Research of pulsed laser induced ripple damage on the surface of copper film mirror

The damaged spot of copper film mirror irradiated by pulsed Nd:YAG laser is analyzed using an optical microscope. The periodic damage ring is found at the laser irradiation spot, the average spacing of these rings is about 45μm. The damage ring can be observed obviously only in the periphery of the laser spot under lower laser energy density； as laser fluence increases, the periodic damage rings cover the whole laser spot； under higher laser energy density, the whole film is damaged and the rings disappear. A Fresnel diffraction model is used to explain the appearance of these rings. The laser beam is diffracted by the aperture in the optic path, laser intensity distribution at the copper film mirror becomes periodic rings and damage ripple pattern forms instantly with the laser ablatio