ABSTRACT
The idea of the fibre having photonic bandgap (PBG) cladding was first put forward by Professor P. Russell in 1991 and it materialized a few years later in 1996. The major characteristics of the PCF are encrypted in its photonic crystal cladding formed by a periodic array of air holes. In general, the simplest photonic crystal cladding is a biaxially periodic, defect-free, composite material with its own well-defined dispersion and band structure. The bands of photonic states arise from the coupled resonances of individual rods. For effective indices above the background index, these resonances are the waveguide modes of the rods, but the rod modes retain their identity below the cut-off, as leaky modes. Unlike the PBG fibre, the kagome structure does not exhibit any band structure; rather, the core-guidance frequencies and indices are such that the photonic structure exhibits a nonzero density of photonic states.
