Representation of the vapor-phase axial deposition helical layer structure through a ‘‘spiral transformation’’

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Abstract
A general transformation representative of the helical nature of the VAD (vapor-phase axial deposition) process is presented which provides the basis for a general 3D characterization of the helical layer structure and associated properties throughout the VAD boule based upon rpm, axial growth rate, shape, and properties of the deposition surface. Graphical representations provide an illustration of the basic concept and dependencies. An analytical function for the 3D VAD refractive index profile based on completely invariant steady-state deposition conditions is presented which properly exhibits both azimuthal and longitudinal phase dependencies; the VAD refractive index representation utilized by Okamoto et al. [Appl. Opt. 20, 2314 (1981)] to predict VAD transmission characteristics is shown to be a subset of this general form. Preliminary experimental substantiation of the transformations validity for the representation of (VAD) helical layer structure is provided in the ripple characteristics of preform profiles as well as cross-polarizer photographs. A practical application exists in extending the characterization of the deposition process such that the resultant VAD helical structure in the boule/preform/fiber is predictable and controllable. The distribution of the helical structure in the preform/fiber in fact fingerprints several aspects of the deposition process.