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The next generation of stellar interferometer arrays will develop methods for observing fainter sources with greater resolution and create synthesized images at optical and IR wavelengths like those obtained from radio telescope arrays. Techniques using single-mode (SM) fiber optics offer significant advantages for future ground and space-based interferometers.
SM fibers and integrated optics components can be used for nearly lossless transport and combination of light beams in a stellar interferometer, avoiding the multiple lossy reflective surfaces that must be kept in precise alignment with conventional optics. Furthermore, SM fibers spatially filter light corrupted by atmospheric seeing fluctuations or optical aberrations, increasing the fringe visibility and potentially improving measurement accuracy for bright sources by an order of magnitude. Controlling the dispersion and polarization properties of SM fibers is possible, but the poor coupling efficiency to aberrated light is a major limitation.
MC fibers consist of a symmetrical arrangement of SM fiber cores inside a common cladding. One MC fiber is placed at the focus of each telescope, where light couples into the fiber and propagates through the cores for a short distance. Each MC fiber is then drawn apart into individual single-core fibers, and the resulting SM fiber beams are interfered pair-wise with beams from other telescopes. MC fibers are predicted to have an improved coupling efficiency over standard SM fibers, and the MC fiber geometry is well suited for transporting and combining light beams with minimal losses in an interferometer array.
Computer simulations of fiber-linked interferometer arrays were performed to evaluate the performance of MC and standard SM fibers with different conditions of atmospheric, photon and detector noise. The effects of waveguide and material dispersion over a broad band at visible wavelengths are also included. The simulations determine the fiber modes, calculate the light coupling, propagate light through the fibers, and measure the beam correlations. Photon and detector noise are added and noisy estimates of the fringe power and bispectrum are found for the interferometer baselines. Statistics are then calculated over large ensembles and the measurements are processed to reconstruct an image of the source object.
MC fibers are found to have greatly improved coupling efficiency over conventional SM fibers to aberrated light, and the noise sensitivity of visibility measurements and images also improves under certain conditions. Simulated images are shown and attempts to make MC fibers are discussed.
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