Thermal annealing of laser damage precursors on fused silica surfaces

Abstract

Previous studies have identified two significant precursors of laser damage on fused silica surfaces at fluenes below ~35J/cm 2 , photoactive impurities in the polishing layer and surface fractures. In the present work, isothermal heating is studied as a means of remediating the highly absorptive, defect structure associated with surface fractures. A series of Vickers indentations were applied to silica surfaces at loads between 0.5N and 10N creating fracture networks between ~10m and ~50m in diameter. The indentations were characterized prior to and following thermal annealing under various time and temperature conditions using confocal timeresolved photo-luminescence (CTP) imaging, and R/1 optical damage testing with 3ns, 355nm laser pulses. Significant improvements in the damage thresholds, together with corresponding reductions in CTP intensity, were observed at temperatures well below the glass transition temperature (T g ). For example, the damage threshold on 05.N indentations which typically initiates at fluences <8 J/cm 2, could be improved >35 J/cm 2 through the use of a ~750 °C thermal treatment. Larger fracture networks required longer or higher temperature treatment to achieve similar results. At an annealing temperature >1100°C, optical microscopy indicates morphological changes in some of the fracture structure of indentations, although remnants of the original fracture and significant deformation was still observed after thermal annealing. This study demonstrates the potential of using isothermal annealing as a means of improving the laser damage resistance of fused silica optical components. Similarly, it provides a means of further understanding the physics associated with optical damage and related mitigation processes.

Producing fused silica optics with high UV-damage resistance to nanosecond pulsed lasers

“…al. [21]. So it may be better to use HF-etching to enhance the damage resistance of cracks instead of annealing.…”

Full Aperture CO2Laser Process to Improve Laser Damage Resistance of Fused Silica Optical Surface

“…But the reason for this improvement is still uncertain. The possible mechanism includes three aspects: firstly, some defects might be eliminated because of laser annealing [11]; secondly, microstructure of the surface might be changed, such as the passivation of dangling bonds or the change of distance between atoms [12]; at last, the surface might be strengthened due to the compressive stress like the laser peening of metal [13]. In the future, systematical experiment will be carried out for confirming the exact mechanism.…”

Improving UV laser damage threshold of fused silica optics by wet chemical etching technique

“…Fused silica is prone to suffer from laser-induced damage when subjected to 355 nm laser irradiation, limiting the lifetime and performance of optics. Therefore, improving fused silica laser-induced damage threshold (LIDT) to enhance the lifetime and operation of optics under high UV fluence has been an research subject during the past decades [1,2] .…”