Noise reduction in an optical interferometer for picometer measurements

Picometer interferometry of moving objects is demonstrated experimentally over the frequency range 0 to 160 Hz. Active feedback is adopted to reduce the lowest frequency noise component due to coupling between geometrical fluctuations of the laser beam incident on the interferometer optics and imperfections in its optical elements. Passive measures to reduce the noise due to thermally induced changes of optical elements both in the geometrical arrangement and in the optical properties are introduced by decreasing the sensitivity to temperature change. With these methods the resultant noise is reduced to 3 pm