Abstract: Numerical investigation on fundamental mode of PBG-PCF with six-fold rotation symmetry is carried out with full-vector plane wave and multipole method. Band gap of two-photonic crystal, constituting PBG-PCF, is determined by full-vector plane wave method. Frequency of possible existing fundamental modes(bound modes) are not only in band gap but also above line k₀α=βα. Radiation modes exist out of band gap. Multipole method is performed with input frequency as variables. Field distribution of fundamental mode is plotted by MATLAB. It indicates that fundamental modes do not exist in all possible domains. It gives a feasible method to find fundamental modes of PBG-PCF.

Key words: photonic band gap guiding photonic crystal fibers (PBG-PCF), full-vector plane wave method, multipole method, fundamental mode