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Title: High power laser heating of low absorption materials

A model is presented and confirmed experimentally that explains the anomalous behavior observed in continuous wave (CW) excitation of thermally isolated optics. Distributed Bragg Reflector (DBR) high reflective optical thin film coatings of HfO₂ and SiO₂were prepared with a very low absorption, about 7 ppm, measured by photothermal common-path interferometry. When illuminated with a 17 kW CW laser for 30 s, the coatings survived peak irradiances of 13 MW/cm², on 500 μm diameter spot cross sections. The temperature profile of the optical surfaces was measured using a calibrated thermal imaging camera for illuminated spot sizes ranging from 500 μm to 5 mm; about the same peak temperatures were recorded regardless of spot size. This phenomenon is explained by solving the heat equation for an optic of finite dimensions and taking into account the non-idealities of the experiment. An analytical result is also derived showing the relationship between millisecond pulse to CW laser operation where (1) the heating is proportional to the laser irradiance (W/m²) for millisecond pulses, (2) the heating is proportional to the beam radius (W/m) for CW, and (3) the heating is proportional to W/m∙ tan⁻¹(√(t)/m) in the transition region between the two.
Authors:
;  [1] ;  [2] ;  [3]
  1. Electrical Engineering, University of Minnesota, Minneapolis, Minnesota 55455 (United States)
  2. Naval Postgraduate School, 1 University Cir, Monterey, California 93943 (United States)
  3. Electro Optics Center, Pennsylvania State University, 222 Northpointe Blvd., Freeport, Pennsylvania 16229 (United States)
Publication Date:
OSTI Identifier:
22305688
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 116; Journal Issue: 12; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ABSORPTION; COATINGS; CROSS SECTIONS; EQUATIONS; EXCITATION; HAFNIUM OXIDES; INTERFEROMETRY; LASER RADIATION; LASER-RADIATION HEATING; OPERATION; RADIANT FLUX DENSITY; SILICON OXIDES; SURFACES; TANTALUM NITRIDES; THIN FILMS