Multiple reflections and Fresnel absorption in an actual 3D keyhole during deep penetration laser welding

In laser welding experiments of glass, keyhole shapes are observed by two high-speed cameras from two perpendicular directions. From the obtained keyhole pictures, it can be seen that in medium- and low-speed laser penetration welding, the main distortion of the keyhole is not the section metamorphosis from rotational symmetry, but the bending of its centre line. Based on such a keyhole photograph, the keyhole profiles and its centre line are determined by the method of polynomial fitting. Then, under the assumption of a circular cross section at each depth of the keyhole, the behaviour of the laser beam in the keyhole is analysed by tracing a ray of light using geometrical optics theory; the Fresnel absorption and multiple reflections in the keyhole are systematically studied, and the laser intensities absorbed on the keyhole walls are calculated. The absorbed laser intensity is not distributed uniformly on the keyhole wall. The keyhole wall absorbs laser intensity mainly on the half-part of the keyhole wall near the front wall. Because of the high absorptivity of the glass, Fresnel absorption from the first incidence of a laser beam plays a dominant role in the final laser intensity distribution on the keyhole wall, multiple reflections have some minor effects on the intensity distribution on the bottom part of the keyhole.