Diffraction losses in ground-based optical interferometers

We present a numerical analysis of free-space propagation of the beams inside a long-baseline optical/infrared interferometer. Unlike the models of beam propagation used in most previous studies, our analysis incorporates the effects of atmospheric seeing, on the wavefronts entering the interferometer. We derive results for the changes in throughput, coherence loss and fringe-detection signal-to-noise ratio arising from diffraction along the propagation path. Our results for conditions of moderate seeing show that although the flux throughput decreases with propagation distance for a given beam diameter, the fringe contrast increases at the same time. In this case it becomes possible for diffraction to increase the signal-to-noise ratio of the fringe measurements. Previous studies have only considered an arrangement where all the apertures in the beam-propagation system have the same diameter. If the light at the end of the propagation path is collected with a fixed size aperture, we find that in many cases the signal-to-noise ratio for fringe detection is maximized when the initial beam diameter is approximately 30 per cent smaller than the final collector diameter. We discuss the implications of our results in the context of future interferometer designs.