Comparison of refractive index measurements of optical fibres by three methods

Abstract

A comparison of three methods of measuring the refractive index profiles of optical fibres is presented. Refracted near-field, and axial interferometry measurements have been compared on a step-index fibre, and parabolic profile fibres which have large areas of uniform refractive index at the core. Only the maximum refractive index differences have been compared so that the interferometry can be more accurately and easily performed. Differences between the measurements, possible difficulties with the calibration methods, and uncertainties are discussed. An RNF calibration error, owing to changes in transmissivity of the instrument with changes in angle of incidence of the refracted rays, have been measured. Measurements made using axial interferometry are considered to have an uncertainty not exceeding ±1%, and should be suitable for calibrating step index calibration fibres. A modified refracted near-field design is proposed under development, which is capable of absolute calibration overcoming some of the difficulties inherent in other instruments. Simultaneous refracted and transmitted near field measurements have been made so that accurate differences could be easily observed. The effects of fibre length and radii of fibre loops are presented, and measurements showed the theoretical leaky-mode correction factors to be too large. With a parabolic profile, measurements of profile and core radius were in best overall agreement with RNF results when a 2 m length of fibre was used.