In the area of long-baseline nulling interferometry, high rejection ratios are needed to cancel out the light of a bright central source and look for nearby faint structures or companions. These rejection requirements directly translate into drastic optical constraints. We discuss the possibility of using single-mode waveguides for such applications. Conversely to simple pinholes, single-mode waveguides efficiently correct wave-front defects of both high- and low-order spatial frequencies, ensure a perfect matching of the amplitude profiles coming from the various beams, and can be used with almost optimum coupling efficiency over a broad optical bandpass (typically an octave). They then appear to greatly enhance the feasibility of high-dynamic interferometric coronagraphs.
The coupling between a turbulence-distorted optical beam and a single-mode waveguide is addressed. The coupling efficiency and the coupled phase are derived, both without aberrations and with small aberrations. These analytical expressions are validated by numerical simulations. Correction with adaptive optics is investigated. In the general case, the Strehl ratio is a pessimistic estimator, and the coupled phase is different and has a smaller variance than the classical phase averaged over the pupil. Application fields are heterodyne detection and stellar interferometry, for which spatial and modal filtering are distinguished.
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