Approximations in the evaluation of the impulse response of nearly parabolic fibres

Formulae are derived giving the flight time of a ray impinging on a nearly-parabolic fibre whose core index profile includes a fourth order perturbation term in addition to the usual second order term of parabolic profiles. The method is applied to the case of parallel beam injection to calculate the impulse responses of nearly parabolic fibres characterized either by a fourth order term or by an value very close to 2. Numerical results are given for parameters typical of silica fibres made by the CVD technique.