An optical dilatometer for picometre displacements

A HeNe laser driven optical dilatometer with differential properties, which can measure displacements with a resolution of 10(-10) m over the flat fre quency range from 0 to 1 kHz, is described. Its principal feature is a para llel arrangement of two, mirror symmetrical, optical paths of which one has an actively operated measurement sample to be studied and the other a simi lar, passive sample. The arrangement ensures good short- and long-term stab ility, thus allowing coherent averaging of sample responses over extended p eriods of time. Resolution thereby increases to 10(-12) m when averaging ov er 10 000 responses. Since both channels are about equally affected by envi ronmental mechanical disturbances, an algorithm compares the outputs from t he parallel channels and rejects correlated vibration and noise components. This increases resolution by an additional factor of five under a reductio n in the frequency response. It brings the total obtainable resolution of t he dilatometer to 2 x 10(-13) m. The instrument is low cost and operates at normal room temperature, pressur e and humidity. Since the spring constant of the lever sensing dilatation i s 0.2 N m(-1), 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.