Sub-nanometer digital positioning of large bodies by Fabry-Pérot interferometry

Sub-nanometer distance stabilization and positioning of large optical structures is a key active control issue for future space telescopes needing picoradian precision. The paper presents solutions and results of a leading experiment, named COSI (Control Optics Structure Interaction), aimed at investigating Fabry-Pérot cavities as active optical links, capable of stiffening large and massive structures operating in vacuum. The optical length of each cavity is actuated by a PZT-driven mechanism and the cavity ensemble is controlled by a model-based Digital Control Unit. The first experiment stabilized the relative tip-tilt motion of two 6.9 kg annular plates, 0.5 m distant. A residual error, lower than 3t-½ pm (1s), t>0.1s, was achieved in presence of severe environment noise and artificial micrometer distance variations, thus fully demonstrating feasibility of COSI concept and technology.