Final laboratory results of visible nulling with dielectric plates

Nullng stellar interferometry may enable the discovery of earth-like planets around other stars. In nulling mode, the zero order fringe is destructive and on axis, thus cancelling light form a bright source and allowing detection of dimer off-axis features. To create a deep on-axis null, the phase must be shifted half a wave achromatically over a broad band. The phase shift is created by adding optical path thickness with dielectric plates. Plates of different materials can balance dispersion. The nulling solutions found for TPF (infrared) and SIM (visible)are promising. This paper presents the final results of a dissertation that developed a nulling beam combiner testbed. The deepest null achieved over the spectral region of 600 to 800 nm was 7×10^-3. The test bed revealed the extreme challenges of this technique and provided very valuable lessons to enable further implementations. The testbed first achromatized the null by actively controlling the optical thicknesses of the plates. The phase as a function of wavelength was measured by PSI on a spectrally dispersed fringe. The phase was fit to a model to determine the optical thicknesses. The eigenfunctions of the model were nearly collinear and consequently the dynamic range required of the phase data was very high and not supported by the hardware. The testbed then searched for the null fringe and locked on the null using a 300 Hz servo loop and on a grey fringe. The OPD was stabilized to 6 nm peak-to valley.