Measurement of thermal energy coupling to metallic materials in millisecond laser based on optical diffraction

A method which is based on the theory of Fraunhofer diffraction, thermal expansion effect, as well as lumped parameter approximation is presented to measure the laser absorptance of metallic materials under the irradiation of pulse laser. Experiments were made in a vacuum condition, using a Q-switched Nd: YAG laser at wavelength of 1064 nm and pulse duration of 0.4 ms. Fine copper wires with bare and oxidized surfaces were respectively studied as samples. In order to eliminate the complicated influence of laser ablation, the intensities used in this work were all below the damage threshold of copper. With this approach the quantitative result of coupling efficiency is obtained by analyzing the fringe spacing of diffraction without the temperature measurement. The experimental result shows a dramatic increase in laser absorptance from 0.03 (bare copper) to 0.09 (oxidized copper), implying that the efficiency of laser energy coupling to metals can be influenced significantly by surface oxidation. In addition, the average temperature rise and diameter variation of samples are calculated by the proposed method and compared with the results of simulation. Furthermore, an additional test that painted sample wire get a permanent damage is performed and discussed.