Optical delay line nanometer level pathlength control law design for space-based interferometry

This article is concerned with the discussion of a control law design for a brassboard optical delay line (ODL) developed for the Interferometry Technology Program at the Jet Propulsion Laboratory to support the space-based optical interferometry missions. Variations on the ODL brassboard design will be flown on the Space Interferometry Mission and New Millenium Separated Spacecraft Interferometer (or Deep Space 3). The brassboard ODL was designed to meet both the performance and environmental requirements for space interferometry. A control experiment was contrived to evaluate how well the brassboard optical delay line can control optical pathlength jitter. Fringe visibility resolution requirements for space interferometry prescribe that the optical pathlength from the two collecting telescope apertures must be equal and stable to within a few nanometers RMS. This paper describes the classical frequency domain loop shaping techniques that were used to design a control law for the experiment. included is a description of a methodology for managing the control authority for the three actuation stages of the ODL, as well as, an input shaping technique for handling the large (1e9) dynamic range issues. Experimental performance results characterizing closed loop control of residual optical jitter in an ambient laboratory environment are reported.