Optical Delay Line Nanometer Level Pathlength Control Law Design For Space-Based Interferometry

The brassboard optical delay line was developed for the Interferometry Technology Program (ITP) at the Jet Propulsion Laboratory to support the space-based optical interferometry missions. This article is concerned with the discussion of control law design; implementation issues; and the quantification of closed loop performance results obtained in a laboratory experiment. 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 described brassboard ODL was designed to meet not only the performance requirements for space interferometry but to also meet environmental requirements. The role of the ODL to equalize the pathlength between two collecting telescopes of an interferometer and a central beam combiner in order to detect and acquire white light fringes is discussed. Fringe visibility resolution requirements for space interferometry ' prescribe that the optical pathlength from the two collecting apertures must be equal and stable to within a few nanometers RMS. A control experiment was contrived to evaluate how well the brassboard optical delay line can control optical pathlength jitter. The classical frequency domain loop shaping techniques that were used to design compensators are examined. Included is a description of a methodology for managing the control authority for the three actuation stages, as well as, an input shaping technique for handling the large (le9) dynamic range issues. Experimental results characterizing closed loop performance control of residual optical jitter in an ambient laboratory environment are included