Effect of thermal anneal on growth behavior of laser-induced damage sites on the exit surface of fused silica

Rajesh N Raman,Christopher W Carr,Raluca A Negres,Manyalibo J Matthews

doi:10.1364/ome.3.000765

Abstract

Thermal anneal is known to arrest the growth of laser-induced damage in optical materials. However, the response of the material which leads to this observed behavior is poorly understood. In this work, we investigate the effect of isothermal anneal at 1100°C for 12 hours on the growth rate of laser-induced damage sites in fused silica. Growth rate was significantly lower for annealed initiated damage sites than that for untreated sites. This decrease in growth rate was associated with the closure of small surface and subsurface cracks, suggesting that aggressive growth rate is due, at least in part, to subsurface fracture complexity.

Highlights

While advances have been made in bulk purification and surface finishing methods for optical materials [(Menapace, Penetrante et al 2002), (Suratwala, Miller et al 2011)], localized damage on optical components used in high power laser systems continues to limit their operational lifetime
One way to minimize size effects is to grow sites at lower fluences so as to keep them within the same size range and monitor growth behavior over successive shots; the smaller change in size associated with the lower growth rate would compromise the signal-to-noise ratio of the experiment
This study showed that a thermal anneal of 1100°C produces a pronounced reduction in the growth rate of freshly initiated sites but not in sites which have been grown with several laser exposures

Summary

Introduction

While advances have been made in bulk purification and surface finishing methods for optical materials [(Menapace, Penetrante et al 2002), (Suratwala, Miller et al 2011)], localized damage on optical components used in high power laser systems continues to limit their operational lifetime. The initial size of such damage sites is highly dependent on laser conditions [(Carr, Trenholme et al 2007), (Carr, Cross et al 2011)] but range from a few microns to a few tens of microns for nanosecond pulses As initiated, these sites are sufficiently sparse that they would pose no significant threat to beam throughput were it not for their propensity to grow exponentially in diameter upon exposure to subsequent pulses (damage growth) [(Norton, Hrubesh et al 2001), (Raze, Morchain et al 2003), (Huang, Han et al 2009), (Negres, Norton et al 2010)]. Sites with nominally identical diameter exposed to nearly identical laser pulses can grow at different rates, and the distributions of these growth rates have been documented [(Negres, Norton et al 2009)] This observation requires that additional innate characteristics of the site, apart from surface diameter, govern growth behavior. Damage site structure may relate to how fast it will grow upon subsequent laser exposure

Methods

Results

Conclusion