Nanometer-level path-length control scheme for nulling interferometry

A method of stabilizing a dual-output rotational shearing inteferometer to the nanometer-level accuracy required for deep starlight nulling in planet searches is presented. In this approach one of the nulling beam combiner s two balanced outputs is used to control the other through a combination of external and internal path-length offsets. The path-length offsets sum to zero for the nulling output and to lambda/4 for the control, or the quadrature, output. In the quadrature output a 1-nm path-length error corresponds to a 1% output-power variation, thus allowing subnanometer control.