Abstract

The four-quadrant phase mask (FQPM) is an exciting new approach to coronagraphy, a classic astronomical technique for detecting faint companions very close to bright stars. Starlight rejection is potentially very high, and inner working distances are substantially smaller than those achieved with classical Lyot coronagraphy. The key component of the original FQPM scheme is a transparent mask divided into quadrants delivering relative phase shifts alternately 0/π/0/π, inserted in an intermediate focal plane of the telescope. Monochromatic masks of this kind have been successfully demonstrated in laboratory and telescope tests. Fabrication of masks with achromatic π phase shifts is challenging but of great interest for optimum astronomical sensitivity. In this paper I present a novel concept for achromatic FQPM operation that utilizes intrinsic phase relationships between transmitted and reflected beams in a dielectric beam splitter.

© 2005 Optical Society of America

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References

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    [CrossRef]
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1996 OSA Technical Digest Series

R. G. Dekany, "The Palomar Adaptive Optics System," in Adaptive Optics 13, OSA Technical Digest Series, 402 (1996).

Astrophys. J.

A. Sivaramakrishnan, C. D. Koresko, R. B. Makidon, T. Berkefeld, and M. J. Kuchner, "Ground-based Coronagraphy with High-order Adaptive Optics," Astrophys. J. 552, 397 (2001).
[CrossRef]

E. E. Bloemhof, "Remnant Speckles in a Highly Corrected Coronagraph," Astrophys. J. 610, L69 (2004).
[CrossRef]

Nature

J. K. Wallace, G. Hardy, and E. Serabyn, "Deep and stable interferometric nulling of broadband light with implications for observing planets around nearby stars," Nature 406, 700 (2000).
[CrossRef] [PubMed]

J. J. Lissauer, "Extrasolar planets," Nature 419, 355 (2002).
[CrossRef] [PubMed]

Opt. Express

Opt. Lett.

PASP

D. Rouan, P. Riaud, A. Boccaletti, Y. Clénet, and A. Labeyrie, "The Four-Quadrant Phase-Mask Coronagraph. I. Principle," PASP 112, 1479 (2000).
[CrossRef]

P. Riaud, A. Boccaletti, D. Rouan, F. Lemarquis, and A. Labeyrie, "The Four-Quadrant Phase-Mask Coronagraph. II. Simulations," PASP 113, 1145 (2001).
[CrossRef]

P. Riaud, A. Boccaletti, J. Baudrand, and D. Rouan, "The Four-Quadrant Phase Mask Coronagraph. III. Laboratory Performance," PASP 115, 712 (2003).
[CrossRef]

A. Boccaletti, P. Riaud, P. Baudoz, J. Baudrand, D. Rouan, D. Gratadour, F. Lacombe, and A.-M. Lagrange, "The Four-Quadrant Phase Mask Coronagraph. IV. First Light at the Very Large Telescope," PASP 116, 1061 (2004).
[CrossRef]

Proc. SPIE

P. Haguenauer, E. Serabyn, E. E. Bloemhof, J. K. Wallace, R. O. Gappinger, B. P. Mennesson, M. Troy, C. D. Koresko, and J. D. Moore, "An off-axis Four-Quadrant Phase-Mask coronagraph for Palomar: high contrast near bright stars imager," Proc. SPIE 5905, 59050S-1 (2005).

J. P. Lloyd, D. T. Gavel, J. R. Graham, P. E. Hodge, A. Sivaramakrishnan, and G. M. Voit, "Four Quadrant Phase Mask: Analytical Calculation and Pupil Geometry," Proc. SPIE 4860, 171 (2003).
[CrossRef]

M. Troy, R G. Dekany, B. R.Oppenheimer, E. E. Bloemhof, T. Trinh, F. Dekens, F. Shi, T. L. Hayward, and B. Brandl, "Palomar Adaptive Optics Project: Status and Performance," Proc. SPIE 4007, 31 (2000).
[CrossRef]

D. Mawet, C. Lenaerts, V. Moreau, Y. Renotte, D. Rouan, and J. Surdej, "Achromatic Four Quadrant Phase Mask using the Dispersion of Form Birefringence," Proc. SPIE 4860, 182 (2003).
[CrossRef]

R. M. Morgan, J. H. Burge, and N. Woolf, "Final laboratory results of visible nulling with dielectric plates," Proc. SPIE 4838, 644 (2003).
[CrossRef]

J. D. Phillips, "Beamsplitters for Astronomical Optical Interferometry," Proc. SPIE, 2477, 132 (1995).
[CrossRef]

Other

W. A. Traub, "Beam Combination and Fringe Measurement," in Principles of Long Baseline Stellar Interferometry, Course Notes from the 1999 Michelson Summer School, P. R. Lawson, ed., Chap. 3, p. 31 (1999).

E. E. Bloemhof, "Design of a 'self-nulling' beam combiner needing no external phase inversion," Opt. Comm., in press.

P. Yeh, Optical Waves in Layered Media, (Wiley, New York, 1988).

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Figures (2)

Fig. 1.
Fig. 1.

Schematic of operation of a coronagraph. The classical Lyot coronagraph and the recently-devised four-quadrant phase mask (FQPM) coronagraph share this architecture, but use different masks in the intermediate focal plane FP1. (The Lyot coronagraph uses an opaque spot of diameter ~λ/D; the FQPM uses a phase mask that evenly divides the intermediate focal plane and shifts phase in alternating quadrants by 0/π/0/π.) The FQPM works best with a circular, unobscured entrance pupil. Either Lyot or FQPM diverts light from an unresolved on-axis star to the outer edge of the reimaged pupil, P2, where it may be blocked by a “Lyot” stop of diameter somewhat less than that of the pupil.

Fig. 2.
Fig. 2.

Schematic achromatic implementation of the FQPM coronagraph using innate phase-shifting properties of beam splitters. A converging telescope beam is focused on the first beam splitter and produces a transmitted and a reflected beam. Masks immediately following define the four FQPM quadrants; these are simple intensity masks consisting of alternating opaque and transparent areas. Each beam is refocused on a second beam splitter, matched to the first, by suitable mirrors M1 that are matched to each other. Then the upper output beam is proportional to either rr’ (for clear quadrants of the mask in the upper path) or tt (for clear quadrants of the mask in the lower path). For general dielectric (non-absorbing) beam splitters, which need not be symmetric, these two terms are automatically out of phase by π at any wavelength or polarization (see text). With quadrant mask images suitably aligned, a broadband FQPM focal-plane mask results. (Subsequent optics, not shown, form a reimaged pupil with Lyot stop and a final image plane in which light from an on-axis point source will be coronagraphically suppressed.) The on-axis attenuation achieved will be limited mainly by the tolerance to which beam splitter coatings can deliver R=T over a broad spectral band.

Equations (3)

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Δ φ = 2 π λ ( n 1 ) Δ t ~ π ;
R S ( Δλ ) 48 π 2 ( λ Δλ ) 2 .
tt ' * + rr * = 1 ; tr ' * + rt * = 0 ,

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