Correlation processing of signals of a single-fibre multimode interferometer upon excitation of few modes during measurements of deformation effects

The features of the correlation processing of a speckle pattern obtained after transmission of radiation from a single-fibre multimode interferometer (SMI) with few modes through a diffusion transparency are studied theoretically and experimentally. It is shown that this approach eliminates the influence of polarisation mode beats and of higher-order modes typical for the two-mode excitation of a SMI and allows the measurement of the elongation of a sensitive fibre piece by exciting few modes in a multimode interferometer.