Damage threshold of microstructured optical fibres

Laser damage thresholds of 10 and 20 micron-core diameter solid-core photonic crystal fibres (PCF) and hollowcore photonic band gap (PBG) fibres have been measured. The studies were carried out using a Nd:Yag laser (30nsec pulses at 10Hz), which is optimally coupled into the fibres by careful mode matching, providing a coupling efficiency greater than 90%. It has been shown that the damage threshold of the 10-micron PBG fibre occurs for pulse energies close to 1 mJ, equivalent to a fluence well in excess of W/cmz propagating down the fibre. This is a factor of 4 larger than the damage threshold of the 10-micron diameter solid-core PCF. However, the damage threshold of the large-cord PBG is smaller than that of the PCF. Theoretical modelling based only on the optical modal properties of the single-mode PBG fibre shows that an enhancement by a factor 25 should be obtainable. -Thus there are different mechanisms potentially responsible for the fragility of larger core PBG fibres. In an experimental study of bend losses it has been found that it is possible to bend the 10-micron PBG fibre up to the breaking point bend radius (less than 1mm). The critical bend radius for the 20-micron PBG is close to 2 mm; which is 50 times smaller than. the critical bend radius of a 20-micron PCF. A summary will be presented of the results of the experimental and theoretical studies, highlighting possible reasons for the observed trends for the two. different forms of fibre.