Silica-core photonic bandgap fibres: Properties and a criterion for single-mode operation

We have studied the properties of silica-core photonic bandgap fibres with a small diameter to pitch ratio (no greater than 0.4) of high-index rods and small refractive-index difference (no greater than 0.03) in the cladding. Theoretical analysis and experimental data demonstrate that the number of core modes depends only on the number of pure-silica elements that form the core and is independent of cladding parameters. If there is one such element, only one dispersion curve of a core mode falls in the fundamental bandgap and the fibre is single-mode. The fibres having seven such elements are multimode.