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Fresnel Rhombs As Achromatic Phase Shifters for Infrared Nulling Interferometry

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Fresnel Rhombs As Achromatic Phase Shifters for Infrared Nulling Interferometry Fresnel rhombs as achromatic phase shifters for infrared nulling interferometry Dimitri Mawet1, Charles Hanot1,Cedric´ Lenaerts1, Pierre Riaud3, Denis Defrere` 1, Denis Vandormael2,Jer´ omeˆ Loicq2, Karl Fleury2, Jean-Yves Plesseria2, Jean Surdej1 and Serge Habraken1;2 1University of Liege,` 17 allee´ du 6 Aout,ˆ B-4000, Sart Tilman, Belgium 2Centre Spatial de Liege,` Avenue du Pre-Aily,´ B-4031, Liege-Angleur,` Belgium 3Observatoire de Paris-Meudon, 5 place Jules Janssen, 92190 Meudon, France [email protected] Abstract: We propose a new family of achromatic phase shifters for in- frared nulling interferometry. These key optical components can be seen as optimized Fresnel rhombs, using the total internal reflection phenomenon, modulated or not. The total internal reflection indeed comes with a phase shift between the polarization components of the incident light. We propose a solution to implement this vectorial phase shift between interferometer arms to provide the destructive interference process needed to disentangle highly contrasted objects from one another. We also show that, modulating the index transition at the total internal reflection interface allows compen- sating for the intrinsic material dispersion in order to make the subsequent phase shift achromatic over especially broad bands. The modulation can be induced by a thin film of a well-chosen material or a subwavelength grating whose structural parameters are thoroughly optimized. We present results from theoretical simulations together with preliminary fabrication outcomes and measurements for a prototype in Zinc Selenide. © 2007 Optical Society of America OCIS codes: (120.3180) Interferometry; (050.5080) Phase Shift. References and links 1. R.N. Bracewell, “Detecting Non Solar Planets by Spinning Infrared Interferometer,” Nature 274, 780–781 (1978). 2. A. Leger,´ 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). 3. A.L. Mieremet, J.J. Braat, H. Bokhove, K. Ravel, “Achromatic phase shifting using adjustable dispersive ele- ments,” Proc. SPIE 4006, 1035–1041 (2000). 4. J. Gay, Y. Rabbia, “Principe d’un coronographe interferentiel,”´ C. R. Acad. Sci. Paris 322, 265–271 (1996). 5. E. Serabyn, M.M. Colavita, “Fully Symmetric Nulling Beam Combiners,” Appl. Opt. 40, 1668–1671 (2001). 6. B. Chazelas, F. Brachet, P. Borde,´ B. Mennesson, M. Ollivier, O. Absil, A. Labeque,` C. Valette, A. LegerA.,´ “Instrumental stability requirements for exoplanet detection with a nulling interferometer: variability noise as a central issue,” Appl. Opt. 45, 984–992 (2006). 7. F. Henault,´ “Design of achromatic phase shifters for spaceborne nulling interferometry,” Opt. Lett. 31, 3635– 3637 (2006). 8. C. Hanot et al., Proc. SPIE 6693-62, in press (2007). 9. A.L. Karlsson, O. Wallner, J.M. Perdigues Armengol, O. Absil,“Three telescope nuller based on multibeam injection into single-mode waveguide,” Proc. SPIE 5491, 831–841 (2004). 10. D. Mawet, C. Lenaerts, P. Riaud, J. Surdej, S. Habraken, D. Vandormael, “Use of subwavelength gratings in TIR incidence as achromatic phase shifters,” Opt. Express 13, 8686–8691 (2005). 11. M.G. Moharam and T.K Gaylord, “Rigourous Coupled-Wave Analysis of planar grating diffraction,” J. Opt. Soc. Am. 71, 811–818 (1981). 12. M. Born & E. Wolf, “Reflection and refraction of a plane wave,” in Principles of Optics eds. (Cambridge Uni- versity press, seventh edition, 1997), 49–53. 13. K. B. Rochford, A. H. Rose, P. A. Williams, C. M. Wang, I. G. Clarke, P. D. Hale, G. W. Day, “Design and performance of a stable linear retarder,” Appl. Opt. 36, 6458–6465 (1997). 14. R. Anderson, “Quarterwaveplate and Fresnel rhomb compared in the 10-Mum CO2 laser emission region,” Appl. Opt. 27, 2746–2747 (1988). 15. P. Clapham, M. Downs, R. King, “Some applications of thin films to polarization devices,” Appl. Opt. 8, 1965– 1974 (1969). 16. J.C. Lagarias, J. A. Reeds, M. H. Wright, and P. E. Wright, “Convergence Properties of the Nelder-Mead Simplex Method in Low Dimensions,” SIAM Journal of Optimization 9 (1), 112–147 (1998). 17. Gary J. Hawkins, Spectral Characterisation of Infrared Optical Materials and Filters (PhD Thesis - The Univer- sity of Reading UK, 1998). 18. F. Peter, Z Phys 15, 358-368 (1923). 19. 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). 20. W. J. Tropf, M.E. Thomas, M.J. Linevsky, “Infrared refractive indices and thermo-optic coefficients for several materials,” Proc. SPIE 3425, 160–171 (1998). 21. A. Labeque,` B. Chazelas, F. Brachet, C. Commeaux, P. Blache, A. Leger,´ M. Ollivier, T. Lepine, C. Valette, “The Nulltimate project: building and testing, at low temperature, achromatic phase shifters to prepare the Darwin mission,” Proc. SPIE 5491, 999–1010 (2004). 22. P. Lalanne, P. Pichon, P. Chavel, E. Cambril, H. Launois, “Interferometric Characterization of Subwavelength Lamellar Gratings,” Appl. Opt. 38, 4980–4984 (1999). 23. W. R. Chen, S. J. Chang, Y. K. Su et al., “Refractive Ion Etching of ZnSe, ZnSSe, ZnCdSe and ZnMgSSe by H2/Ar and CH4/H2/AR,” Jpn. J. Appl. Phys. 39, 3308–3313 (2000). 24. K. Kurisu, T. Hirai, K. Fuse et al., “Development of a Diffractive Optical Element for Laser Processing,” SEI Technical Review 53, 86–91 (2002). 25. G.R. Mariner and K. Vedam, “Stress-optic coefficient of ZnSe at 10.6 microns,” Appl. Opt. 20, 2878–2879 (1981). 26. C. Yang and P. Yeh, “Artificial uniaxial and biaxial dielectrics with the use of photoinduced gratings,” J. Appl. Phys. 81, 23–29 (1997). 1. Introduction Despite the tremendous progress in the indirect detection techniques and the increasing number of detected extrasolar planetary objects, exoplanet hunters are still struggling with the techni- cal challenges hampering the conception of an observatory that will allow the direct imaging and characterization of Earth-like planets. Indeed, even best telescopes can’t see them directly because of the large flux ratio between the planet and its parent star. For example, an Earth-like exoplanet is typically » 109 times fainter than its host star in the visible spectrum, » 106 in the thermal infrared. Among all raised ideas to reach the needed very high dynamic range, such as visible coronagraphy, infrared nulling interferometry proposed by R. Bracewell in 1978 [1] appears to be a most promising technique. The nulling interferometry consists in adjusting the phases (with a p phase shift in the two-telescope case) of the beams coming from various tele- scopes (two or more) to produce a fully destructive interference on the optical axis, nulling the starlight while letting the planetary signal pass through. One of the most critical units of a nulling interferometer is the phase shifter. The difficulty is to conceive it to perform well enough over the whole operational spectral range. Remotely characterizing exoplanet atmospheres, which is our ultimate goal, requires spectroscopic analy- sis over large spectral bands. For example, the Darwin Infrared Space Interferometer [2] con- sidered by the European Space Agency (ESA), or the Terrestrial Planet Finder-Interferometer (TPF-I) considered by NASA, will operate in the infrared wavelength range spanning from 6 to 18 (or 20) micrometers. Several studies have been initiated by both agencies, with the purpose of selecting the best achromatic phase shifter (APS). Exhaustive listing or detailed trade-off between different existing APS is beyond the scope of this paper. However, let us cite the four most promising concepts that are currently being examined: - the dispersive plate APS [3], directly inspired by the techniques used by optical designers to minimize lens chromatic aberrations, uses a given number of glass or dielectric plates, whose materials and thicknesses are optimized together with the free-air optical path difference (OPD), mutually neutralizing the various dispersion gradients to produce a given phase shift over the desired spectral band; - the focus-crossing APS [4], where an achromatic phase shift of p is produced by making the beam cross a focus along one of the interferometer arm; - the field-reversal APS [5], which is based on the achromatic reversal of the electric-field vector on one of the two interfering waves, by means of a rotational shearing interferom- eter with a fixed shear of 180 degrees; - the vectorial APS, subject of this paper, which consists in spatially distributing a phase retardance that is primarily affecting the orthogonal polarization components. According to [6], for an interferometer with stellar leakages similar to those of a Bracewell two-telescope interferometer, a sensible value for the mean instrumental leakage contribution to the null depth would be µ ¶ l 3:37 N(l) = 10¡5 (1) 7 mm In addition to the APS imperfections there are many systematic sources of deterioration of the null depth: e.g., telescope pointing errors, photometric unbalance, OPD errors, etc. Con- sequently, the requirement for the limitation on the null depth due to the APS alone, must be significantly lower than expressed in Eq. 1, i.e., N = 10¡6 or better. In practice, we will always apply a reasonable security coefficient for the performances of the APS by requiring a few 10¡7. As we will show, the vectorial APS family is a promising alternative to the three first men- tioned APS. Indeed, as compared to the difficulty of finding the dispersive plate APS working point, the vectorial APS implementation is straightforward. Moreover, unlike the focus-crossing and field-reversal APS, vectorial APS do not come with a subsequent pupil flip. The pupil in- version, creating a pair of images for one single planet, induces several limitative consequences. First, the spatial coherence g of the interfering beams must be of the order of the null specifi- cation on the APS, i.e.
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