Multiple-Pulse Laser-Induced Damage To Metal Mirror Surfaces

The accumulation dependence of damage and laser induced damage morphologies on single crystal metal surfaces have been observed under Q-switched 1064 nm Nd:YAG laser irradiation at 10 nsec pulses at a 10 Hz rate. Several different damage morphologies were observed: slip-line formation, ripple patterns, flat-melting, and boiling. Damage probability versus fluence curves and accumulation curves are plotted to investigate damage behavior as it correlates with morphology. Accumulation curves showed different damage behavior for crystals of different orientation. Threshold decrease due to accumulation was the largest on (111) Cu and Al surfaces, whidh also had the lowest single shot damage threshold. The threshold reduction in accumulation follows the equation FN = F1 NS-1 where FN is the N-pulse damage threshold, N the pulse number, and S the slope of the accumulation curve. Accumulation appears to be the result of the storage cycle of thermal stress-strain energy induced by each laser pulse, and multi-pulse damage results from plastic slip and fatigue.

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