Exploring the World of Micromaterials Using Laser-Speckle Techniques
Sprache des Titels:
Englisch
Original Buchtitel:
Speckles, from Grains to Flowers
Original Kurzfassung:
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.