A displacement-measuring instrument utilizing self-mixing interferometry

We develop a self-mixing laser interferometer for the measurement of displacements on a generic target surface. The measurement is based on the bright-speckle tracking, a technique we have recently proposed to solve amplitude fading associated with the speckle statistics when the displacement to be covered is well in excess of the speckle longitudinal size. We implement the dynamical tracking of speckle maxima with piezo actuators and a phase-sensing loop. Also, we use an automatic gain control, based on a liquid crystal attenuator, to improve the amplitude statistics. Details of digital signal acquisition with adaptive signal processing through a field programmable gate array are discussed. The resulting instrument offers sub-(mu)m resolution in the measurement of displacement up to 500 mm of total shift, has virtually no need for alignment, and has very relaxed target-surface requisites, yet works with a very simple and inexpensive set-up.