Optical heterodyne interferometry

Abstract

We describe a new approach to precision interferometry that is in principle different from previous techniques in that it is free from the systematic errors usually associated with interferometry. Optical heterodyne techniques are used to directly measure the empirical mode spacing of an interferometer over a broad (10%) frequency range. The frequency difference between any two modes can then be computed by summing the spacing of the modes that lie between them, in this way optical frequency differences of up to 50 THz can be measured without the usual corrections for mirror phase shift or diffraction. The heterodyne principle is realized by a dual frequency modulation technique which resonates 4-GHz sidebands of a stabilized dye laser with adjacent interferometer modes. The interferometer has a finesse of 20,000 and a linewidth of only 10 kHz, and measurements show that its mode spacing varies quadratically around its center frequency with a maximum deviation of a few parts in 10 −8 .