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We are interested in maximizing the performance of fiber lasers and amplifiers,
particularly for amplification of ps-ns pulses. The observed pulse energies from large
mode area fiber amplifiers routinely exceed the reported bulk damage threshold of silica.
We have undertaken a program to establish the intrinsic damage thresholds of silica that
are relevant for fiber applications. We use a single transverse / single longitudinal mode
Q-switched Nd:YAG laser focused to an 8-µm spot several Rayleigh ranges deep in silica
windows for the nanosecond measurement, and a Q-switched, mode locked Nd:YAG
laser for the picoseconds measurements. Our key findings include:
1. The damage threshold is deterministic rather than statistical for both ns and ps
pulses. The threshold varies less than 1% from location to location.
2. The intrinsic damage threshold of silica is 475±25 GW/cm2 (fluence = 3850
J/cm2) for 8 ns pulses and approximately 3 times higher for 14 ps pulses.
3. There is no difference in damage thresholds among Corning's A0, B1, C1, D1,
D2, and D5 grades of silica.
4. A tight focus is required to avoid large self focusing corrections and to avoid SBS
for the 8-ns pulses.
5. Damage morphologies are reproducible from pulse to pulse but change with focal
spot size and pulse duration. In all cases, damage appears to begin exactly at the
focus and then move upstream approximately one Rayleigh range.
6. The dependence of the damage threshold fluence on pulse duration is nearly linear
for pulse durations longer than 50 ps. The square root of duration dependence
reported by several investigators for the 50 ps to 10 ns range is refuted.
7. The variation of damage fluence with pulse duration from 20 fs to 20 ns and
beyond is well described by a single electron avalanche rate equation with three
fixed rates for the avalanche, multiphoton ionization, and electron recombination
terms.
8. Our damage threshold is consistent with the most reliable DC field breakdown
threshold.
9. We verified in detail the self focusing corrections and the SBS thresholds for our
measurement conditions.
10. The damage threshold is affected little by mechanical strain at levels similar to
those in polarization-preserving fiber.
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