Distortion of light signals transmitted via, multlmode graded-index fibres

Light signals with very narrow spectra are randomly distorted when transmitted via multimode graded-index fibres. Detailed experimental results are given for the harmonic distortion, which can be as high as −20 dB, after transmission through 1 km of fibre. The distortion is reduced for laser sources with more longitudinal modes or fibres with higher dispersion.