Optical fibres: the best electromagnetic waveguides ever

The publication by Kao and Hockham in 1965 of what has become accepted as the first serious analysis of the prospects for optical fibre communications had on its first page “Solving the Maxwell equations under the boundary conditions imposed by the physical structure….”, followed by the famous expressions for the optical modes in a cylindrical geometry and the observation that the lowest order HE11 mode in a fibre had no cut-off. Since that time, mode control in optical fibres has been key to optical fibre development, be it limiting the modal diffusion (coupling) in multimode fibres, to minimising bend or microbend losses, or to controlling the ‘modality’ and modal instability in high-power lasers operating at kilowatt levels.The progress of optical fibres is charted over the decades through understanding the guidance conditions set out by Maxwell, or their ‘weakly-guiding’ simplifications. Particular examples are at the two extremes of fibre performance, the 0.146 dB/km of today’s ULL (ultra-low loss) fibres and the remarkable kWatt power handling ability of large core fibres. The prospects for new air core fibres with very different guidance mechanisms than that analysed by Kao and Hockham will be examined and some predictions for the future will be made.