Optical fibre beam delivery of high-energy laser pulses: beam quality preservation and fibre endpreparation

This paper investigates the extension of optical fibre beam delivery to high-brightness applications, in particular laser percussion drilling, where both a good beam quality and high peak power are required. Beam quality preservation through a number of optical fibres is studied both experimentally and by using a ray propagation model. It is determined that in order to achieve the beam quality required for percussion drilling (M2<30) the largest fibre which can be used is 400µm diameter. The laser-induced damage threshold is measured for a number of 400µm fibres, and a CO2 laser-annealing technique is shown to increase the damage threshold by a factor of 10, allowing 28J, 1ms pulses to be transmitted.