  Multimode fibers (MMFs) show great promise as miniature probes for sensing,
imaging and spectroscopy applications. Different parameters of the fibers, such
as numerical aperture, refractive index profile and length, have been already
optimized for better performance. Here we investigate the role of the core
shape, in particular for wavefront shaping applications where a focus is formed
at the output of the MMF. We demonstrate that in contrast to a conventional
round-core MMF, a square-core design doesn't suffer from focus aberrations.
Moreover, we find that how the interference pattern behind a square-core fiber
decorrelates with the input frequency is largely independent of the input light
coupling. Finally, we demonstrate that a square core shape provides an
on-average uniform distribution of the output intensity, free from the
input-output correlations seen in round fibers, showing great promise for
imaging and spectroscopy applications.
