A holographic vortex coronagraph (HVC) has been proposed as an improvement over conventional coronagraphs for use in high-contrast astronomical imaging for detecting planets, dust disks, and other broadband light scatterers in the vicinities of stars other than the Sun. Because such light scatterers are so faint relative to their parent stars, in order to be able to detect them, it is necessary to effect ultra-high-contrast (typically by a factor of the order of 1010) suppression of broadband light from the stars. Unfortunately, the performances of conventional coronagraphs are limited by low throughput, dispersion, and difficulty of satisfying challenging manufacturing requirements. The HVC concept offers the potential to overcome these limitations.

In a Holographic Vortex Coronagraph, a telescope would focus light onto a blazed first-order vortex grating. A beam block would absorb residual undiffracted light. Lens A would collimate the first-order-diffracted light, forming an exit pupil wherein a Lyot stop would be placed. Lens B would reimage the light transmitted through the Lyot stop to the final focal plane.
A key feature of any coronagraph is an occulting mask in the image plane of a telescope, centered on the optical axis of the telescope. In a conventional coronagraph, the occulting mask is an opaque amplitude mask that obstructs the central starlight when the optical axis points toward the star in question. In the HVC, the occulting mask is a holographic vortex grating, which may be created by etching or otherwise forming the interference pattern of a helical phase ramp and a plane wave beam into an optical surface. As shown in the figure, the interference pattern has a forked appearance.

Light incident perpendicular to the grating is diffracted into discrete orders at angles given by θn = sin–1(nλ / d), where n is the diffraction order, λ is the wavelength of the diffracted light, and d is the lateral distance between adjacent grooves in the grating. In addition, the grating could be blazed to concentrate the diffracted light primarily into one order. If the grating is blazed to concentrate the light into the first (n = 1) order, then almost all of the light from a star or any other on-axis source will be transformed into a beam having a helical wavefront. Total destructive interference occurs along the axis of the helix over a broad wavelength band, attenuating the light from the star or other on-axis source.

The holographic vortex grating in the HVC is placed at the focus of the telescope and is designed and fabricated so as to almost completely suppress light from an on-axis star without significantly affecting images of planets or other light scatterers near the star. The starlight removed from the exit pupil appears outside exit pupil, whereas the light from scatterers near the star appears within the exit pupil. A Lyot stop — an aperture stop to block the starlight while passing the light from nearby scatterers — is placed in the exit pupil.

On the basis of previous research, it is anticipated that in comparison with a conventional coronagraph, the HVC would be less sensitive to aberrations, would yield higher throughput of light from scatterers near stars, and would offer greater planet/star contrast. On the basis of previous achievements in the fabrication of gratings similar to holographic vortex gratings, it appears that the grating for the HVC could readily be fabricated to satisfy initial requirements for imaging of extrasolar planets.

This work was done by David Palacios 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-45047

Topics:
Fabrication Forming Imaging and visualization Logistics Manufacturing processes Optics Photonics Physical Sciences Research and development Sun and solar Telescopes
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Holographic Vortex Coronagraph

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    This article first appeared in the July, 2010 issue of Photonics Tech Briefs Magazine (Vol. 34 No. 7).

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    Overview

    The document discusses the Holographic Vortex Coronagraph (HVC), a cutting-edge technology developed to enhance the detection and characterization of faint exo-solar planetary systems, a key objective of NASA's Terrestrial Planet Finder Coronagraph (TPF-C) mission. The challenge in detecting these distant planets lies in their low brightness compared to their parent stars, necessitating advanced starlight suppression techniques with a contrast ratio on the order of 10^-10.

    Conventional coronagraphs face limitations such as low throughput, dispersion, and complex manufacturing requirements, which hinder their effectiveness in ultra-high contrast imaging. The HVC addresses these issues through its innovative design, which includes a blazed first-order vortex grating (VG) that allows for higher light throughput and improved planet/star contrast. The document details a preliminary mask design for the VG, which was modeled using PMMA glass and specific groove spacing, achieving a monochromatic contrast of 2x10^-12 at a critical position, surpassing the TPF-C mission's requirements.

    The HVC is particularly advantageous for broadband operation, as it is naturally achromatized, maintaining consistent topological charge across different wavelengths. However, the document notes that the first-order diffraction angle's dependence on wavelength can lead to image smearing, which is a challenge for broadband performance. The use of a specially shaped Lyot stop can mitigate some of these issues, although it may reduce throughput by approximately 15% for a 10% bandwidth.

    Despite the challenges of broadband operation, the HVC is predicted to achieve a contrast of 5.1x10^-9 even with light leakage from higher diffraction orders. This performance has not yet been demonstrated by existing ultra-high contrast imaging techniques, making the HVC a promising solution for future planet-finding missions.

    Overall, the Holographic Vortex Coronagraph represents a significant advancement in the field of astronomical imaging, offering a robust solution to the limitations of traditional coronagraphs. Its unique features, including low sensitivity to aberrations and enhanced pupil nulling, position it as a leading candidate for ultra-high contrast imaging applications, ultimately aiding in the exploration of exo-solar planetary systems.