Moiré Interferometry Analysis of Laser Weld Induced Thermal Strain

Abstract

An experimental method is presented to study laser weld induced thermal strain using digital image analysis enhanced moiré interferometry. A phenomenon that occurs in the assembly of optical components is that the final optimum coupled power will randomly change upon completion of the laser weld process. The change in power is due to residual thermal stresses being generated in the welded components. For single mode devices, relative motions of the components in the order of 1 μm could result in a 1 dB degradation of coupled power. The behavior of thermal strain is unpredictable since the relative orientation of the optical components at the optimum alignment is random. The goal of this investigation was to perform a baseline study of parameters affecting laser weld thermal strain. The first phase of the work was to study thermal strain induced by a single weld on a flat Kovar plate. The results show that thermal strain is independent of material inhomogeneity. However, this investigation did reveal asymmetry of the power distribution in the weld laser with a principal axes offset +30 deg from horizontal. The second phase of the experiment was to characterize thermal strain resulting from welding on an interface of two Kovar plates. The results indicate that thermal strain at the center of two welds is not affected by welds that are greater than 1 mm apart. Also, thermal strain levels at locations adjacent to the weld are not significantly affected by weld separation distance. This study successfully demonstrated that digital image analysis enhanced moiré interferometry can be used in the study of laser weld induced thermal strain. Digital image processing, fractional fringe analysis, and high frequency specimen gratings increase sensitivity levels to enable the technique to be used to characterize submicron thermal distortions.