Speckle and Modal Noise in Optical Fibres Theory and Experiment

Abstract

Modal noise has been observed in optical fibres excited by partially coherent sources. In this paper the theory of this noise is presented. Following the basic guidelines of the speckle theory, the statistical properties of the fluctuations of the power transmitted through a circular aperture of arbitrary size are evaluated. It is found that, for fibres having an α -exponent index-profile, the maximum attainable signal-to-noise ratio, when the receiving aperture area is equal to the core area, is given by the square root of the number of the guided modes. The hypotheses on which the theory is based are discussed and an experiment, confirming the theoretical results, is presented.