<title>Multipulse laser-induced failure prediction for Mo metal mirrors</title>

Abstract

In combination with known thermomechanical fatigue data for Mo we have applied the Transient Photothermal Deflection (TPD) technique to develop a model f the N-on-i damage ofMo mirrors to predict their multipulse lifetimes. In laser-damage experiments to verify the model mechanically polished Mo mirrors were irradiated with 10 ns Nd:YAG laser pulses at 1064 nm at a 10 Hz rep rate. In the TPD experiments the approximately 600 jtm diameter Nd:YAG laser spot was probed off axis by a smaller HeNe laser beam whose deflection was detected by a fast bicell photodetector and amplifier. Digitized photodetector waveforms indicated that the surface angular deflection could be converted into surfae disp1zement. In addition thermal modelling of the vertical heat distribution enabled the peak surface-deflection signal to be converted into peak surfwe temperawre. The thermomechanical model was verified by both the experimental and model results. Conventional mechanical fatigue data for Mo were used to derive a predictive equation for the laser-accumulation lifetime of Mo mirrors. Experiments were performed with one to 1O'' pulses per site yielding laser-damage thresholds and accumulation curves. The accumulation behavior predicted from measurennts of mechanical fatigue was in excellent agreement with the measured behavior. It is possible that data on high-cycle ( mechanical fatigue can be used to predict the performance of optical surfaces at equally large values of N.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.