TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Instrument Technology and Advanced Concepts
and is intended ultimately to demonstrate 10 -10 light suppression to 3λ/D or better. The testbed
demonstrates proof-of-concept control of the amplitude, phase, spectral band pass, and
polarization of the light to achieve target performance levels.
Approach
The visible nulling testbed achieves high contrast imaging via interferometry. 35,36,37 Using the
telescope pupil, we synthesize a “nulling interferometer-based coronagraph” by dividing the light
into two or more copies, applying π phase changes to selected copies, and recombining them
with a lateral shear proportional to the required baseline. The pupil overlap region is then
projected into the far-field, i.e., sent to an image plane, so that the resulting image is the
superposition of the star and planet system with an interference fringe pattern. The star is in the
dark portion of the fringe and is deeply attenuated, whereas the planet falls within the light or
unattenuated location of the fringe.
Previous nulling experiments used a rotational shearing interferometer and a single mode optical
fiber. 38,39 For TPF applications, rotational shearing is not acceptable because of the multiple
baselines induced. Consequently, a linear shear is introduced via a modified Mach-Zehnder
interferometer. 40
A coherent array of single mode fibers filters starlight over a wide field of view. 41 Its principal
function is loosely analogous to the filtering of scattered starlight in the Lyot plane of a
‘conventional’ coronagraph. Starlight that leaks past the diffraction suppression of the nulling
interferometer is spatially filtered by each optical fiber in the array while the planet light is
allowed to propagate without attenuation. The residual leaked starlight is also incoherent with
planet light, thus the filtered planet light will focus into an image (a single pixel in the field)
while the leaked starlight will be evenly distributed over the field of view. Thus to achieve the
10 -10 contrast between star and planet, it is sufficient for the nuller to operate at 10 -7 with the
residual light spread over 1000 sub-apertures in the single mode fiber array (SMFA). 42 This
fiber array may also be a valuable component in a conventional coronagraphic imaging system.
35 Angel, R. (1990), “Use of a 16-m Telescope to Detect Earthlike Planets,” Proceedings of the Workshop
on The Next Generation Space Telescope, P. Bely and C. Burrows, eds., Space Telescope Science
Institute, pp. 81–94.
36 Angel, J.R.P, and Woolf, N.J. (1997),”An Imaging Nulling Interferometer to Study Extrasolar
Planets,”Astrophysical Journal, v475, pp. 373-379.
37 Shao, M., (1991), “Hubble Extra Solar Planet Interferometer,” SPIE v1494.
38 Serabyn, E., Wallace, J.K., Hardy, G.J., Schwindthin, E.G.H., and Nguyen (1999), “Deep Nulling of
Visible LASER Light,” Appl. Opt., v38, p7128.
39 Wallace, K., Hardy, G, and Serabyn, E. (2000), “Deep and stable interferometric nulling of broadband
light with implications for observing planets around nearby stars,” Nature, v406.
40 Serabyn, E. and Colavita, M.M. (2001), “Fully Symmetric Nulling Beam Combiners,” Applied Optics,
v40, pp. 1668–1671.
41 Shao, M., Serabyn, E., Levine, B.M., Mennesson, B.P., and Velusamy, T. (2002), “Visible nulling
coronagraph for detecting planets around nearby stars,” SPIE v4860.
42 Levine, B.M., Shao, M., Liu, D.T., Wallace, J.K., and Lane, B.F. (2003), “Planet Detection in Visible
Light with a Single Aperture Telescope and Nulling Coronagraph,” SPIE v5170.
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