I. Introduction
Photonic bandgap fibers (PBGFs) with a totally different guiding mechanism than conventional fibers have a potential of revolutionizing the fiber industry. Both air core [1]–[4] and all-solid PBGF [5], [6] have been proposed and demonstrated experimentally. Hybrid photonic crystal fibers in which light can be guided by both PBG and total internal reflection have been reported [7]. Air-core guiding PBGFs have attracted a lot of research attention because of their potential of achieving much lower loss [1], [2] and nonlinearity [3] than the conventional glass-core fibers. The lowest loss air-core PBGF reported so far is 13 dB/km in a seven-hole core fiber [1] and 1.2 dB/km in a 19-hole larger core fiber [4].