Low loss ZBLAN waveguides for mid-infrared astronomical interferometry

The mid-infrared spectral region is the ideal waveband for hunting young exoplanets, because the planet is still warm from its formation and exhibits a peak in its black-body radiation at ~4 µm. This results in an improved contrast with respect to the parent star. Furthermore, the earths atmosphere exhibits a transmission window at 4 µm. Even though the contrast further improves at longer wavelengths, the thermal emission background during terrestrial observation introduces noise and the telescope has a reduced angular resolution. The direct imaging of exoplanets at small angular separations of < 1 λ/D (wavelength λ, telescope mirror diameter D) requires powerful techniques such as stellar interferometry [1,2]. Stellar interferometry combines the light from several telescopes or multiple sub-apertures of the same telescope to enable sub-diffraction limited imaging. By exploiting photonic components, such as fibres, waveguides, splitters, couplers and phase-shifters, a significant performance improvement of stellar interferometers can be achieved. This is due to spatial filtering of the light by single-mode fibres resulting in light with a plane wavefront and thereby improved fringe stability. Furthermore, stellar interferometry benefits from the inherent environmental stability offered by integrated optics. In order to capitalise on these advantages photonic components for astronomical instruments must exhibit low losses, including coupling, Fresnel and bend; Polarization dependent losses and birefringence should be minimized and the single-mode operation range should be maximized.