The objective of this work is to understand catastrophic optical damage in nanosecond pulsed fiber amplifiers. We used a pulsed, single longitudinal mode, TEM00 laser at 1.064 Âµm, with 7.5-nsec pulse duration, focused to a 7.45-μm-radius spot in bulk fused silica. Our bulk damage threshold irradiance is corrected to account for self focusing. The pulse to pulse variation in the damage irradiance in pure silica is less than 1%. Damage is nearly instantaneous, with an induction time much less than 1 ns. These observations are consistent with an electron avalanche rate equation model, using reasonable rate coefficients. The bulk optical breakdown threshold irradiance of pure fused silica is 5.0x1011 ±7% Watts/cm2. We also measured the surface damage threshold irradiance of 1% Yb3+ doped fused silica preform of Liekki Yb1200 fiber, and found it is equal to that of pure silica within 2%. The optical damage morphology is reproducible from pulse to pulse. To facilitate the morphology study we developed a technique for locating the laser focus based on the third harmonic signal generated at the air-fused silica interface. This gives a very small uncertainty in focal position (~ 10 μm) which is important in interpreting the damage structure. The surface third harmonic method was also used to determine the laser focus spot size and verify beam quality. Earlier reports have claimed that the damage irradiance depends strongly on the size of the focal spot. We varied the focal volume to look for evidence of this effect, but found none.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.