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

Thermal and mechanical stresses developed in concentric three-layered optical fiber-core, and inner and outer cladding, have been thoroughly studied for various concentrations of dopants and geometric structures. In order to examine the parametric results of thermal stresses in preforms, the stresses were measured with a polariscope. The results agreed well with the theoretical calculations. The thermal stresses were calculated for three temperature ranges in which the glass in each layer has a different thermal expansion coefficient. The mechanical stresses were studied considering the normal stress in the molten neck down region and its development with time. In order to include the time dependence of the stress below softening point, Maxwell's one dimensional viscoelasticity was applied. In a parametric study, the analyzes were carried out based on the fiber parameters such as relative index difference, ratio of clad to core, and depressed relative index difference. With an increase of core index above the silica, the thermal stresses in core increased linearly, but the depressed inner clad does not affect the stresses in core. From the parametric studies and modeling it was found that when the depressed inner cladding (DIC) layer has a large cross-section or high dopant concentration, the mechanical stress in core change from compression to tension.