Exploring the World of Micromaterials Using Laser-Speckle Techniques

Laser speckle based methods for measuring strain within specimen have been devised by several authors using a wide variety of optical arrangements.1–3 Almost all of the proposed methods aim at measuring strain over an extended baselength given, in case of the laser speckle strain gauge4 by the distance at the specimens surface of two impinging beams of laser light that is usually on the order of 5 mm to 50 mm. Others reported on encouraging results using a set–up employing a single illuminated spot at the specimens surface.1, 5 Still the extend the mechanical strain is averaged over is given by the beam diameter which using HeNe lasers is somewhat limited to approximately 1 mm. In this proposed paper we report on the development and application of a laser speckle shift strain sensor that employs a laser beam focussed down to only several tens of micrometers thus allowing a very localized strain reading.5 Although as is known from the fourier optical analysis the average speckle size is inversely proportional to the spot diameter and directly proportional to the projection distance by miniaturizing the sensor a true microscopic strain gauge can be devised. Thus some problems in material physics can by addressed, like measuring strain — mostly caused by thermal imbalance — within an extended micro chip, or measuring mechanical strain within thin fibres or foils, or determining strain caused by the mismatch of thermal expansion coefficients between a copper substrate and AgSn solder in electronic circuits, where averaging the strain reading over extended strain fields would definitely underestimate true mechanical (over–) loads that could lead to catastrophic failures.