Delivery of nanosecond pulses through hollow core photonic crystal fibres and the associated damage limitations

Hollow core photonic crystal fibres (HC-PCFs) show significant improvement over standard solid-core single-mode fibres and although short pulses (around 60 ns pulse width) and energies greater than 0.5 mJ were delivered in a single spatial mode through the hollow-core fibre, providing the pulse energy and high beam quality required for micro-machining of metals, the predicted performance (10's of mJ's) has not yet been achieved. The damage threshold limitations of the HC-PCF were investigated, both by coupling the laser into the fibre core, and by focusing the laser spot directly onto the photonic cladding structure surrounding the hollow core to elucidate the fundamental damage mechanism of this 'web-like' structure. For 1064nm delivery damage occurs exclusively at the launch end face with either partial or complete ablation of the photonic crystal cladding around the core. The pulse energies at which this occurs have been identified using Q-switched Nd:YAG lasers either pulsed from 10 Hz to 100 kHz (10 ns and 60 ns pulse width) or in single-shot mode to isolate the initial damage event. It is proposed that a contributing factor to the damage is the mode-mismatch between the gaussian profile of the incident laser beam and the fundamental mode of the HC-PCF (which is unlike that of conventional fibre). Pulse delivery and damage thresholds for HC-PCF designed for 532 nm operation are also reported. These fibres have noticeably lower damage thresholds compared with the 1064 nm fibre and in this instance damage occurs exclusively along the length of the fibre, yet appears to be independent of bend radius. It is proposed that these fibres may be failing at imperfections within the fibre introduced during the fabrication process.