Solitary thermal shock-waves and optical damage in optical fibres

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Description/Abstract

Thermally-induced catastrophic optical breakdown is clearly of great importance for fiber-based laser-light delivery systems. Fiber core can be destroyed irreparably at rates of 1 m/s by breakdown that starts at a locally heated point and travels back towards the laser; that the damage tracks left behind are often elegantly uniform and periodic is only a slight recompense. Breakdown can occur at relatively modest intensities (we have recorded 2.8 mW/µm in a multimode fibre at blue/green wavelengths), and has been observed in many different fibres at both Ar+ and Nd:YAG laser wavelengths. We initiate the effect by heating the fibre with a small flame whilst the laser light is propagating within it. A solitary thermal shock-wave is created which propagates along the fibre towards the laser, leaving the core permanently damaged and unable to guide light. Associated with this shock-wave is a bright spot of side-scattered light which can be observed propagating along the fibre; for this reason we have named the effect the "fibre-fuse". Similar thermal shock-waves have previously been seen in gases ("laser-induced deflagration waves")