Experimental results from a visible nulling interferometer

Direct detection of planets around nearby stars requires the suppression of the parent starlight to many orders of magnitude (9 orders for visible, 6 for the MidIR). Myriad techniques have been proposed to accomplish this task including: pupil masking, coronagraphy and nulling interferometry. All of these techniques are currently undergoing rapid development. Nulling interferometry in the visible part of the spectrum is viewed as particularly difficult for two reasons: 1) fundamentally, the planet/star contrast is greater by roughly three orders of magnitude and 2) practically speaking, the expected residual fabrication errors produce larger phase error at shorter wavelengths. For these reasons, nulling interferometry experiments are currently planned for 7-12 /spl mu/m, where both of these factors are mitigated. However, progress may not be smooth at these mid IR wavelengths due to the lack of readily available components, and the difficulty of working with a large thermal background. Therefore, nulling interferometry in the visible continues to be compelling, particularly if it can be done deeply. Our team has proposed an instrument known as a visible nulling coronagraph. This instrument resides behind a single aperture and provides deep and stable interferometric nulls that are consistent with the requirements for direct detection. Its two key components are a nulling interferometer and a single mode fiber array (for reducing scattered light after the interferometer). In this paper, we first summarize the benefits of a nulling coronagraph. We subsequently show experimental measurements made with the visible nulling interferometer. We have demonstrated the feasibility of the shearing nulling concept, and are the first to control the null with a (laser) metrology system. As a result of these experiments, the visible nulling coronagraph continues to be a compelling technique for planet detection and spectroscopy.