Optical Vortex Coronagraph Figures

Optics Letters Preprint (4.8 Mb) --- To Appear 15 December (ol.osa.org)
This work first appeared in the Master Degree Thesis of Greg Foo:
OSC QC350.O77 Vol. 353, 2005.

AIP Press Release

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Schematic of the Lyot solar coronagraph. In 1939 Lyot placed an occulting mask
in the image plane of the sun to allow only the sun's corona to reach the camera.

 

Detailed schematic of a stellar cornoagraph. A vortex coronagraph is achieved
when a vortex phase mask is placed in the first focal plane (FP1). Other planet
detection schemes have been proposed using other types of masks. The vortex
cornonagraph has the advantage that no planet light is lost to absorption, and
all of the starlight is rejected when the star lies along the optical axis.
The vortex phase mask creates a hole the size of the exit pupil (see (a) below)
and thus the Lyot stop in the plane PP2 may be made as large as the exit pupil.
All other stellar coronagraph techniques require small Lyot stops and require the
planet image to occur in a "sweet zone".

 

Experimental images for a mock planet (red) and star (green) system.
(a) Intensity distribution in the plane of the Lyot stop. An apeture is
      placed in this plane to transmit the planet light (red) but obstruct the starlight (green).
(b) Image of the star-planet system without the vortex mask in place.
(c) Same as (b) but with the vortex mask in place. The vortex coronagraph
      has removed the starlight (green).
These photos were taken by Jaehoon Lee, with the assistance of Greg Foo
(both from the College of Optical Sciences, University of Arizona).

 

Measured surface profile of the helical vortex mask used to obtain the images above.
The pitch of the mask corresponds to two full waves of phase retardation for green light.
This false color image was obtained by Joanna Schmit (Veeco, Inc.) using a Veeco interferometric
profilometer. The mask was made by Eric Johnson, CREOL, University of Central Florida.

 

Non-coronagraphic photograph showing Saturn through a vortex phase mask.
This image demonstrates the ease at which a vortex mask can be placed in a telescopic system.
This photo was made with the help of Eric Christensen and Greg Foo.

 

Computed intensity of vortex coronagraph for a monochromatic point source.
The analytically derived distribution predicts that the intensity is zero across the
black central disk, and that the intensity decreases as r ^{- 4} beyond the edge of the disk.
The parameter r is the radial distance from the center of the disk.

 

Numerically generated images of a star-planet system.
Only the planet is seen owing to the vortex coronagraph.
If the star passes within the resolution angle (~ &lambda / D)
or if the position of the star is displaced from the optical axis,
the intensity will vary as the square of the angular displacement.
Otherwise essentially no starlight is seen and all of the planet light is transmitted.
These calculations assume no aberrations or other scattering sources,
and they assume the vortex mask can be made achromatic.

 

Vortex coronagraphs for microwave or far infrared radiation based on annular gratings
are independently being explored by D. Mawet, P. Riaud, O. Absil, and J. Surdej,
Univ. Liege, Belgium.

 

For more information, contact Grover Swartzlander at
The College of Optical Sciences, University of Arizona
(520) 626-3723
grovers@optics.arizona.edu

This research was supported by the US Army Research Office and the State of Arizona.