for the characterization of multimode fiber propagation and intermodal power exchange effects.