High power laser heating of low absorption materials
Abstract
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 SiO2 were 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.
Document Details
- Document Type
- Pub Defense Publication
- Publication Date
- Sep 25, 2014
- Source ID
- 10.1063/1.4896750
Entities
People
- A. Ogloza
- J. Talghader
- James P. Thomas
- K. Olson
Organizations
- Naval Postgraduate School
- Office of Naval Research
- Pennsylvania State University
- University of Minnesota