High-accuracy Fabry-Perot displacement interferometry using fiber lasers

We describe ongoing work in a Fabry-Perot interferometry system designed to measure displacements over a range of 50 mm with sub-pm uncertainty. The apparatus involves probing two nearby modes of the Fabry-Perot cavity with narrow-linewidth fiber telecom lasers and measuring both the mode spacing and the absolute mode frequencies relative to a third, frequency-stabilized, fiber laser. We explore the improvement in resolution obtained as the frequency separation between the two modes is increased and employ an internal consistency requirement to infer the magnitude of residual systematic errors. The measurement uncertainty is sufficiently small that we are easily able to see the Gouy phase shift as the cavity length is changed over several millimeters.