Characterisation Of Optical Fibres By Measurement Of The Transmitted Near Field

Abstract

Methods of deriving propagation parameters of optical fibres from measurements of the transmitted near field are described. The transmitted near field of an optical fibre is dependent on the illumination wavelength spectrum and this is used to determine refractive index and mode spot profiles. When an optical fibre is multimoded by illuminating with white light the transmitted near field intensity distribution is directly related to the refractive index profile. This can be used to achieve a reasonably accurate measure of the refractive index profile of both multimode and monomode fibres. Both the theoretical considerations and the practical conditions for performing this measurement are described. When the optical fibre is single-moded the transmitted near field intensity distribution is the mode spot distribution. Many of the propagation characteristics can be determined from this distribution. For standard single mode fibres, such as step index or depressed cladding, the spot size and hence the equivalent step index are easily derived from this distribution. Advanced single mode fibres can be characterised from a more detailed knowledge of this distribution. Other techniques have been suggested and used for these measurements, they are all theoretically interrelated. By considering the relationship between the refractive index profile, the transmitted near field intensity distribution and the far field intensity distribution it is possible to estimate the relative merits and errors of these techniques.