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 z ero for the nulling output and to lambda/4 for the control, or the quadratu re, output. In the quadrature output a 1-nm path-length error corresponds t o a 1% output-power variation, thus allowing subanometer control. (C) 1999 Optical Society of America.