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 &mgr;m, with 7.5-nsec pulse
duration, focused to a 7.45-&mgr;m-radius spot inside a fused silica window, to measure the single shot optical
breakdown threshold irradiances of 4.7E11 and 6.4E11 W/cm2 respectively for pure fused silica, and for a
1% Yb3+ doped fused silica preform of Liekki's Yb1200 fiber. These irradiances have been corrected for
self focusing which reduced the area of the focal spot by 10% relative to its low field value. Pulse to pulse
variations in the damage irradiance in pure silica was >2%. The damage induction time appears to be much
less than 1 ns.
We found the damage morphology was reproducible from pulse to pulse. To facilitate our morphology
study we developed a technique for locating the position of the focal waist based on the third harmonic
signal generated at the air-fused silica interface. This gives a precise location of the focal position (± 10
&mgr;m) which is important in interpreting the damage structure. The surface third harmonic method was also
used to determine the diameter of the focal waist.
Earlier reports have claimed the damage irradiance depends strongly on the size of the focal waist. We
varied the waist size to look for evidence of this effect, but to date we have found none. We have also
studied the temporal structure of the broadband light emitted upon optical breakdown. We find it consists
of two pulses, a short one of 16 ns duration, and a long one of several hundred ns. The brightness, spectra,
and time profiles of the white light provide clues to the nature of the material modification.