Determination of laser damage initiation probability and growth on fused silica scratches Page: 4 of 10
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2. EXPERIMENTAL
2.1 Experiment overview
Since damage initiations at working fluences will be dominated by the scratch density on the fabricated optic we have
developed methods to prepare samples with realistic scratches. The samples once fabricated must be tested under the
desired laser conditions and then results must be quantified. This requires methodologies to measure scratch lengths and
widths and to identify damage induced by the laser shots.
2.2 Sample preparation
For off-line evaluation of various surface preparations it is convenient to utilize 2-inch fused silica substrates. The 1-cm
thick samples were scratched by sliding a sample over a dry polyurethane pad loaded with a number of fused silica
200 m spheres; this is illustrated in Figure la. The resulting scratches are easily revealed after a light etch as shown in
the micrograph of Figure lb. It should be noted that even the best conventionally finished optic has scratches which are
only revealed by a light etch. [2]
FS sample Polyurethane pad (Suba 550)
/ with 200sm silica spheres
Ctnnnn
(a)
' F;
! i F '
1mm . I _
S - - y
(b)
Figure 1. Scratch sample technique and micrograph.
Each sample prepared with this technique has a unique array of scratches but they have in common a distribution of
lengths and widths that encompass those scratches that might be present on a full size optic. The scratch morphology is
primarily composed of brittle, trailing Hertzian fractures. The method produces mostly parallel lines of scratches which
ease the measurement of lengths and widths of the ensemble. A typical part will have a total scratch length of over a
meter in the region that is ultimately tested with the laser as will be described below.
2.3 Laser testing
The laser damage testing procedure is an adaptation to our previously developed technique for studying isolated damage
sites [4]. In order to test a statistically representative scratch distribution we use the Optical Sciences Laser (OSL) [5]
which provides a 3-cm beam on target at fluence and pulse shapes of interest. The OSL is a master oscillator power
amplifier (MOPA) system producing 200 J of light at 1.053 m which is frequency converted to -100 J at 351 nm. Pulse
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Norton, M. A.; Carr, C. W.; Cross, D. A.; Negres, R. A.; Bude, J. D.; Steele, W. A. et al. Determination of laser damage initiation probability and growth on fused silica scratches, article, October 26, 2010; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc830278/m1/4/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.