Direct display of the deflection function of optical fiber preforms

An astigmatic optical system to provide a direct display of the deflection function of an optical fiber preform is described. The deflection function thus obtained can be used to evaluate the quality of the preform directly or it can be digitized and recorded into the computer for the reconstruction of the refractive-index profile. This method has the feature that it can resolve the ray crossing problem that other methods fail to do. Therefore, this system can measure with more accuracy the profile of a preform with a boron-doped layer than other methods can, notably the focusing method. A detailed experimental comparison between this method and the focusing method is given. An error analysis of the system from the uncertainty principle is also given.