Abstract

Nulling interferometry constitutes a very promising technique in observational astrophysics. This method consists in attenuating the signal of a bright astrophysical object in order to detect much fainter nearby features, e.g. exoplanets around their host star. An on-axis destructive interference is created by adjusting the phases of the beams coming from various telescopes. The huge flux ratio between the parent star and the planet (106 in the thermal infrared) requires unprecedented high performance broadband phase shifters. We present a new design for these key components called Achromatic Phase Shifters (APS). We propose to use subwavelength diffractive optical elements under total internal reflection (TIR) incidence. Our component can be seen as an evolution of the Fresnel Rhomb technology.

© 2005 Optical Society of America

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References

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Appl. Opt.

Icarus

A. Léger, J.M. Mariotti, B. Mennesson, M. Ollivier, J.L. Puget, D. Rouan, J. Schneider, �??Could We Search for Primitive Life on Extrasolar Planets in the Near Future,�?? Icarus 123, 249�??255 (1996).
[CrossRef]

J. Opt. Soc. Am.

Nature

R.N. Bracewell, �??Detecting Non Solar Planets by Spinning Infrared Interferometer,�?? Nature 274, 780�??781 (1978).
[CrossRef]

Opt. Eng.

W. J. Tropf, �??Temperature-dependent refractive index models for BaF2, CaF2, MgF2, SrF2, LiF, NaF, KCl, ZnS and ZnSe,�?? Opt. Eng. 34, 1369-1373 (1995).
[CrossRef]

Principles of Optics

M. Born & E. Wolf, �??Reflection and refraction of a plane wave,�?? in Principles of Optics eds. (Cambridge University press, seventh edition, 1997), pp. 49�??53.

Z Phys

F. Peter, Z Phys 15, 358-368 (1923).
[CrossRef]

Other

Gary J. Hawkins, Spectral Characterisation of Infrared Optical Materials and Filters (PhD Thesis - The University of Reading UK, 1998).

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Figures (4)

Fig. 1.
Fig. 1.

Schematic representation of a subwavelength grating. The main parameters of the structure are: the grating vector ∣K∣ = 2π/Λ, perpendicular to the grating lines, with Λ being the spatial period, the grating depth h and the filling factor f, such that fΛ is the width of the grating ridges. TE and TM are the vectorial orthogonal polarization components of the θ-incident light. ni and nt are the refractive indices of the incident (substrate) and emergent (transmitting) media, respectively.

Fig. 2.
Fig. 2.

Schematic of the TIRG APS component. The TIRG APS is analog to a Fresnel rhomb which TIR interfaces are engraved with an optimized subwavelength grating. A TIRG APS component calculated for a π phase shift possesses two TIR interfaces, each providing a π/2 phase shift such that the resultant is ΔΦ TE-TM,1+2 = ΔΦ TE-TM,1 + ΔΦ TE-TM,2 + π/2 + π/2 = π. Such a component is to be inserted in each interferometer arm and orthogonally from one another.

Fig. 3.
Fig. 3.

Continuous lines: CdTe TIRG APS performances in terms of Null Depth (logarithmic scale). Dotted lines: CdTe Fresnel Rhomb Null Depths (logarithmic scale). Left: 6–11 microns band, the TIRG APS mean Null Depth is μND = 3.8×10-9. Right: 11–18 microns band, the TIRG APS mean Null Depth is μND = 1.8×10-9.

Fig. 4.
Fig. 4.

Electromagnetic field RCWA visualization in the CdTe TIRG APS case at 6 microns. Top left: TE component field. Top right: TM component. Bottom left: TE - TM phase shift field visualization. Bottom right: TIRG APS TE -TM phase shift versus wavelength together with the effective indices.

Tables (2)

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Table 1. Temperature-dependant coefficients for material index representations.

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Table 2. Null Depths for the optimal Fresnel Rhomb configurations and TIRG APS ones and corresponding grating periods for the selected materials.

Equations (2)

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Δ ϕ TE TM = 2 arctan [ sin 2 θ n ti 2 n ti 2 cos θ ] 2 arctan [ sin 2 θ n ti 2 cos θ ]
n Diamond , ZnSe , CdTe , Ge λ T = ( A + B λ 2 λ 2 C + D λ 2 λ 2 E + F λ 2 λ 2 G ) 1 2

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