POLARIZATION EFFECTS AND THEIR MINIMIZATION IN AN INFRARED SINGLE-MODE FIBEROPTIC DOUBLE-FOURIER STELLAR INTERFEROMETER

In astronomical double-Fourier interferometry, stretching a single-mod e fibre to generate optical path length modulation is an attractive al ternative solution to the more classical bulk delay line. However, suc h an operation is not free from side effects, one of them being the pe rturbation of the radiation state of polarization by the waveguide. We present both theoretical and experimental investigations of the polar ization behaviour of single-mode fibres in a double-Fourier interferom eter. Changes in the state of polarization can be induced by asymmetri es in the fibre parameters, and macrobending, twisting, etc., but the main cause of perturbation is the stretch of the fibre which is wrappe d onto a piezoelectric tube driven by a high voltage. Not only are the se fibre-stretch-induced polarization effects chromatic, but also they depend on the stretching state. The general consequences of the polar ization perturbations for double-Fourier interferometric imaging are t o distort the source's reconstructed spectral and spatial brightness d istribution, depending upon the fringe detection scheme. Polarization effects can be minimized by introducing a polarization controller or b etter yet, as is proposed here, the fibre can be wrapped in a specific way to neutralize the perturbations.