Physical foundations of achromatic nulling interferometry for stellar coronagraphy
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Physical Foundations of Achromatic Nulling Interferometry for Stellar Coronagraphy A. V. Tavrov Space Research Institute, Russian Academy of Sciences, Moscow, 117997 Russia Moscow Power Engineering Institute, Moscow, 111250 Russia National Astronomical Observatory, 2-21-1 Osawa, Mitaka Tokyo, 181-0015 Japan e-mail: [email protected], [email protected], [email protected] Received January 30, 2008
Abstract—We propose an achromatic interferometer for the observation of a faint off-axis light source against the background of a bright on-axis light source. The on-axis source and its copy acquire an achromatic phase shift by 180° and interfere with a phase difference of π. The achromatic phase shift is attributable to the geometric phase in the three-dimensional interferometer scheme. Interference spatially separates the nulled and bright fields of the on-axis source, redirecting them on opposite sides of the beamsplitter. Interference does not attenuate the field of the off-axis source and redirects it with an equal intensity on both sides of the beamsplitter. We consider the principle of operation of the nulling interferometer and constraints on the attenuation of an extended source due to the decrease in coherence. The laboratory breadboard and experiment are briefly described. PACS numbers: 42.25.Hz, 42.25.Ja, 42.25.Kb, 07.60.-j, 95.55.-n DOI: 10.1134/S1063776108120042
1. INTRODUCTION Achromatic instruments are used in a wide range of optical measurements, from ultrashort pulse devices to astronomical instruments [1–3]. In astronomical observations, nonsolar planets have a total brightness that is lower than the brightness of a star by 6–10 orders of magnitude, depending on the wavelength range, respectively, from the infrared (IR) to the visible. In Bracewell’s method [4], a long-baseline interferometer increases the resolution using two telescopes: in this interferometer, the light from the background on-axis source (star) has a phase shift by π radians and interferes with a phase difference of π. At the same time, the light from the off-axis source (planet) interferes with another phase difference, so that the off- axis source is attenuated only slightly and has a signal level sufficient for photodetection. In recent years, various optical devices have been proposed to attenuate the background signal of the on-axis source. These include phase [5, 6] and focal [7] masks, entrance pupil apodization [8], delay lines [9], achromatic phase-shifting devices [10], and their combinations [11]. A nulling interferometer that directly solves the problem of stellar coronography has been considered as a basic device that removes the background light through an achromatic phase delay by π radians. The achromatic interference coronagraph [12] uses the scheme of a Michelson interferometer with a zero optical path difference. One arm of the interferometer contains spherical and parabolic mirrors for focusing and collimation, while its other arm contains only flat
mirrors. The difference of the interferomet
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