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An optical vortex coronagraph for high-contrast imaging

Optical vortex coronagraphy, a new technique for detecting Earth-like extrasolar planets, has several key advantages over other high-contrast imaging techniques.
20 November 2006, SPIE Newsroom. DOI: 10.1117/2.1200611.0474

Since ancient times humankind has pondered the question, Are we alone in the universe? Until recently this question was considered to be purely philosophical. However, modern scientists are now on the verge of finding an answer. To date, hundreds of extrasolar planets the size of Jupiter or larger have been discovered through ground-based astronomy. Unfortunately, present techniques can detect only planets significantly larger than Earth. In order to find Earth-like planets, we must build a telescope that can detect a planet signal that is 10 billion times fainter than the star it's orbiting! If we are to have any hope of finding terrestrial planets, we must suppress the star's light while leaving the planet's light relatively intact.

A stellar coronagraph is one device that makes this goal realizable. This instrument uses an occulting mask placed in the image plane of a telescope, creating an artificial stellar eclipse. The intercepted starlight is scattered into a bright ring, which is then blocked by an aperture known as a Lyot stop. Special amplitude occulting masks that highly attenuate the on-axis starlight have been proposed, but current designs are flawed.1 The masks suffer from an inability to work in close proximity to a star, low planet light throughput, and high sensitivity to system aberrations. There is, however, a unique solution that solves all of these problems simultaneously: the optical vortex coronagraph (OVC).2–6

An OVC operates differently from a traditional coronagraph. Normally, the star must be eclipsed by a large dark spot, which blocks starlight much as the moon blocks sunlight during a solar eclipse. Because coronagraphs are not perfect, some of the light from nearby planets will also be eclipsed, decreasing the throughput of planet light. Furthermore, the Lyot stop that obstructs the scattered starlight also limits planet light throughput in the excluded area. This degrades the coronagraph's ability to see planets close to the star. In an OVC, the amplitude-occulting mask is replaced with a helical shaped phase plate known as an optical vortex mask (OVM). The OVM creates a hole in the starlight instead of eclipsing the star. As a result, nearby planet light will not be as heavily attenuated as with a conventional device. Additionally, an OVC scatters starlight outside the pupil, maximizing planet light throughput and increasing our ability to observe planets closer to the parent star. Compared with existing instruments, an OVC could provide the same (or better) scientific gain with a telescope half the size.

An ideal OVC completely eliminates starlight while leaving planet light untouched. However, the shape of a manufactured OVM will be pixilated by the etching process, which decreases performance. We simulated this effect with a currently manufacturable non-ideal OVM composed of a grid of square, 0.2 × 0.2μm pixels. To validate the utility of an OVC, we simulated the case of a terrestrial planet 10 billion times fainter than its parent star, separated by an angle of 2λ/D, where λ is the wavelength of light and D is the diameter of the telescope. In Figure 1, panels (a) and (b) contain images simulated before and after implementation of an OVC. We performed the same simulations for an eighth-order band-limited coronagraph, which is currently the best-performing traditional coronagraph; panels (c) and (d) show the results. It is clear that the planet cannot be detected with the traditional coronagraph. But with an OVC, it can be.

Figure 1. These simulated images compare the effects of an optical vortex coronagraph (OVC) and an eighth-order band-limited coronagraph (8th BLC): before and after (a and b) implementation of an (OVC) and before and after (c and d) implementation of an 8th BLC.

In our current SPIE article,6 we present these results in more detail and discuss further benefits of an OVC. Current NASA missions, such as the Terrestrial Planet Finder Mission, could benefit from employing such a device, both for scientific gain and mission design enhancements. Through this mission and the use of an OVC, we may soon know whether we are alone in the universe.

I would like to thank S. Hunyadi for her suggestions and comments. This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

David Palacios
Jet Propulsion Laboratory
Pasadena, CA

David Palacios is a member of the Active Optical Systems group at JPL. He specializes in coronagraphy and active wavefront sensing and control. His present work on vortex coronagraphy is an extension of his PhD dissertation, ‘An Optical Vortex Spatial Filter.’