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A polarization interferometer which measures optical path differences with improved accuracy control is presented. The approach makes use of four signals in quadrature, computing the phase with an algorithm insensitive to laser power drifts. Experimental results of the interferometer’s performance are given under laser warmup conditions, and with long-term monitoring of optical path differences.

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We discuss a simple experiment for detecting small deformations by speckle interferometry. The optical setup uses a laser diode for the source together with a lens to expand the laser beam and a beam splitter dividing the beam and illuminating two scattering surfaces. A low-cost, commercial charge coupled device ͑CCD͒ photocamera provides images of the two superimposed speckle fields through the beam splitter. We first take a picture with the system at rest, and then take a second one after a deformation is made in a surface. By simple subtraction of the digital pictures, we obtain a fringe pattern that gives us information about the deformation.

Cases of long-term deformation of fused silica flats are reported. The phenomenon is detected at the scale of the nanometer, and exhibits a time constant of the order of 9 years. The observed deformation appears related to gravity and constraints, but a change of physical properties locally resulting in non-homothetic behavior is also hypothesized.