Tabletop mid-infrared nulling testbed for the Keck interferometer and the Terrestrial Planet Finder

Abstract

A tabletop rotational-shearing interferometer experiment has been constructed and operated at JPL to serve as a testbed for the mid-infrared (~10 μm) nulling beam combiners on the Keck Interferometer and the Terrestrial Planet Finder. The testbed is a pupil-plane combiner in which destructive combination of the incoming wavefronts is achieved using a rooftop mirror system in which the polarization vector is flipped along the vertical axis on one arm and the horizontal axis on the other. The optical pathlength along one arm is adjustable using a linear stage driven by picomotor and piezoelectric actuators. The combined light is focussed onto a single-pixel LN_2-cooled HgCdTe detector. In order to provide adequate sensitivity in the presence of the very bright thermal emission from the room-temperature optics, the light source is modulated and the output is demodulated using a lock-in amplifier. The optical pathlength difference (OPD) is stabilized under computer control by slowly dithering the actuated arm and balancing the leakage signal on either side of the null. The system has produced a stabilized null depth of < 10^(-4) using a diode laser source emitting at a wavelength of 9.2 μm, and transient nulls of 10^(-2) with a broadband thermal IR source in a 6.4% optical bandpass.

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