TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 4
Null depth
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
1.E-08
6 8 10 12
Wavelength (um)
Figure 4-12. Calculated null depth across the band for NaCl and AgCl wedges between 6 and 13 μm.
A second key component of a beam combiner design is the field-inversion system which inverts the
electric field of one of each pair of incoming beams by exactly π across the waveband, allowing the beam
combiner to form a null (destructive fringe) rather than a constructive fringe. Several different methods of
field inversion have been proposed, and at least two have been tested in the laboratory. One method uses a
periscope-like arrangement of mirrors to produce the field inversion. This has the advantage of being
effective across a very wide waveband. A second method uses pieces of glass of carefully controlled
thickness known as phase plates to produce the field inversion. Figure 4-12 shows the calculated null
depth for a set of silver chloride (AgCl) and sodium chloride (NaCl) phase plates. Other glass
combinations potentially yielding deep nulls are also known.
A third key component for the beamcombiner is the ability to control the optical path and maintain the
null fringe on the star. Light at a different wavelength from the nulling band is used for this purpose, for
example between 2 and 4 μm. If this light follows as much of the beam train as possible, then the phase of
the null fringe can be better controlled, leading to deeper more stable nulls, important for faint planet
detection. In the flight design study described next, the fringe tracking light follows the science path
almost exactly and is interfered on the same beamsplitters as the nulling light.
4.6.1 Co-axial Combiners for a Formation-Flying Interferometer
For the formation-flying interferometer design work, a simplified nuller was used rather than the highly
symmetric MMZ design. The main benefits would be that only one internal laser metrology beam would
be needed for each beam of the interferometer rather than two, the attenuation or round-trip insertion loss
would be less for the metrology light, and the science light would emerge on only two beams rather than
four, offering a small but significant improvement in signal-to-noise ratio at the detector. There would
also be associated engineering simplifications. Because of the wide spectral band to be covered by TPF-I,
the nuller is actually divided into two units stacked next to one another, one operating on the wavelength
range 7 to 11 μm and the other on the range 11 to 17 μm. This arrangement necessitates an additional
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