Adaptive Nulling for Interferometric Detection of Planets

An adaptive-nulling method has been proposed to augment the nulling-optical-interferometry method of detection of Earth-like planets around distant stars. The method is intended to reduce the cost of building and aligning the highly precise optical components and assemblies needed for nulling.

Typically, at the mid-infrared wavelengths used for detecting planets orbiting distant stars, a star is millions of times brighter than an Earth-sized planet. In order to directly detect the light from the planet, it is necessary to remove most of the light coming from the star. Nulling interferometry is one way to suppress the light from the star without appreciably suppressing the light from the planet.

altIn nulling interferometry in its simplest form, one uses two nominally identical telescopes aimed in the same direction and separated laterally by a suitable distance. The light collected by the two telescopes is processed through optical trains and combined on a detector. The optical trains are designed such that the electric fields produced by an on-axis source (the star) are in anti-phase at the detector while the electric fields from the planet, which is slightly off-axis, combine in phase, so that the contrast ratio between the star and the planet is greatly decreased. If the electric fields from the star are exactly equal in amplitude and opposite in phase, then the star is effectively “nulled out.”

Nulling is effective only if it is complete in the sense that it occurs simultaneously in both polarization states and at all wavelengths of interest. The need to ensure complete nulling translates to extremely tight demands upon the design and fabrication of the complex optical trains: The two telescopes must be highly symmetric, the reflectivities of the many mirrors in the telescopes and other optics must be carefully tailored, the optical coatings must be extremely uniform, sources of contamination must be minimized, optical surfaces must be nearly ideal, and alignments must be extremely precise. Satisfaction of all of these requirements entails substantial cost.
In the proposed method, a compensator would be inserted into each optical train, upstream of the location where the output beam from the two telescopes are combined. Each compensator would be an optical subsystem that would control the amplitude and phase of the electric field of the spatial mode that couples into the detector, and would do so independently at each wavelength for each of the two polarization states of the beam. The compensator would correct for the imperfections in the optical train and in the beam combiner, making it possible to obtain a deep null from an imperfect instrument.

In one conceptual compensator (see figure), the uncompensated beam from the telescope would be split by a birefringent optical element into vertically and horizontally polarized components, which would be dispersed into wavelength components. The light of the various wavelength components would be focused by a paraboloidal mirror onto a deformable mirror, forming two bright lines, each corresponding to the dispersed spectrum for each polarization state. That is to say, each combination of polarization and wavelength would be focused to a different point on the mirror. The local piston displacement and local slope of the deformable mirror would be controlled to control the phase and amplitude, respectively. Then the light would be re-collimated by the paraboloidal mirror, the wavelength components would be recombined by another dispersive optical element, and then the horizontal and vertical polarization components would be recombined by another birefringent element to produce a single, corrected output beam. The sensing of the amplitude and phase errors and the control of the deformable mirror would be effected by use of a combination of previously developed nulling and wavefront-sensing-and-control techniques. This approach has been successfully demonstrated in the laboratory, both at nearinfrared and mid-infrared wavelengths.

This work was done by Oliver P. Lay and Robert D. Peters of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-40152

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Adaptive Nulling for Interferometric Detection of Planets (reference NPO-40152) is currently available for download from the TSP library.

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