Lighting the fuse

26 September 2012

Optical network operators could find it easier to predict the limits of their infrastructure in the future thanks to an experimentally proven model of fibre fuse effect dynamics. Researchers from the University of Aveiro in Portugal have shown how the velocity of the fuse zone depends on the optical intensity of the signal, and have identified a ‘saturation region’ in Erbium doped fibres for the first time.

Point of failure

The fibre fuse effect, which was first published in 1988 by Kashyap in Electronics Letters, is a self-destruction process induced in optical fibres by propagating light signals with power over 1 watt. The cause is a local heating point, usually in a damaged or dirty connector or in a tight bend in the fibre and, once initiated, an optical discharge travels along the fibre leaving a train of holes or ‘voids’ where the silica has been vaporised.  

Although first reported over 20 years ago, this phenomenon has only recently become a major concern as techniques such as dense wavelength multiplexing and non-linear amplification (e.g. Raman amplifiers) have significantly increased the signal power in optical communication networks, and there has been rapid development of high-power light sources.

Much of the research over the last ten years has been focused on practical techniques to prevent and stop a fibre fuse, along with theoretical studies and microscopic analysis. Several studies of the fibre fuse discharge zone in different types of fibres concluded that the relationship between the propagation velocity and the optical signal intensity is linear.  

A simpler model

Last year the University of Aveiro team, inspired by the models of combustion flame propagation, modelled the optical fibre fuse to an ordinary differential equation, using a travelling variable. Their simplified model disagreed with the previous work in that it showed that the relation between the optical discharge zone and the optical signal power density is not linear.

In their latest work in this issue of Electronics Letters, the Aveiro team have presented experimental evidence that their model predicts the evolution of the fuse effect. They injected powers in the range of 2.0-4.5 W which corresponds to the intensity ranges of 2.5-5.6 MW/cm2 for single-mode fibres and 7.3-16.5 MW/cm2 for Erbium-doped fibres.

In standard single-mode fibres they observed that the fuse zone velocity is linearly related to the optical intensity. However, for Erbium doped fibres they observed an initial linear dependence that switched to saturation at optical intensities above 12.5 MW/cm2. This is the first time that such a saturation zone has been seen and it confirms the predictions made by the researchers’ model. They are now working on further refining their model to include the changes of phase induced by the high temperatures reached in the fibre core, and to account for higher heat capacities.

Increasing understanding

With the issue of fibre degradation becoming increasingly important, the team are dedicating their efforts to the study of void formation dymamics, the thermal characterisation of the optical discharge zone, and seeking fuse effect mitigation mechanisms. They are also extending their studies to new fibres such as bend insensitive fibres.

Overall this field is making rapid progress, but further fundamental studies of the fuse effect are crucial as the process that leads to the ignition is still not clear. Understanding this will help with the challenge of designing new fibres that have a higher ignition threshold and will better cope with the growing demands of communication traffic.

The Letter presenting the results on which this article is based can be found on the IET Digital Library.

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Cover of Electronics Letters, volume 50, issue 19

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