TPF-I SWG Report - Exoplanet Exploration Program - NASA

D ESIGN AND A R C H I T E C T U R E T RADE S TUDIES
4.6 Beam Combiner Design
In nulling interferometry, excess starlight is blocked using an interferometric cancellation technique,
improving the star-to-planet contrast ratio to enable planet detection over an observation time typically of
several hours. To reduce the stellar leakage signal to a level comparable to the zodiacal background the
nuller should be capable of reaching null depths of at least 10 -5 . To reach deep nulls, a number of
parameters must be controlled, some inside the nuller and some in other associated systems. An important
feature of a successful nulling beam combiner will be identical treatment of both input beams in terms of
phase shifts, reflections and transmissions, angles of incidence, polarizations, etc. Different nuller designs
are being tested in the laboratory and in the field. A modified Mach-Zehnder design (MMZ) has been
deployed on the Keck telescopes, and this design seeks to achieve a high degree of symmetry in the beam
combination by causing each input beam to be both transmitted once and reflected once from nearidentical
beamsplitters (this design achieved 2 million to 1 laser nulls at 10.6 μm).
A different design (also using conventional beamsplitters), the rooftop nuller, was built at JPL for SIM
and achieved high performance nulls (laser transient nulls at 633 nm near 10 -6 and 18% bandwidth redlight
nulls of 10 -4 ) in the visible waveband. A newer concept, the fiber beam combiner is currently being
tested in the laboratory for deployment on a large ground-based telescope. This design seeks to achieve
high beam symmetry by dispensing with the beamsplitters and combining the incoming beams directly on
the tip of a single-mode optical fiber, and this design has already achieved laser nulls of 10 5 in the visible.
Bright
outputs
Beamsplitter 4
Common
mirrors
Nulled
outputs
Inner mirror
Input beams
Beamsplitter 1
Outer mirror
Figure 4-11. Modified Mach-Zehnder nuller used on Keck Telescopes.
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