Variation of the speckle contrast in a graded-index fibre by misalignment of the incident beam

The speckle pattern observed at the output end of a multimode fibre is closely related to the modal noise. In this paper, the average contrast of the speckle pattern is studied as a function of the radial position in the output end plane for three kinds of misalignment of the incident beam, i.e. defocusing, axial misalignment and angular misalignment. It is found that a variation of the speckle contrast is very sensitive to all kinds of misalignment of the incident beam and that its varying form strongly depends on the kind of misalignment of the incident beam. The speckle contrast at the centre of the output end plane is also examined as a function of the magnitude of misalignment. It is found that the contrast increases from a minimum value in the correct alignment to a certain saturated value with an increase of misalignment.