TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 6
6.2.4 Adaptive Nuller Testbed (AdN)
Deep broadband nulls require that the amplitude and phase of the electric field from each arm of the
interferometer are very carefully matched over the full range of wavelengths and two polarization states.
This becomes very challenging with large beamtrains — the tolerances on the surface figure and coatings
of the many optical components is very tight. The Adaptive Nuller (Peters et al 2006) is a device inserted
into each beamtrain that compensates the amplitude and phase errors independently for each spectral
channel and polarization state. It effectively converts the broadband nulling challenge into a parallel set of
narrow-band nulls, and enables robust deep nulling with mismatched optics and coatings. A deformable
mirror is used to compensate amplitude and phase individually at each wavelength and polarization. The
concept was first tested successfully at near-infrared wavelengths before moving to the mid-infrared
system shown in Fig. 6-5. Testing is ongoing, but a null of 50,000:1 has already been demonstrated with
mismatched optics over a bandwidth of 8–12 microns. Extending this to larger bandwidths should be
simply a matter of adding more actuators to the deformable mirror.
Figure 6-5. The Adaptive Nuller Testbed (AdN). The Adaptive Nuller has as its goal to demonstrate
phase and intensity compensation of beams within a nulling interferometer to a level of 0.12% in
amplitude and 5 nm in phase. This version of the AdN operates over a wavelength range of 8–12 μm.
The AdN separates the two linear polarizations and produces spectra that are imaged one above the
other at the surface of a deformable mirror. The long-focus parabolas, used in AdN are seen on the
upper right. The PI of the AdN is Robert Peters.
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