Simple method of measuring vibration amplitudes at better than nanometer sensitivity

It is shown that vibrations driven at fixed frequencies of a few kilohertz can be studied, with displacement sensitivities in the nanometer range, by thermally modulating the length of one arm of a Michelson interferometer, and analyzing the photomultiplier output with a sampling oscilloscope and a spectrum analyzer. This method of measurement has the advantage that the high sensitivity can be attained without a constant temperature enclosure for the interferometer.