Residual stresses in a doubly clad fiber with depressed inner cladding (DIC)

Thermal and mechanical stresses developed in concentric three-layered optic al fiber-core, and inner and outer cladding, have been throughly studied fo r various concentrations of dopants and geometric structures. In order to e xamine the parametric results of thermal stresses in preforms, the stresses were measured with a polariscope. The results agreed well with the theoret ical calculations. The thermal stresses were calculated for three temperatu re ranges in which the glass in each layer has a different thermal expansio n coefficient. The mechanical stresses were studied considering the normal stress in the molten neck down region and its development with time. In ord er to include the time dependence of the stress below softening point, Maxw ell's one dimensional viscoelasticity was applied, In a parametric study, t he analyzes were carried out based on the fiber parameters such as relative index difference, ratio of clad to core, and depressed relative index diff erence. With an increase of core index above the silica, the thermal stress es in core increased linearly, but the depressed inner clad does not affect the stresses in core, From the parametric studies and modeling it was foun d that when the depressed inner cladding (DIC) layer has a large cross-sect ion or high dopant concentration, the mechanical stress in core change from compression to tension.