Growth of laser damage in fused silica: diameter to depth ratio Page: 4 of 11
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follows a different path to the beam combiner located before the test chamber, the individual path lengths are controlled
to keep the arrival times simultaneous within a few nanoseconds. Three important characteristics of the test beams result
from using this scheme: first, the depleted fundamental spatial profile tends to have better spatial uniformity than the
unconverted beam, second, the temporal pulse width of the depleted fundamental is longer than the unconverted beam
third, the relative polarizations of the frequency converted beam and the depleted fundamental are parallel to each other.
Both wavelengths are spatially filtered before they are recombined at the sample chamber. The key components of the
layout for these experiments with the SLAB laser are shown in figure 1. The sample is located in an image relay plane of
the laser and the beam size for all wavelengths on the sample in the test chamber is nominally 5 mm x 5 mm. The beams
are spatially and temporally overlapped at the sample. The laser is incident on the sample at 150. The fluence of each
wavelength is independently controlled with a polarizer/wave plate combination.
1053 m input tun o ng working distance microscope
351M 10531
Samaple
1053 rmn spa1a filter
Fequency
conrerter
Cori~ni fidntcal 351 rm spatial filter 351 n 1053
mpucaMlera mput mera 3 c
Figure 1. Schematic layout for the experiments.
The sample is housed in a stainless steel vacuum chamber, which is located in a class 100 area where samples up to
150 mm x 150 mm in size are handled during loading. The tests are conducted in air, at 2.5 torr, by introducing dry
filtered high purity air into the chamber after it is pumped out to vacuum.
Laser beam measurements for both wavelengths on the part include measurements of the temporal pulse shapes, energy,
and incident beam spatial intensity distributions measured with 16-bit scientific-grade CCD cameras. The cameras are
calibrated both for energy and for magnification and are used to set and record the fluence on the sample for each shot.
Diagnostics to measure the growth include a white light illuminated, long working distance microscope and CCD camera
and a scientific grade CCD camera recording the image of the damage site using the transmitted laser beam. In practice,
the camera viewing the beam transmitted through the sample is used to locate the starting damage and the input cameras
are used to set the local fluence in a 1-mm patch surrounding the site. The lateral growth of the damage site can be
measured either from the transmitted camera or from the microscope. For the measurements reported here both the
lateral and the depth measurements are made off line using a high resolution optical microscope. A typical image of the
converted beam and temporal waveforms on the sample for the combined 351 nm and 1053 nm tests are shown in figure
2a and 2b respectively. The calculated statistics for the 351 nm beam shown in figure 2a yield a contrast ratio
(rms/average) of 18% over the central 60% of the beam area and ~8% in the 1-mm patch surrounding the growing site.
The beam contrast for the depleted beam is somewhat better. Typical temporal waveforms are shown in figure 2b, with
the average FWHM=11 ns for the 351 nm beam and 18 ns for the 1053 nm beam; also seen in the plot is the relative time
of arrival of the two beams with the 1053 nm beam arriving approximately 2 ns before the 351 nm beam.
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Norton, M. A.; Adams, J. J.; Carr, C. W.; Donohue, E. E.; Feit, M. D.; Hackel, R. P. et al. Growth of laser damage in fused silica: diameter to depth ratio, article, October 29, 2007; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc894932/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.