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 HfO 2 and SiO 2 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/cm 2 , on 500 lm 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 lm 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/m 2 ) 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 Á tan À1 ð ffi ffi t p =mÞ in the transition region between the two. V C 2014 AIP Publishing LLC.

Advanced optical interference filters based on metal and dielectric layers

“…Thus, large efforts have been placed on minimizing the absorption and scattering losses in these filters. Actually, many developments have been carried out towards the decrease of absorption losses, especially for high energy and high power lasers [11] and other efforts have been made in order to minimize scattering losses that can bring very large parasitic light especially in imaging systems for space or astronomy applications [10,12]. By the combination of two dielectric materials, it is then possible to design and fabricate most of lossless optical functions.…”