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 hulling 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 λ/4 for the control, or the quadrature, output. In the quadrature output a l-nm path-length error corresponds to a 1% output-power variation, thus allowing subnanometer control.
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