Nulling 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 from 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 7x10-3. The test bed revealed the extreme challenges
of this technique and provided very valuable lessons to enable
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.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.