Amplitude fluctuation in laser signal transmitted through a long multimode fiber

Theoretical investigation has been made on fluctuation in detected signal at the end of a multimode fiber. It has been predicted that the fluctuation is caused by oscillation frequency fluctuation of a laser diode in association with interference, taking place at the detector surface, among nonorthogonal axial Components of field vectors for different modes. The prediction has been confirmed using a two-mode fiber and multimode graded-index fibers, by measuring amplitude and frequency of the signal fluctuation at the output end of the fiber. A method to suppress the amplitude fluctuation has been proposed for transmission characteristics measurement on long fibers.