  An efficient technique to solve polynomial eigenproblems is shown to result
in an accurate method for computing leaky modes of optical fibers. The
nonlinear eigenproblem arises from a frequency-dependent perfectly matched
layer and is solved using a contour integral eigensolver. Extensive
computations on an antiresonant fiber with a complex transverse microstructure
are reported. Even when employing up to one million degrees of freedom, the
fiber model appears to remain in a preasymptotic regime where computed
confinement loss values are likely to be off by orders of magnitude. This and
other difficulties in computing mode losses, together with some techniques to
overcome them, are detailed.
