TPF-I SWG Report - Exoplanet Exploration Program - NASA

T E C H N O L O G Y R OADMAP FOR TPF-I
2. The best broad-band mid-infrared results were obtained in January 2007 with the Adaptive Nuller
tesbed, noted in Fig. 6-1 as “Peters 2007.” A null depth of 50,000:1 was obtained with a 32%
bandwidth. This rejection ratio is only a factor of 2 from the 100,000:1 required for the stretched
X-array. Broad-band nulling results to date are therefore extremely promising. Also of particular
note is the result by Samuele et al. (2007) showing nulling at almost 10 -6 with a bandwidth of
15%. This result, at a wavelength 20 times shorter than mid-infrared wavelengths, suggests there
is no fundamental reason why similar null depths cannot be achieved in the mid-infrared.
The best broad-band mid-infrared null is arguably only a factor of two removed from Pre Phase A
requirements for TPF-I, if one takes into account the relaxation of requirements due to advances in our
understanding of instability noise. The literature references for the points noted in Fig. 6-1 are listed at
the end of this Chapter. Descriptions of the TPF-I starlight suppression testbeds are given in the
following pages.
Table 6-2 Comparison of 2005 Flight Requirements with Pre-Phase A Nulling Testbed
Requirements
Parameter
Flight
Performance
Achromatic
Nuller
Planet Detection
Testbed
Adaptive
Nuller
Null depth 7.5 × 10 -7 1 × 10 -6 1 × 10 -6 1 × 10 -5
Amplitude control 0.13% Derived 0.12% 0.2% (static)
Phase control 1.5 nm Derived 2 nm 5 nm (static)
Stability timescale 50,000 s + 100 s 5,000 s 6 h
Bandwidth 7–17 μm 25% λ = 10.6 μm 30 %
6.2.2 Achromatic Nulling Testbed (ANT)
Perhaps the most fundamental technical demonstration for TPF-I is to show that deep broad-band nulling
is possible. For the ANT, a bandwidth of 25% was chosen at a central wavelength of 10 μm and a target
null depth of 1 × 10 -6 . The nuller being used is a compact two-beam Mach-Zehnder interferometer using
opposite field flips in each arm with an arrangement of periscope mirrors. The interferometer is pictured
in Fig. 6-2. The ANT has also implemented a dispersive plate nuller and a through-focus nuller. The best
results to date have come from the periscope nuller, yielding broad-band nulls of 15,000:1 in unpolarized
light. Our efforts to characterize the limitations of the periscope nuller have shown that the null is not
dispersion limited; dispersion and chromatic effects are very well compensated within the nuller, and
decreasing the bandwidth of the light source from 25% to 10% does not improve the null. However, there
is evidence that polarization-dependent amplitude and phase errors are present in the testbed. Tests using
the same optics, 10-μm laser light, and mid-infrared polarizers have yielded null depths of 200,000:1. It is
difficult to model and predict these effects, as the predictions require knowledge of the material properties
of the beamsplitter coatings, which unfortunately remains proprietary. The laser nulling results suggest,
however, that a factor of 10 improvement in broad-band nulling is possible in the near future. This work
is now in progress.
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