Mode classification and loss mechanism in air-core Bragg fibers

With an equivalent mode-solving model, the mode spectra in air-core Bragg fibers are systematically studied by using an improved full-vector finite-difference method. All supported modes are classified into four categories, namely, guided modes, cladding modes, leaky modes and PML modes, among which the leaky modes can be further subdivided into radiation-like and evanescence-like leaky modes. To ensure that the modes are solved accurately and efficiently with this model, the strategy for choosing model parameters is suggested. Benefiting from this convenient mode solver, the characteristics of cladding modes are observed in details, and potential applications are suggested. Moreover, the rapid loss of a hybrid mode at lower frequencies is explained by an evanescence-like leaky mode induced cutoff, which is different from that of TE01 mode due to the failure of band gap confinement.