  In this paper we propose a new hypothesis of out of plane band gap formation
in all solid photonic band gap (PBG) fibers. At first, one considers the
process of the core modes formation as a result of excitation of continuous
mode spectrum of single dielectric rod of the cladding. It is shown that the
core modes can be considered as transverse resonance of a set of the radiation
modes decoupled from the continuum provided that the phase matching condition
for the propagation constants are fulfilled. An analogy between the core mode
formation in all solid PBG fibers and formation of bound states in the
continuum in photonics are drawn. Based on these conclusions, the light guiding
in all solid PBG fiber with disordered cladding structure and quasi 3D photonic
band gap formation in all solid PBG fibers with a small index (1%) can be
explained.
