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An optical dilatometer for picometre displacements

A HeNe laser driven optical dilatometer with differential properties, which can measure displacements with a resolution of m over the flat frequency range from 0 to 1 kHz, is described. Its principal feature is a parallel arrangement of two, mirror symmetrical, optical paths of which one has an actively operated measurement sample to be studied and the other a similar, passive sample. The arrangement ensures good short- and long-term stability, thus allowing coherent averaging of sample responses over extended periods of time. Resolution thereby increases to m when averaging over 10 000 responses. Since both channels are about equally affected by environmental mechanical disturbances, an algorithm compares the outputs from the parallel channels and rejects correlated vibration and noise components. This increases resolution by an additional factor of five under a reduction in the frequency response. It brings the total obtainable resolution of the dilatometer to m. The instrument is low cost and operates at normal room temperature, pressure and humidity. Since the spring constant of the lever sensing dilatation is , thus exerting negligible force on the sample, the instrument is particularly suitable for the study of small (piezoelectric) strains of soft biological tissues, but is not limited to such applications.