A Tale of Two Limits: Fundamental Properties of Photonic-Crystal Fibers

We present analytical results that shed new light on the properties of photonic-crystal fibers (optical fibers with periodic structures in their cladding). First, we discuss a general theorem, applicable to any periodic cladding structure, that gives rigorous conditions for the existence of cutoff-free guided modes—it lets you look at a structure, in most cases without calculation, and by inspection give a rigorous guarantee that index-guiding will occur. This theorem especially illuminates the long-wavelength limit, which has proved difficult to study numerically, to show that the index-guided modes in photonic-crystal fibers (like their step-index counterparts) need not have any theoretical cutoff for guidance. Second, we look in the opposite regime, that of very short wavelengths. As previously identified by other authors, there is a scalar approximation that becomes exact in this limit, even for very high contrast fibers. We show that this “scalar” limit has consequences for practical operation at finite wavelengths that do not seem to have been fully appreciated: it tells you when band gaps arise and between which bands, reveals the symmetry and “LP” degeneracies of the modes, and predicts the scaling of cladding-related losses (roughness, absorption, etc.) as the size of a hollow core is increased.