The optical-damage threshold in bulk fused silica is important for establishing the performance limits of high-power laser systems, but the mechanism is poorly understood. Now, researchers at Sandia National Laboratories (Albuquerque, NM) have established a rate equation governing catastrophic optical damage in fused silica. Sandia researcher Arlee Smith and colleagues measured optical damage thresholds of bulk fused silica at 1064 nm for 8 ns pulses from a single-mode Q-switched laser and 14 ps pulses from a mode-locked Q-switched laser focused to 7.5 mm spots and found damage thresholds of 3854 ± 85 J/cm2 and 25.4 ± 1.0 J/cm2 respectively, corrected for self-focusing.
The observations were described by a simple electron-avalanche model, which stated that the electron-avalanche coefficient, a, must overcome the large free-electron lifetime, t, to reach the threshold, in contrast to commonly published claims that the damage-threshold fluence scales as the square root of the pulse durations (t½) in the picosecond/nanosecond regime. The group reported a bulk-damage-threshold irradiance in fused silica of 475 × 109 W/cm2 ± 7% for linearly or circularly polarized light, at a pulse duration of 7.5 ns and a focal waist of 7.45 mm. Contact Jeff Koplow at [email protected].