Noise analysis of refractive index profile measurements of an optical fibre and preform by deflection function techniques

It is shown that many of the existing index profiling methods for optical fibre and preform are related to one another through the deflection function from which the index profile is reconstructed. These methods are classified into direct deflection function measurement, integral (or phase) measurement, and differential (or intensity) measurement. The deflection function from the integral method is obtained by measuring the phase distribution of the exit rays followed by numerical differentiation. Thus the reconstructed index profile has Gaussian noise. The deflection function from the differential method is obtained by measuring the light intensity distribution followed by a numerical integration. This process enhances low frequency noise but suppresses high frequency noise. The noise from direct method behaves in the similar manner as in the differential method except that it is less pronounced. Studies of the choice of methods for profiling of preform, multimode fibre and single mode fibre are also given.