An experimental equivalent-step-index (ESI) technique for the characte
risation of an arbitrary singlemode fibre is proposed, which is based
on equalising the Petermann spot sizes of the actual fibre and the ESI
fibre. Numerical results for several common profile shapes are presen
ted and discussed. This technique is particularly suitable for predict
ing the properties of singlemode fibres at 1.55 mu m that were origina
lly designed to operate at 1.3 mu m.