23 January 2007 Nanosecond laser-induced breakdown in pure and Yb3+-doped fused silica
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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.
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Arlee V. Smith, Arlee V. Smith, Binh Do, Binh Do, Mikko Soderlund, Mikko Soderlund, } "Nanosecond laser-induced breakdown in pure and Yb3+-doped fused silica", Proc. SPIE 6403, Laser-Induced Damage in Optical Materials: 2006, 640321 (23 January 2007); doi: 10.1117/12.696261; https://doi.org/10.1117/12.696261

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