Continuous-wave laser particle conditioning: thresholds and time scales
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Authors
Brown, Andrew
Ogloza, Albert
Olson, Kyle
Talghader, Joseph
Subjects
Laser damage
Laser conditioning
Continuous wave
Laser conditioning
Continuous wave
Advisors
Date of Issue
2017
Date
Publisher
Elsevier
Language
Abstract
The optical absorption of contaminants on high reflectivity mirrors was measured using photo thermal
common-path interferometry before and after exposure to high power continuous-wave laser light. The
contaminants were micron-sized graphite flakes on hafnia-silica distributed Bragg reflectors
illuminated by a ytterbium-doped fiber laser. After one-second periods of exposure, the mirrors
demonstrated reduced absorption for irradiances as low as 11 kW cm−2 and had an obvious threshold
near 20 kW cm−2. Final absorption values were reduced by up to 90% of their initial value for
irradiances of 92 kW cm−2. For shorter pulses at 34 kW cm−2, a minimum exposure time required to
begin absorption reduction was found between 100 μs and 200 μs, with particles reaching their final
minimum absorption value within 300 ms. Microscope images of the surface showed agglomerated
particles fragmenting with some being removed completely, probably by evaporation for exposures
between 200 μs to 10 ms. Exposures of 100 ms and longer left behind a thin semi-transparent
residue, covering much of the conditioned area. An order of magnitude estimate of the time
necessary to begin altering the surface contaminants (also known as ”conditioning”) indicates about
200 μs seconds at 34 kW cm−2, based on heating an average carbon particle to its sublimation
temperature including energy loss to thermal contact and radiation. This estimation is close to the
observed exposure time required to begin absorption reduction.
Type
Article
Description
The article of record as published may be found at http://dx.doi.org/10.1016/j.optlastec.2016.09.030
Series/Report No
Department
Electrical Engineering
Organization
Naval Postgraduate School (U.S.)
Identifiers
NPS Report Number
Sponsors
Office of Naval research (ONR)
Funding
N00014-12-1-1030 (ONR)
Format
4 p.
Citation
A. Brown, A. Ogloza, K. Olson, J. Talghader, "Continuous-wave laser particle conditioning: thresholds and time scales," Optics and Laser Technology, v. 89 (2017), pp. 27-30.
Distribution Statement
Rights
This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.