This innovation will replace a beam combiner, a phase shifter, and a mode conditioner, thus simplifying the system design and alignment, and saving weight and space in future missions. This nuller is a dielectric-waveguidebased, four-port asymmetric coupler. Its nulling performance is based on the mode-sorting property of adiabatic asymmetric couplers that are intrinsically achromatic. This nuller has been designed, and its performance modeled, in the 6.5-micrometer to 9.25-micrometer spectral interval (36% bandwidth). The calculated suppression of starlight for this 15-cm-long device is 10–5 or better through the whole bandwidth. This is enough to satisfy requirements of a flagship exoplanet-characterization mission.

A 3-dimensional view of the Integrated Optics Achromatic Nuller device. (a) The scales are distorted for visual clarity. The input from the two telescopes is incident on the device from the left. (b) The input field for the case of the two telescope beams arriving in-phase (starlight) and exactly out-of-phase (planet light).
Nulling interferometry is an approach to starlight suppression that will allow the detection and spectral characterization of Earth-like exoplanets. Nulling interferometers separate the light originating from a dim planet from the bright starlight by placing the star at the bottom of a deep, destructive interference fringe, where the starlight is effectively cancelled, or nulled, thus allowing the faint off-axis light to be much more easily seen. This process is referred to as nulling of the starlight.

Achromatic nulling technology is a critical component that provides the starlight suppression in interferometer-based observatories. Previously considered space-based interferometers are aimed at approximately 6-to-20-micrometer spectral range. While containing the spectral features of many gases that are considered to be signatures of life, it also offers better planet-to-star brightness ratio than shorter wavelengths.

In the Integrated Optics Achromatic Nuller (IOAN) device, the two beams from the interferometer’s collecting telescopes pass through the same focusing optic and are incident on the input of the nuller.

The dual-input waveguide structure accommodates two modes, while each of the output waveguides accommodates one mode only. At the input, the waveguide structure is symmetric and, therefore, the fundamental mode of the structure at the input is symmetric and the other mode is anti-symmetric. At the output, one of the waveguides is wider than the other, and therefore has a higher effective refractive index. For the light originating from the star, if the interferometer is perfectly balanced, the input field in the focal plane of the focusing optic at the input of the device is symmetric, while for the light field originating from the planet (assuming the exact π phase shift) it is anti-symmetric. Thus, in the two-mode input waveguide the starlight excites the fundamental mode, while the planet light excites the second, anti-symmetric, mode.

This work was done by Alexander Ksendzov of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47834

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Integrated Optics Achromatic Nuller for Stellar Interferometry

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This article first appeared in the January, 2012 issue of Photonics Tech Briefs Magazine.

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