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.

Theoretical and experimental investigations are described for determining the transmission characteristics of a multimode fiber with microbending for coherent and partially coherent illumination. The measured values of the average excess power loss are shown to be in close agreement with the theory. Also, an estimate of the excess transient loss due to mode coupling is found to be in good agreement with previously published data. Mode–mode interference is shown to be the cause of temporal fluctuations in the microbending loss, from which expressions for modal noise and baseband/subcarrier nonlinearity are derived on a statistical basis. For a given overall loss, the results show that many uniformly distributed small amplitude microbends cause much less modal noise and distortion than a few large amplitude microbends.

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

Power loss, modal noise and distortion due to microbending of optical fibers

An electromagnetic modal theory is presented for characterizing a tilt between the axes of two parabolic-index multimode fibers. The power loss, modal noise and distortion are analyzed by approximating the fiber modes by Laguerre-Gaussian functions. Using a uniform power distribution, this analysis corroborates a previously reported geometrical optics result that fiber axis tilt at the splice is approximately equivalent to an axial offset. It is shown that this equivalence applies to the prediction of loss values only, and that it becomes less accurate as the fiber mode volume decreases. For a given loss, the modal noise and distortion results for tilt offset are shown to usually be higher than those due to axial or longitudinal offsets, both for coherent as well as for partially coherent illumination.

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

Citations

Power loss, modal noise and distortion due to microbending of optical fibers

Modal Analysis Of Loss, Noise And Distortion Due To Tilt Misalignment In Multimode Fiber Splices

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