Laser-induced damage of Kapton™ thin films demonstrating temperature and wavelength dependent absorptance: a case study in remote-sensing material analysis

摘要

Optical properties and laser damage characteristics of thin-film aluminized Kapton were investigated. Optical absorption of virgin and irradiated samples was measured from the Kapton side using a Cary 5000 Grating Spectrophotometer and an ABB/Bomem MB157S FTIR Spectrometer with a combined range of 0.2 to 15 μm at both room-temperature and 150°C. Laser-induced damage parameters of penetration time and maximum temperature were measured in a vacuum environment using an IPG Photonics continuous-wave solid-state laser operating at 1.07 μm and an electric-discharge CO_2 laser operating at 10.6 μm. Rather large differences in damage behavior at the two wavelengths were observed due to the variability in starting absorption properties between the NIR and LWIR. A FLIR Systems Quantum Well Infrared Photometer at 8-9.2 um was used to remotely examine the thin-film temperature evolution based on a known LWIR band of nearly-constant emissivity. A dual-detector FTIR spectrometer was also employed during testing in order to extract spectral emittance information from high-temperature irradiation exposures. Surface emittance was found to change after the material heated past approximately 500°C and during subsequent post-test cooling. This evolving spectral emittance with temperature successfully predicted increases in absorption that led to more rapid penetration times and higher heating rates at increased 1.07-μm laser power. A simplified one-dimensional thermal conduction and radiation model replicated the remotely-sensed temperature as a function of time in tests with constant absorptance and no material breakdown. With the result of evolving emittance data, this model could be modified to capture more realistic heating trends at higher irradiances whereby damage occurs and absorption properties vary spectrally.