Design of effectively single-mode air-core photonic bandgap fiber with improved transmission characteristics for the realization of ultimate low loss waveguide.

In this paper, we study the novel propagation properties of an improved triangular-type air-core photonic bandgap fiber (PBGF) structured with an anti-resonant silica core surround, through a full-vector modal solver based on the finite-element method (FEM). At first, to realize a single-mode operation over a wide wavelength range, the fiber whose core is constructed by removing 1 air-hole and expanded is proposed and structurally-optimized. In particular, the structural parameters for the fiber that prevent the narrow-band transmission due to the existence of the surface modes and enhance the confinement of the power in the air-core are presented. For the realization of an ultimate low loss transmission, a 7-unit-cell PBGF is analyzed and we show that the 7-unit-cell PBGF can achieve not only lower confinement loss than that of regular-type 7-unit-cell PBGF, but also lower power fraction in the silica-ring when compared with the regular 19-unit-cell PBGF with an anti-resonant core surround, exhibiting an effectively single-mode operation.