We compared the 1064 nm surface damage thresholds of fused silica polished by
three different techniques:
1. A conventional polishing technique: that uses loose Alumina abrasives (lapping)
followed by a fine Cerium oxide polish.
2. An alumina polishing process producing surfaces very close to super polished.
3. Process 2 followed by a silica polish until the silica surfaces are super polished.
We employed the same measurement technique that proved successful for the bulk
damage threshold measurement to measure the damage thresholds of bare silica surfaces
polished by the above three polishing techniques. We used an 8-nanosecond, single
transverse and longitudinal mode pulsed laser, from a Q-switched Nd:YAG laser. We
used the surface third harmonic generation technique to precisely place the focus of the
laser beam on the surface of the fused silica window, and to measure the laser focus spot
size which was found to be 8 μm in radius.
We employed the same measurement techniques that have proven successful for bulk
damage thresholds measurements to measure damage thresholds of bare silica surfaces polished
using various methods and to measure damage thresholds for antireflection coated silica, again for
various surface polishes. Light in a single transverse and longitudinal mode, from a Q-switched
Nd:YAG laser is focused to an 8 µm spot on the front and rear surfaces of silica windows polished
using ceria, alumina, or alumina/silica to find the damage threshold. We repeated the exercise for the
same surfaces anti reflection coated with silica/hafnia film stacks. We used surface third harmonic
generation to precisely place the focus on the surfaces. Key findings include:
1. The surface damage threshold can be made equal to the bulk damage threshold. There is a
large difference in single-pulse damage thresholds of bare silica surfaces polished using
ceria, alumina, and alumina followed by silica. The ceria polished samples have a statistical
damage threshold ranging from 50 to 450 GW/cm2. The alumina polished surfaces damage at
200-500 GW/cm2, with half the spots damaging at the bulk threshold of 500 GW/cm2. The
windows polished by alumina followed by silica damage almost universally at the bulk
damage threshold of 500 GW/cm2.
2. There are strong conditioning effects for these surfaces. The ceria polished surfaces have
reduced thresholds for multiple pulses. The alumina polished surfaces attain the bulk damage
threshold at most locations using multiple pulse annealing.
3. The underlying polishes strongly affect the damage thresholds for the AR coatings. The
alumina plus silica polished samples have the highest thresholds, with statistical variations
from 150-380 GW/cm2. The alumina polished samples damage at only 50 GW/cm2, but with
annealing the threshold rises to 200 GW/cm2, while the ceria polished samples damage at 50-200 GW/cm2 with no strong multiple shot effect.
4. We found there was no beam size variation of the damage threshold irradiance for the bare
alumina/silica polished samples.
5. We showed that air breakdown does not limit the surface irradiance, silica breakdown does.
6. We recorded damage morphologies for the different surfaces.
Our objective is to understand the mechanism that generates catastrophic optical damage in pulsed fiber amplifiers. We measured optical damage thresholds of bulk fused silica at 1064 nm for 8 ns and 14 ps pulses. The 8 ns pulse is single longitudinal mode from a Q-switched laser, and the 14 ps pulse is from a Q-switched mode-lock laser. The beams in both cases are TEM00 mode, and they are focused to a 7.5 μm spot inside a fused silica window. The pulse-to-pulse energy variations are 1% for 8 ns pulses and 5% for 14 ps pulses. Under these conditions optical damage is always accompanied by plasma formation at the focal spot; we found the damage threshold fluences are 3854 ± 85 J/cm2 for the 8 ns pulses and 25.4 ± 1.0 J/cm2 for the 14 ps pulses. These fluences are corrected for self focusing. Both damage thresholds are deterministic, in contrast to the claim often made in the literature that optical damage is statistical in the nanosecond range. The measured damage threshold fluences for 8 ns and 14 ps pulses do not fit a square root of pulse duration scaling rule. We interpret the damage in terms of plasma formation initiated by multiphoton ionization and amplified by an electron avalanche. The damage threshold irradiance can be matched with a simple rate equation model that includes multiphoton ionization, electron avalanche, and electron-hole recombination. The damage morphologies are dramatically different in the nanosecond and picosecond cases because of the large difference in deposited energy. However, both morphologies are reproducible from pulse to pulse. We also measured surface damage thresholds for silica windows polished by different methods. We find that cerium oxide polished surfaces damage at approximately 40% of the bulk threshold, with a large statistical spread. Surfaces prepared using an Al2O3 polish damaged between 50% and 100% of the bulk damage limit, with a substantial fraction at 100%. Surfaces polished using first the Al2O3 polish and then an SiO2 polish exhibit surface damage values equal to the bulk damage value at nearly every point. We also measured damage thresholds for different sized focal spots. Some earlier reports have claimed that damage thresholds depend strongly on the size of the focal spot, but we find the surface threshold is independent of the spot size.