Erbium doped fibre optogeometrical and equivalent step index parameters determination from transmitted near-field measurements

We study the optogeometrical and the equivalent step index parameters of a commercial erbium doped fibre. Our set-up is based on the Transmitted Near Field Technique (TNFT). The combination of the TNFT with an infrared camera opens the possibility of yielding very quick measurements. The refractive index profile difference and the mode field diameter have been determined from the near field intensity pattern. Then optogeometrical and two sets of equivalent step-index parameters have been calculated. Finally all these parameters have been used to compute the modelled intensity profiles. We compare these calculated patterns with experiment. All measurements and calculations are carried out at 1310 nm and 1550 nm.