A prime objective of modeling optical fibers is capturing mode confinement losses correctly. This paper demonstrates that specific modeling choices, especially regarding the outer fiber cladding regions and the placement of the computational boundary, have significant impacts on the calculated mode losses. This sensitivity of the computed mode losses is especially high for microstructure fibers that do not guide light by total internal reflection. Our results illustrate that one can obtain disparate mode confinement loss profiles for the same optical fiber design simply by moving the boundary to a new material region. We conclude with new recommendations for how to better model these losses.

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.

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