Attenuation of an optical fibre immersed in a high-index surrounding medium

A theoretical description is given of the propagation characteristics, in particular the attenuation, of an optical fibre immersed in a medium whose refractive index is larger than that of the core medium. The complete range of possible V values is considered, both above and below core-mode cutoff, and approximate analytic expressions are derived for various regions of interest within this range. A variety of analytical techniques are used to obtain solutions even where conventional perturbation techniques fail.